EP4024415A1 - Superconducting magnet system and method for operating the system - Google Patents

Superconducting magnet system and method for operating the system Download PDF

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Publication number
EP4024415A1
EP4024415A1 EP21150247.1A EP21150247A EP4024415A1 EP 4024415 A1 EP4024415 A1 EP 4024415A1 EP 21150247 A EP21150247 A EP 21150247A EP 4024415 A1 EP4024415 A1 EP 4024415A1
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EP
European Patent Office
Prior art keywords
superconducting
current
temperature
mpcs
superconducting magnet
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EP21150247.1A
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German (de)
French (fr)
Inventor
Cornelis Leonardus Gerardus Ham
Gerardus Bernardus Jozef Mulder
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Koninklijke Philips NV
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Koninklijke Philips NV
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Priority to EP21150247.1A priority Critical patent/EP4024415A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/006—Supplying energising or de-energising current; Flux pumps
    • H01F6/008—Electric circuit arrangements for energising superconductive electromagnets
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/06—Coils, e.g. winding, insulating, terminating or casing arrangements therefor
    • H01F6/065—Feed-through bushings, terminals and joints
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/04—Cooling

Definitions

  • the invention relates to the field of superconducting magnets, and in particular to medical imaging devices like magnetic resonance examination systems and other systems employing superconducting magnet systems.
  • the invention also relates to a method for controlling a ramp-up or ramp-down process for a superconducting magnet system.
  • the invention further relates to a magnetic resonance examination system, the magnetic resonance examination system comprising such a superconducting magnet system.
  • the invention also relates to a method for energizing a superconducting magnet system and an associated computer program product.
  • the present invention refers to MRI magnets, but is not limited to those magnets and is therefore not limited to the use of superconducting magnets in medical systems. Nevertheless modern MR systems have a superconducting magnet made from NbTi. Once the magnet has been ramped-up to field, the current is running in persistent mode. The superconducting coil forms a closed loop without electrical resistance and the current runs forever without the need to add power to it.
  • a magnet persistent mode switch (MPCS) is added, which can be opened during the ramp up or ramp down procedure.
  • MPCS magnet persistent mode switch
  • the MPCS is actually an amount of superconducting wire with a heater to make that specific part of the superconducting wire to become normal and having an electrical resistance.
  • the magnet coil is actually a huge self-inductance.
  • the magnet power supply to energize the magnet delivers for instance a constant voltage and in combination with the high self-inductance the current increases linearly.
  • a typical ramp voltage is 5V.
  • the duration to bring the a typical MRI magnet to field is less than 1 hour for a 1.5T magnet up to about 3 hours for a 3T magnet.
  • Magnet for spectroscopy may have different dimensions and may have a different duration to ramp-up or ramp-down.
  • Conventional magnets have their superconducting coils in a bath of liquid helium.
  • the temperature of those coils is equal to that of the helium bath and is about 4.2K (liquid helium at atmospheric pressure i.e., ⁇ 1bar).
  • the pressure may vary within a (small) pressure range but might also be controlled around a pressure set point. Since the boiling point of helium is a function of the gas pressure, the helium temperature is also within a (small) range or might even be fixed (e.g., 4.2K).
  • a 1.5T magnet loses typically 30L of liquid helium per ramp cycle (ramp-up and ramp-down) and a 3T magnet typically 100L of liquid helium.
  • the magnet power supply may deliver its power with a fixed voltage, in first order leading to a linear increase of the current as function of time (more or less constant dl/dt).
  • the voltage at the magnet itself is lower because of the ohmic voltage losses over the ramp cables.
  • This ohmic voltage increases when the current increases, leading to a lower dl/dt.
  • this magnet power supply could have been programmed for other voltage (or current) profiles as a function of time.
  • the sealed magnet is an example of magnet types where the superconducting coils are not sitting in a bath of liquid material.
  • This can for instance be magnets with superconducting coils made from LTS materials like NbTi as used for conventional magnets, but also from other materials, such as Nb 3 Sn, MgB2, or even more exotic HTS materials.
  • LTS materials like NbTi as used for conventional magnets, but also from other materials, such as Nb 3 Sn, MgB2, or even more exotic HTS materials.
  • These magnets do not be limited to MRI. It can also be used for (high field) spectroscopy, trains, accelerators, etc.
  • the term sealed magnet is mentioned, this must be read as the above described generalization.
  • the energize process is started at a sufficiently low temperature T energize .
  • the ⁇ T (T AutoDischarge - T energize ) must be large enough to deal with the dissipation during the energize process, with dissipation contributions due to eddy currents, hysteresis and dissipation in the MPCS. Therefore the magnet may have a higher critical temperature than a conventional magnet and/or be cooled to a lower temperature than a conventional magnet. This is achieved by adding more superconducting wire to the magnet design.
  • There are two problems involved with the previously described sealed magnet with ⁇ T is: The magnet first needs to be cooled to a pretty low temperature T energize where the cold head has little cooling power. This takes a lot of time to reach that temperature. This can be seen in Fig. 5 described later in the description.
  • the critical temperature of the superconducting coils at the end of the energization process must have a sufficiently large margin compared to the temperature at the end of the energize process (T AutoDischarge ) in order to avoid a Loss of Field (quench).
  • T AutoDischarge the temperature at the end of the energize process
  • a higher critical temperature is achieved by adding more superconductor wire to the coils and this is expensive.
  • a superconducting magnet system comprising: a superconducting coil with a least a superconducting coil winding for generating a magnetic field, wherein the superconducting coil winding is located in a first temperature region, an electrical current lead assembly for transferring electrical current to the superconducting coil winding, a power supply for providing an electrical current to the electrical current lead assembly, the electrical current lead assembly comprising:
  • the first temperature region having a temperature below a critical temperature of a low temperature superconductor, LTS.
  • the second temperature region having a temperature below a critical temperature of a high temperature superconductor, HTS.
  • At least a part of the high temperature superconducting, HTS material of the magnet persistent current switch, MPCS and/or the high temperature superconducting current leads connecting the MPCS with the superconducting coil winding comprises additional electrically conductive material preferred a metal, more preferred copper, wherein the additional electrically conductive material is cladded to the HTS material or embedded as filaments into the HTS material.
  • additional electrically conductive material is cladded to the HTS material or embedded as filaments into the HTS material.
  • the superconducting magnet system comprises a circuit with a contactor, wherein the circuit is arranged in parallel to the MPCS between the first electrical current lead and the second electrical current lead in a third temperature region.
  • the third temperature region has a normal temperature around room temperature or has the same temperature as the first temperature region or as the second temperature region.
  • the high temperature superconducting material, HTS of the magnet persistent current switch, MPCS has a high resistance in normal state. It is advantageous if the copper cladding/filaments do not form one long conductor, in order to achieve a high resistance in normal state.
  • the high temperature superconducting material, HTS of the magnet persistent current switch, MPCS is made of a ceramic material.
  • the high temperature superconducting material, HTS of the magnet persistent current switch, MPCS has a resistance suitable to minimize dissipation when energizing the superconducting magnet coil.
  • Most of the high temperature superconducting materials are ceramic materials having a very high resistance in normal state. In case the HTS material of the MPCS becomes normal, its resistance becomes very high. In that case the dissipation due to the energize process is very low, apart from the heater to keep it in the normal state. In normal operation, i.e., when the magnet is in persistent mode, the magnet current runs through the MPCS. This is also the case for the HTS MPCS.
  • MPCS has a resistance suitable to minimize dissipation when energizing the superconducting magnet coil to prevent an excessive heat in the HTS material that leads to electrical arcing and most probably to a damage of the magnet to a level that it needs to be replaced.
  • the high temperature superconducting, HTS current leads and the high temperature superconducting, HTS magnet persistent current switch, MPCS are separate components.
  • HTS current leads and the high temperature superconducting, HTS magnet persistent current switch, MPCS are integrated components forming one long high temperature superconducting, HTS lead. This has the advantage that unnecessary contacts and welds are avoided.
  • the invention further relates to a magnetic resonance examination system, the magnetic resonance examination system comprising a superconducting magnet system as described above.
  • the invention also relates to a method for controlling a ramp-up process for a superconducting magnet system, the method comprising the following steps:
  • the invention also relates to a method for controlling a ramp-down process for a superconducting magnet system, the method comprising the following steps:
  • the invention also relates to a method for energizing a superconducting magnet system, the method comprising the following steps:
  • the current I remains below a max I(T) including a margin to Ic(T), wherein Ic(T) is the maximum current that can be carried by the superconducting magnet coil without generating a Loss of Field.
  • an energizing speed dI/dT is determined by the speed of cooling down of the superconducting magnet coil.
  • the method for energizing a superconducting magnet system can be used independently of the superconducting magnet system described above. In an embodiment of the invention, however, it may be provided that the superconducting magnet system is a previously described superconducting magnet system with a magnet persistent current switch as described above.
  • the step of providing a superconducting magnet system comprises the step of providing a superconducting magnet system, wherein the MPCS is made of a high temperature superconductor, HTS.
  • the invention also relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method for energizing a superconducting magnet system as described above.
  • Fig. 1 schematically depicts a superconducting magnet system 1 with a magnet persistent mode switch 8 in a first temperature region and a magnet power supply 4 according to the state of the art.
  • the superconducting magnet system 1 comprises a cryostat, including a cooling system for cooling different regions 5, 6, 7 of the superconducting magnet system to different temperatures.
  • the system further comprising a vessel 19 in which the different temperature regions 5, 6, 7 are arranged and a superconducting coil 2 with at least a superconducting coil winding 3 for generating a magnetic field.
  • the coil winding 3 is located in a first temperature region 5, the first temperature region 5 having a temperature below a critical temperature of a low temperature superconductor (LTS). This part of the cryostat is therefore cooled to a temperature of about 4K (2 nd stage of cold head).
  • LTS low temperature superconductor
  • the system further comprises an electrical current lead assembly 9 for transferring electrical current to the superconducting coil winding 3, wherein the assembly comprises a first current lead 10 and a second current lead 11, wherein each lead 10, 11 is electrically connected with the superconducting coil winding 3.
  • the current leads 10, 11 run all the way up to the outside of the magnet cryostat to a region that is close to room temperature (300K) and are connected to a power supply 4.
  • a second temperature region 6 is cooled to about 40K (1 st stage of the cold head).
  • the superconducting magnet system 1 further comprises a magnet persistent current switch 8 (MPCS) located in the first temperature region 5 having a temperature below a critical temperature of a low temperature superconductor (LTS).
  • MPCS magnet persistent current switch 8
  • the MPCS 8 is arranged between the first electrical current lead 10 and the second electrical current lead 11 and the MPCS 8 being adapted to switch between an open state and a closed state of the superconducting magnet coil 2.
  • the MPCS 8 in Fig. 1 is drawn in its open state, i.e., where the superconductor of the MPCS 8 has been warmed up to become normal.
  • the energize or discharge procedure is quite simple.
  • the energize procedure starts with the magnet persistent mode switch 8 (MPCS): the superconducting persistent current loop has to be opened by heating a part of this closed loop.
  • MPCS magnet persistent mode switch 8
  • the magnet power supply 4 can apply voltage to the selfinductance of the superconducting coil 2, but the resistive part of the circuit 8 (MPCS) is connected in parallel of this.
  • the applied voltage leads to a current increase in this magnet self inductance.
  • a part of the current is running through the normal wire of the MPCS 8 and dissipates energy and this keeps the MPCS at a sufficiently high temperature to keep it normal.
  • the heating element is usually not needed at this phase of the process.
  • Fig. 2 schematically depicts a superconducting magnet system 1 with a magnet persistent mode switch 8 (MPCS) in a high temperature superconducting (HTS) temperature region 6 and a circuit 14 with a contactor 15 parallel to the MPCS 8 according to an embodiment of the invention.
  • Fig. 2 shows a superconducting coil 2 with a least a superconducting coil winding 3 for generating a magnetic field.
  • the superconducting coil winding 3 is located in a first temperature region 5 wherein the first temperature region 5 has a temperature below a critical temperature of a low temperature superconductor, LTS.
  • the superconducting magnet system 1 comprises an electrical current lead assembly 9 for transferring electrical current to the superconducting coil winding 3.
  • the electrical current lead assembly 9 consists of a first current lead 10, 12 and a second current lead 11, 13, wherein each current lead 10, 11 is electrically connected with the superconducting coil winding 3.
  • the first current lead 12 and the second current lead 13 are made of a high temperature superconductor, HTS.
  • the superconducting magnet system 1 shown in Fig. 2 further comprises a magnet persistent current switch 8, MPCS made of a high temperature superconductor, HTS, and thermally connected to a second temperature region 6.
  • the second temperature region 6 has a temperature below a critical temperature of a high temperature superconductor, HTS.
  • the region is e.g., cooled to about 40K.
  • the superconducting magnet system 1 further comprises a circuit 14 with a contactor 15, wherein the circuit 14 is arranged in parallel to the MPCS 8 between the first electrical current lead 10 and the second electrical current lead 11 in a third temperature region 7. Wherein a third temperature region is a region that is close to room temperature (300K).
  • the superconducting magnet system 1 comprising a power supply 4 for providing an electrical current to the electrical current lead assembly 9.
  • the first electrical current lead 10 and the second electrical current lead 11 each comprising a contactor 16, 17 wherein the contactors 16, 17 are arranged in the third temperature region 7 between the power supply 4 and the circuit 14 with the contactor 15.
  • the normal temperature of the parallel contactor 15 may only be the case during the energize/discharge process. Depending where it is connected to the current leads 10,11 (above or below contactors 16, 17), the parallel contactor 15 is at the third temperature region 7 (normal temperature) or at the second temperature region 6 ( ⁇ 40K).
  • the high temperature superconducting material is a copper-oxide based high temperature superconducting material.
  • the high temperature superconducting material, HTS of the magnet persistent current switch 8, MPCS has a resistance suitable to minimize dissipation when energizing the superconducting coil 2.
  • HTS current leads 12, 13 connecting the MPCS 8 with the superconducting coil winding 3 comprises metal especially copper.
  • high temperature superconductor filaments can be embedded in a copper matrix.
  • Figs. 3 a) and 3 b) schematically depicts electrical current lead assemblies 9 of a superconducting magnet system 1 according to an embodiment of the invention.
  • the high temperature superconducting current leads 12, 13 and the high temperature superconducting magnet persistent current switch 8 are separate components as shown in Fig. 3 a) .
  • Fig. 3 b) an alternative is shown where the high temperature superconducting current leads 12, 13 and the high temperature superconducting magnet persistent current switch 8 are integrated components forming one long high temperature superconducting lead. This has the advantage that unnecessary contacts and welds are avoided.
  • Fig. 4 shows two flow charts of a method for controlling a ramp-up process shown in Fig. 4 a) and for controlling a ramp-down process shown in Fig. 4 b) of a superconducting magnet system 1 according to an embodiment of the invention.
  • a zero boil-off (ZBO) magnet current leads are inserted prior to the actual energize or discharge procedure and removed after it.
  • the energize procedure starts with the magnet persistent mode switch 8, MPCS: the superconducting persistent current loop has to be opened by heating a part of this closed loop.
  • MPCS the superconducting persistent current loop has to be opened by heating a part of this closed loop.
  • the magnet power supply 4 can apply a voltage to the self-inductance of the magnet coil 2, but the resistive part of the circuit (MPCS) is connected in parallel of this.
  • the applied voltage leads to a current increase in this magnet self-inductance.
  • a part of the current is running through the normal wire of the MPCS 8 and dissipates energy and this keeps the MPCS 8 at a sufficiently high temperature to keep it normal.
  • the heating element 18 is usually not needed at this phase of the process.
  • the current is kept constant (dissipation in MPCS 8 stops) and if the heating element 18 would still be needed, this is switched off.
  • the MPCS 8 is cooling down and is getting superconducting again, the magnet coil 2 is energized.
  • the discharge process is similar, but at the start of this process the current from the magnet power supply 4 has to be at a similar level as the current in the magnet coil 2.
  • the method for controlling a ramp-up process for a superconducting magnet system 1 starts with step 400 by providing a superconducting magnet system 1 according to claim 1 of the invention.
  • Fig. 4 a) describes the ramp-up process starting at zero current.
  • the magnet whould have been discharged partially, e.g., for shimming the magnet, the energizing process looks a bit different. Then the current from the power supply 4 must be set at the right level before opening the MPCS 8, similar to the first part of the ramp-down process.
  • step 410 the contactor 15 of the circuit 14 arranged in parallel to the magnet persistent current switch 8, MPCS in the third temperature region 7, is opened.
  • the MPCS 8 is opened by switching on the heating element 18 of the MPCS 8.
  • step 430 the contactors 16, 17 of the first current lead 10 and the second current lead 11 are closed. Since at the start of the ramp-up process there is no current in the superconducting coil 2, the order of steps 410 to 430 does not matter.
  • the contactors 16, 17 are implemented in the third temperature region 7.
  • the third temperature region can be e.g., a third temperature region around 300 K.
  • the contactors 16, 17 can be implemented at the second temperease 6 region at 40K as well.
  • the practical limitation is that the contactors 16, 17 are preferably placed in a low-field area and that is close to the outside of the cryostat.
  • step 440 the superconducting magnet coil 2 is energized until full current.
  • step 450 the contactor 15 of the circuit 14 arranged in parallel to the MPCS 8 in the third temperature region 7 is closed.
  • the MPSC 8 is closed by switching off the heating element 18 of the MPSC 8.
  • step 470 the current from the power supply 4 is set to zero.
  • step 480 the contactors 16, 17 in the third temperature region 7 of the first current lead 10 and the second current lead 11 are opened. The superconducting magnet system 1 is now ramped-up.
  • Fig. 4 b shows a flow chart of a method for controlling a ramp-down process for a superconducting magnet system 1.
  • the method starts in step 500 with providing a superconducting magnet system 1 according to claim 1 of the invention.
  • step 510 the contactors 16, 17 of the first current lead 10 and the second current lead 11 are closed.
  • the contactors 16, 17 are implemented in the third temperature region 7.
  • the third temperature region can be e.g., a temperature region around 300K.
  • the contactors 16, 17 can be implemented at the second temperautre region at 40K as well.
  • the practical limitation is that the contactors 16, 17 are preferably placed in a low-field area and that is close to the outside of the cryostat.
  • step 520 closing the contactor 15 of the circuit 14 arranged in parallel to the magnet persistent current switch 8, MPSC in the third temperature region 7 is closed.
  • step 530 a current from the power supply 4 is set to the same current as in the superconducting coil 2.
  • step 540 the MPSC 8 is opened by switching on the heating element 18 of the MPSC 8.
  • step 550 the superconducting magnet coil 2 is discharged until zero current.
  • step 560 the MPSC 8 is closed by switching off the heating element 18 of the MPSC 8.
  • step 570 the contactor 15 of the circuit 14 arranged in parallel to the MPSC 8 is closed.
  • step 580 opening the contactors 16, 17 in the third temperature region 7 of the first current lead 10 and the second current lead 11 are opened. Since the current is zero the order of steps 560 to 580 does not matter.
  • the superconducting magnet system 1 is now ramped down.
  • Fig. 5 shows a diagram of the temperature and the current plotted against the time in seconds according to the state of the art.
  • the critical temperature of the superconducting coil 2 at the end of the energization process must have a sufficiently large margin compared to the temperature at the end of the energize process (T AutoDischarge ) in order to avoid a Loss of Field (quench).
  • T AutoDischarge the temperature at the end of the energize process
  • a higher critical temperature is achieved by adding more superconductor wire to the coils and this is expensive.
  • the energize may start when the temperature drops below 7K, i.e., where the maximum allowed current becomes >0.
  • the maximum allowed current increases further and the energize current can be increased to the maximum allowed value.
  • the additional energize current is kind of "titrated" to the magnet, based on the actual temperature and the related maximum allowed current.
  • Fig. 6 shows a diagram of a critical current and a simplified curve for maximum allowed current as function of temperature according to an embodiment of the invention.
  • Fig. 7 shows a flow chart of a method for energizing a superconducting magnet system 1 according to another embodiment of the invention.
  • the method starts with step 700 by providing a superconducting magnet system 1 wherein the superconducting magnet system 1 comprises a superconducting coil 2 with at least a superconducting coil winding 3 for generating a magnetic field, wherein the superconducting coil 2 is in thermal contact with a first temperature region 5 having a temperature below a critical temperature of a low temperature superconductor, LTS, the system further comprising a power supply 4 and a magnet persistent current switch 8, MPCS, wherein the MPCS 8 is arranged to switch between an open state and a closed state of the superconducting magnet coil 2.
  • MPCS magnet persistent current switch
  • the method for energizing a superconducting magnet system 1 can be used independently of the superconducting magnet system 1 as described in claim 1. In an embodiment of the invention, however, it may be provided that the superconducting magnet system 1 is a previously described superconducting magnet system 1 with a magnet persistent current switch (MPCS) 8 as claimed in claim 1.
  • MPCS magnet persistent current switch
  • a current generated by the power supply 4 is set to an initial current value.
  • step 720 the MPCS 8 is activated to its open position and connecting the superconducting magnet coil 2 and the power supply 4, when the actual temperate T actual of the superconducting coil winding 3 falls below the superconducting critical temperature T c (I) at the actual electrical current density minus a safety margin ⁇ T which is higher than the ultimate operating temperature T op .
  • step 730 the superconducting magnet coil 2 is energized with a current, wherein the current and the energizing speed dl/dt is adjusted according to the temperature decrease during cooling of the superconducting coil winding 3, while keeping the actual temperature within the safety margin.
  • the step of energizing the superconducting magnet coil 2 with a current takes place at a higher temperature than the superconducting critical temperature T c (I) at full current.
  • the MPCS 8 is activated to its close position when the target magnetic field strength is reached, thereby placing the superconducting magnet coil 2 in a closed circuit.
  • Fig. 8 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system 1 according to an embodiment of the invention.
  • the ramp speed is now a function of the cool down speed and is in general much lower than based on according to the 5V output of the magnet power supply.
  • Fig. 9 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system 1 with reduced T AutoDischarge according to an embodiment of the invention.
  • Fig. 9 is similar to Fig. 8 , but with a 1K lower T AutoDischarge . Even for this situation the magnet is faster on field than with the existing energize procedure and with the original T AutoDischarge . This is the case when less superconductor would be used. That is the big (financial) advantage of using this energize idea.
  • Fig. 10 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system 1 according to an embodiment of the invention.
  • Advantages that can be seen in Fig. 10 are shorter shim iterations and a faster taking the magnet back to field after a turn off of the magnetic field by initiating an automated ramp-up, minimizing operational downtime. This is the situation of faster shimming (discharge and faster energize again). You do not have to wait with cooling down until T energize has been reached. Furthermore, a shorter installation time and major cost down on superconductors can be achieved.

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Abstract

The invention relates to a superconducting magnet system (1) the system comprising a superconducting coil (2) with a least a superconducting coil winding (3) for generating a magnetic field, wherein the superconducting coil winding (3) is located in a first temperature region (5), a magnet persistent current switch (8), MPCS, wherein the MPCS is made of a high temperature superconductor, HTS, and thermally connected to a second temperature region (6), the second temperature region (6) having a temperature below a critical temperature of a high temperature superconductor, HTS, wherein the MPCS (8) is connected to the superconducting coil winding (3) by the electrical current lead assembly (9), wherein the first current lead (12) and the second current lead (13) are made of a high temperature superconductor, HTS. In this way, the superconducting magnet system (1) can be energized more easily without creating a heat load on the low temperature region of the superconducting magnet coil (2). The invention also relates to a method for controlling a ramp-up or ramp-down process for a superconducting magnet system (1). The invention further relates to a magnetic resonance examination system, the magnetic resonance examination system comprising such a superconducting magnet system (1). The invention further relates to a method for energizing a superconducting magnet system (1) and an associated computer program product. Hence, an improved method for energizing a superconducting magnet system (1) is provided.

Description

    FIELD OF THE INVENTION
  • The invention relates to the field of superconducting magnets, and in particular to medical imaging devices like magnetic resonance examination systems and other systems employing superconducting magnet systems. The invention also relates to a method for controlling a ramp-up or ramp-down process for a superconducting magnet system. The invention further relates to a magnetic resonance examination system, the magnetic resonance examination system comprising such a superconducting magnet system. The invention also relates to a method for energizing a superconducting magnet system and an associated computer program product.
    • BACKGROUND OF THE INVENTION
  • A number of important applications exist for superconductive magnet systems. These include imaging systems, as for medical imaging, as well as spectrometry systems, typically used in materials analysis and scientific research applications. The present invention refers to MRI magnets, but is not limited to those magnets and is therefore not limited to the use of superconducting magnets in medical systems. Nevertheless modern MR systems have a superconducting magnet made from NbTi. Once the magnet has been ramped-up to field, the current is running in persistent mode. The superconducting coil forms a closed loop without electrical resistance and the current runs forever without the need to add power to it. In order to ramp-up, with different words to energize or to ramp-down, with different words to discharge the magnet, a magnet persistent mode switch (MPCS) is added, which can be opened during the ramp up or ramp down procedure. "Opened" means that the switch has a finite resistance, which is very large compared to the zero resistance of the superconducting magnet itself. The MPCS is actually an amount of superconducting wire with a heater to make that specific part of the superconducting wire to become normal and having an electrical resistance. The magnet coil is actually a huge self-inductance. The magnet power supply to energize the magnet delivers for instance a constant voltage and in combination with the high self-inductance the current increases linearly. A typical ramp voltage is 5V. The duration to bring the a typical MRI magnet to field is less than 1 hour for a 1.5T magnet up to about 3 hours for a 3T magnet. Magnet for spectroscopy may have different dimensions and may have a different duration to ramp-up or ramp-down.
  • Conventional magnets have their superconducting coils in a bath of liquid helium. The temperature of those coils is equal to that of the helium bath and is about 4.2K (liquid helium at atmospheric pressure i.e., ∼1bar). The pressure may vary within a (small) pressure range but might also be controlled around a pressure set point. Since the boiling point of helium is a function of the gas pressure, the helium temperature is also within a (small) range or might even be fixed (e.g., 4.2K).
  • During the energize/discharge process dissipation occurs in the 4K structure (coil + helium vessel) due to eddy currents in metal construction and in the superconductor, hysteresis losses in the superconductor and dissipation to open the MPCS and dissipation in the MPCS during the energize or discharge procedure. When the superconductor in the MPCS gets normal, its resistance R is several ohms for zero boil-off (ZBO) magnets and maybe even tens of ohms for sealed magnets, wherein a sealed magnet is an example of magnet types where the superconducting coils are not sitting in a bath of liquid material. The ramp voltage of about 5 V leads to dissipation P=V2/R of a few W for sealed magnet to order of magnitude of 10W for ZBO magnets.
  • For conventional magnets, this dissipation does not lead to temperature increase, but it will lead to additional helium boil-off while the temperature remains constant (assuming constant pressure, see above). A 1.5T magnet loses typically 30L of liquid helium per ramp cycle (ramp-up and ramp-down) and a 3T magnet typically 100L of liquid helium.
  • As stated, the magnet power supply may deliver its power with a fixed voltage, in first order leading to a linear increase of the current as function of time (more or less constant dl/dt). The voltage at the magnet itself is lower because of the ohmic voltage losses over the ramp cables. This ohmic voltage increases when the current increases, leading to a lower dl/dt. This leads to a small deviation from the linear current increase. This depends on the cable resistance (as function of length, diameter, material property and temperature). Apart from ramp-up or ramp-down with a constant voltage (more or less linear current in/decrease, this magnet power supply could have been programmed for other voltage (or current) profiles as a function of time.
  • As already described above the sealed magnet is an example of magnet types where the superconducting coils are not sitting in a bath of liquid material. This can for instance be magnets with superconducting coils made from LTS materials like NbTi as used for conventional magnets, but also from other materials, such as Nb3Sn, MgB2, or even more exotic HTS materials. These magnets do not be limited to MRI. It can also be used for (high field) spectroscopy, trains, accelerators, etc. For the time being, wherever the term sealed magnet is mentioned, this must be read as the above described generalization.
  • For the sealed magnet, the situation is completely different. The state of the art ramp procedure as used for a sealed magnet uses similar ramp voltages as for conventional magnets, i.e., constant voltages and linearly increasing current. However, the temperature of the superconducting coils is not equal to the helium bath, since there is no helium bath anymore. Therefore its temperature does not remain constant as for conventional magnets. In case of dissipation at the superconducting coils of a sealed magnet, their temperature will increase. When the magnet is at full current, its temperature should be with a sufficient margin below the critical temperature. When the magnet temperature is above this level the smart magnet electronics will automatically discharge the magnet in order to prevent a Loss of Field event (quench). Therefore this temperature is called the auto discharge temperature TAutoDischarge.
  • In order to energize the magnet, the energize process is started at a sufficiently low temperature Tenergize. The ΔT = (TAutoDischarge - Tenergize) must be large enough to deal with the dissipation during the energize process, with dissipation contributions due to eddy currents, hysteresis and dissipation in the MPCS. Therefore the magnet may have a higher critical temperature than a conventional magnet and/or be cooled to a lower temperature than a conventional magnet. This is achieved by adding more superconducting wire to the magnet design. There are two problems involved with the previously described sealed magnet with ΔT is:
    The magnet first needs to be cooled to a pretty low temperature Tenergize where the cold head has little cooling power. This takes a lot of time to reach that temperature. This can be seen in Fig. 5 described later in the description.
  • The critical temperature of the superconducting coils at the end of the energization process must have a sufficiently large margin compared to the temperature at the end of the energize process (TAutoDischarge) in order to avoid a Loss of Field (quench). A higher critical temperature is achieved by adding more superconductor wire to the coils and this is expensive.
  • One could argue to add a pause during the energize process to allow the magnet to cool down again. This may reduce ΔT and therefore reduce the additional cost due to the additional superconductor. But pausing also increases the total time to energize the magnet. Usually a magnet is cooling down (at installation or e.g., after a LoF (quench)) and one first needs to wait until the magnet has been cooled down below a threshold temperature and then start energize. This threshold temperature Tenergize has been chosen in such a way that the temperature at the end of the energize process remains below TAutoDischarge.
  • Alternatively, current can be removed or added to superconducting magnet systems very slowly without causing enough heating to boil off the liquid cryogen. In these situations, it takes many hours to completely add or remove the current, making rapid turning the magnetic field on or off in this manner not feasible.
    • SUMMARY OF THE INVENTION
  • It is an object of the invention to provide a superconducting magnet system, whereby the superconducting magnet system can be energized more easily without creating a heat load on the low temperature region of the superconducting magnet coil. It is also a task of the invention to provide an improved method for energizing a superconducting magnet coil.
  • According to the invention, this object is addressed by the subject matter of the independent claims. Preferred embodiments of the invention are described in the sub claims.
  • Therefore, according to the invention, a superconducting magnet system is provided, the system comprising:
    a superconducting coil with a least a superconducting coil winding for generating a magnetic field, wherein the superconducting coil winding is located in a first temperature region,
    an electrical current lead assembly for transferring electrical current to the superconducting coil winding, a power supply for providing an electrical current to the electrical current lead assembly, the electrical current lead assembly comprising:
    • a first electrical current lead and a second electrical current lead, wherein each current lead is electrically connected with the superconducting coil winding, the first electrical current lead and the second electrical current lead each comprising a contactor between the power supply and the superconducting coil winding, the superconducting magnet system further comprising a magnet persistent current switch, MPCS, wherein the MPCS is arranged between the first electrical current lead and the second electrical current lead and the MPCS being adapted to switch between an open state and a closed state of the superconducting coil, the MPCS comprising a heating element,
    • wherein the MPCS is made of a high temperature superconductor, HTS, and thermally connected to a second temperature region,
    • wherein the MPCS is connected to the superconducting coil winding by the electrical current lead assembly, wherein the first current lead and the second current lead are made of a high temperature superconductor, HTS.
  • Hence, it is an essential idea of the invention to implement a magnet persistent current switch at a higher temperature as the superconducting magnet coil itself, wherein the MPCS is made of a high temperature superconductor, HTS.
  • According to a preferred embodiment of the invention the first temperature region having a temperature below a critical temperature of a low temperature superconductor, LTS.
  • In another preferred embodiment of the invention the second temperature region having a temperature below a critical temperature of a high temperature superconductor, HTS.
  • According to a preferred embodiment of the invention at least a part of the high temperature superconducting, HTS material of the magnet persistent current switch, MPCS and/or the high temperature superconducting current leads connecting the MPCS with the superconducting coil winding comprises additional electrically conductive material preferred a metal, more preferred copper, wherein the additional electrically conductive material is cladded to the HTS material or embedded as filaments into the HTS material. Adding a metal especially copper to the HTS material of the MPCS has the advantage that the heater is still capable to open the MPCS and that the HTS is still able to recover to superconducting state after the energize process. Adding a metal to the HTS current leads protects the magnet from a quench of the HTS current leads.
  • According to a preferred embodiment of the invention the superconducting magnet system comprises a circuit with a contactor, wherein the circuit is arranged in parallel to the MPCS between the first electrical current lead and the second electrical current lead in a third temperature region.
  • According to an embodiment of the invention the third temperature region has a normal temperature around room temperature or has the same temperature as the first temperature region or as the second temperature region.
  • Preferably the high temperature superconducting material, HTS of the magnet persistent current switch, MPCS has a high resistance in normal state. It is advantageous if the copper cladding/filaments do not form one long conductor, in order to achieve a high resistance in normal state.
  • More Prefereably the high temperature superconducting material, HTS of the magnet persistent current switch, MPCS is made of a ceramic material. The high temperature superconducting material, HTS of the magnet persistent current switch, MPCS has a resistance suitable to minimize dissipation when energizing the superconducting magnet coil. Most of the high temperature superconducting materials are ceramic materials having a very high resistance in normal state. In case the HTS material of the MPCS becomes normal, its resistance becomes very high. In that case the dissipation due to the energize process is very low, apart from the heater to keep it in the normal state. In normal operation, i.e., when the magnet is in persistent mode, the magnet current runs through the MPCS. This is also the case for the HTS MPCS. Therefore it is advantageous in cases when parts of the HTS material becomes normal, leading to a Loss of Field event (quench), if the superconducting material, HTS of the magnet persistent current switch, MPCS has a resistance suitable to minimize dissipation when energizing the superconducting magnet coil to prevent an excessive heat in the HTS material that leads to electrical arcing and most probably to a damage of the magnet to a level that it needs to be replaced.
  • According to one preferred embodiment of the invention the high temperature superconducting, HTS current leads and the high temperature superconducting, HTS magnet persistent current switch, MPCS are separate components.
  • As an alternative to the solution mentioned above the the high temperature superconducting, HTS current leads and the high temperature superconducting, HTS magnet persistent current switch, MPCS are integrated components forming one long high temperature superconducting, HTS lead. This has the advantage that unnecessary contacts and welds are avoided.
  • The invention further relates to a magnetic resonance examination system, the magnetic resonance examination system comprising a superconducting magnet system as described above.
  • The invention also relates to a method for controlling a ramp-up process for a superconducting magnet system, the method comprising the following steps:
    • Providing a superconducting magnet system as described above, wherein the superconducting magnet system further comprises a circuit with a contactor, wherein the circuit is arranged in parallel to the MPCS between the first electrical current lead and the second electrical current lead in a third temperature region,
    • Opening the contactor of the circuit arranged in parallel to the magnet persistent current switch, MPCS in the third temperature region, opening the MPCS by switching on the heating element of the MPCS, closing the contactors in the third temperature region of the first current lead and the second current lead, where the steps can be performed in any order,
      • Energizing the superconducting magnet coil until full current,
      • Closing the contactor of the circuit arranged in parallel to the MPCS in the third temperature region,
      • Closing the MPSC by switching off the heating element of the MPSC,
      • Setting the current from the power supply to zero,
      • Opening the contactors in the third temperature region of the first current lead and the second current lead.
  • Furthermore, the invention also relates to a method for controlling a ramp-down process for a superconducting magnet system, the method comprising the following steps:
    • Providing a superconducting magnet system as described above, wherein the superconducting magnet system further comprises a circuit with a contactor, wherein the circuit is arranged in parallel to the MPCS between the first electrical current lead and the second electrical current lead in a third temperature region,
    • Closing the contactors in the third temperature region of the first current lead and the second current lead,
    • Opening the contactor of the circuit arranged in parallel to the magnet persistent current switch, MPSC in the third temperature region,
    • Setting a current from the power supply to the same current as in the superconducting coil,
    • Opening the MPSC by switching on the heating element of the MPCS,
    • Discharging the superconducting magnet coil until zero current,
    • Closing the MPSC by switching off the heating element of the MPSC, closing the contactor of the circuit arranged in parallel to the MPSC, opening the contactors in the third temperature region of the first current lead and the second current lead, where the steps can be performed in any order.
  • Furthermore, the invention also relates to a method for energizing a superconducting magnet system, the method comprising the following steps:
    • Providing a superconducting magnet system wherein the superconducting magnet system comprises a superconducting coil with at least a superconducting coil winding for generating a magnetic field, wherein the superconducting coil is in thermal contact with a first temperature region having a temperature below a critical temperature of a low temperature superconductor, LTS, the system further comprising a power supply and a magnet persistent current switch, MPCS, wherein the MPCS is arranged to switch between an open state and a closed state of the superconducting magnet coil,
    • Setting a current generated by the power supply to an initial current value,
    • Activating the MPCS to its open position and connecting the superconducting magnet coil and the power supply, wherein the activation starts at a temperature above the critical temperature of the superconducting magnet coil at its full current temperature Tc(Ifull),
    • Energizing the superconducting magnet coil, while the superconducting magnet coil is above its critical temperature at its full current Tc(Ifull) during a part of the energization process, where the full current is the current at the target field,
    • Activating the MPCS to its closed position when the target magnetic field strength is reached, thereby placing the superconducting magnet coil in a closed circuit.
  • In an embodiment of the invention the current I remains below a max I(T) including a margin to Ic(T), wherein Ic(T) is the maximum current that can be carried by the superconducting magnet coil without generating a Loss of Field.
  • In a preferred embodiment of the invention an energizing speed dI/dT is determined by the speed of cooling down of the superconducting magnet coil.
  • The method for energizing a superconducting magnet system can be used independently of the superconducting magnet system described above. In an embodiment of the invention, however, it may be provided that the superconducting magnet system is a previously described superconducting magnet system with a magnet persistent current switch as described above. In particular, the step of providing a superconducting magnet system comprises the step of providing a superconducting magnet system, wherein the MPCS is made of a high temperature superconductor, HTS.
  • Finally, the invention also relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method for energizing a superconducting magnet system as described above.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such an embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.
  • In the drawings:
    • Fig. 1 schematically depicts a superconducting magnet system with a magnet persistent mode switch in a first temperature region and a magnet power supply according to the state of the art,
    • Fig. 2 schematically depicts a superconducting magnet system with a magnet persistent mode switch (MPCS) in a high temperature superconducting temperature region and a contactor parallel to the MPCS according to an embodiment of the invention,
    • Fig. 3 schematically depicts an electrical current lead assembly of a superconducting magnet system according to an embodiment of the invention,
    • Fig. 4 Fig. 4 a) shows a flow chart of a method for a ramp-up process and Fig. 4 b) a flow chart of a method for a ramp-down process for a superconducting magnet system according to an embodiment of the invention,
    • Fig. 5 shows a diagram of the temperature and the current plotted against the time in seconds according to the state of the art,
    • Fig. 6 shows a diagram of a critical current and a simplified curve for maximum allowed current as function of temperature according to an embodiment of the invention,
    • Fig. 7 shows a flow chart of a method for energizing a superconducting magnet system according to another embodiment of the invention,
    • Fig. 8 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system according to an embodiment of the invention,
    • Fig. 9 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system with reduced TAutoDischarge according to an embodiment of the invention,
    • Fig. 10 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system according to an embodiment of the invention.
    DETAILED DESCRIPTION OF EMBODIMENTS
  • Fig. 1 schematically depicts a superconducting magnet system 1 with a magnet persistent mode switch 8 in a first temperature region and a magnet power supply 4 according to the state of the art. The superconducting magnet system 1 comprises a cryostat, including a cooling system for cooling different regions 5, 6, 7 of the superconducting magnet system to different temperatures. The system further comprising a vessel 19 in which the different temperature regions 5, 6, 7 are arranged and a superconducting coil 2 with at least a superconducting coil winding 3 for generating a magnetic field. The coil winding 3 is located in a first temperature region 5, the first temperature region 5 having a temperature below a critical temperature of a low temperature superconductor (LTS). This part of the cryostat is therefore cooled to a temperature of about 4K (2nd stage of cold head).
  • The system further comprises an electrical current lead assembly 9 for transferring electrical current to the superconducting coil winding 3, wherein the assembly comprises a first current lead 10 and a second current lead 11, wherein each lead 10, 11 is electrically connected with the superconducting coil winding 3. The current leads 10, 11 run all the way up to the outside of the magnet cryostat to a region that is close to room temperature (300K) and are connected to a power supply 4. A second temperature region 6 is cooled to about 40K (1st stage of the cold head). The superconducting magnet system 1 further comprises a magnet persistent current switch 8 (MPCS) located in the first temperature region 5 having a temperature below a critical temperature of a low temperature superconductor (LTS). The MPCS 8 is arranged between the first electrical current lead 10 and the second electrical current lead 11 and the MPCS 8 being adapted to switch between an open state and a closed state of the superconducting magnet coil 2. The MPCS 8 in Fig. 1 is drawn in its open state, i.e., where the superconductor of the MPCS 8 has been warmed up to become normal.
  • In order to energize a ZBO magnet current leads are inserted prior to the actual energize or discharge procedure and removed after it. The energize or discharge procedure is quite simple. The energize procedure starts with the magnet persistent mode switch 8 (MPCS): the superconducting persistent current loop has to be opened by heating a part of this closed loop. Now the magnet power supply 4 can apply voltage to the selfinductance of the superconducting coil 2, but the resistive part of the circuit 8 (MPCS) is connected in parallel of this. The applied voltage leads to a current increase in this magnet self inductance. A part of the current is running through the normal wire of the MPCS 8 and dissipates energy and this keeps the MPCS at a sufficiently high temperature to keep it normal. The heating element is usually not needed at this phase of the process. When the current has reached its final current, the current is kept constant (dissipation in MPCS 8 stops) and if the heater would still be needed, this is switched off. When the MPCS 8 is cooling down and is getting superconducting again, the magnet is energized and the current is persistent. The discharge process is similar, but at the start of this process the current from the magnet power supply 4 has to be at a similar level as the current in the superconducting magnet coil 2.
  • Fig. 2 schematically depicts a superconducting magnet system 1 with a magnet persistent mode switch 8 (MPCS) in a high temperature superconducting (HTS) temperature region 6 and a circuit 14 with a contactor 15 parallel to the MPCS 8 according to an embodiment of the invention. Fig. 2 shows a superconducting coil 2 with a least a superconducting coil winding 3 for generating a magnetic field. The superconducting coil winding 3 is located in a first temperature region 5 wherein the first temperature region 5 has a temperature below a critical temperature of a low temperature superconductor, LTS. Furthermore the superconducting magnet system 1 comprises an electrical current lead assembly 9 for transferring electrical current to the superconducting coil winding 3. The electrical current lead assembly 9 consists of a first current lead 10, 12 and a second current lead 11, 13, wherein each current lead 10, 11 is electrically connected with the superconducting coil winding 3. The first current lead 12 and the second current lead 13 are made of a high temperature superconductor, HTS.
  • The superconducting magnet system 1 shown in Fig. 2 further comprises a magnet persistent current switch 8, MPCS made of a high temperature superconductor, HTS, and thermally connected to a second temperature region 6. The second temperature region 6 has a temperature below a critical temperature of a high temperature superconductor, HTS. The region is e.g., cooled to about 40K.
  • The superconducting magnet system 1 further comprises a circuit 14 with a contactor 15, wherein the circuit 14 is arranged in parallel to the MPCS 8 between the first electrical current lead 10 and the second electrical current lead 11 in a third temperature region 7. Wherein a third temperature region is a region that is close to room temperature (300K). The superconducting magnet system 1 comprising a power supply 4 for providing an electrical current to the electrical current lead assembly 9. The first electrical current lead 10 and the second electrical current lead 11 each comprising a contactor 16, 17 wherein the contactors 16, 17 are arranged in the third temperature region 7 between the power supply 4 and the circuit 14 with the contactor 15. When the contactors 16, 17 of the current leads 10, 11 are opened (as in normal operation), one branch of the contactor 16, 17 will be at about room temperature, while the other branch will be cooled to the same temperature as the second temperatur region 6. The contactor 15 parallel to the MPCS 8 is connected to this second branch. That means that the temperature of this parallel contactor 15 depends very much on the moment it is used. When the contactors 16, 17 in the current leads 10, 11 are open, the parallel contactor 15 will have a temperature close to the second temperature region 6. When the contactors 16, 17 in the current leads 10, 11 are closed, the temperature of the parallel contactor 15 will be at a much higher temperature, close to the third temperature region 7 at room temperature. The normal temperature of the parallel contactor 15 may only be the case during the energize/discharge process. Depending where it is connected to the current leads 10,11 (above or below contactors 16, 17), the parallel contactor 15 is at the third temperature region 7 (normal temperature) or at the second temperature region 6 (∼40K).
  • In an embodiment of the invention the high temperature superconducting material is a copper-oxide based high temperature superconducting material. In particular the high temperature superconducting material, HTS of the magnet persistent current switch 8, MPCS has a resistance suitable to minimize dissipation when energizing the superconducting coil 2.
  • It may also be provided in an embodiment of the invention that at least a part of the high temperature superconducting material of the magnet persistent current switch 8, MPCS and/or the high temperature superconducting, HTS current leads 12, 13 connecting the MPCS 8 with the superconducting coil winding 3 comprises metal especially copper. In another embodiment high temperature superconductor filaments can be embedded in a copper matrix.
  • Figs. 3 a) and 3 b) schematically depicts electrical current lead assemblies 9 of a superconducting magnet system 1 according to an embodiment of the invention. In an embodiment of the invention the high temperature superconducting current leads 12, 13 and the high temperature superconducting magnet persistent current switch 8 are separate components as shown in Fig. 3 a). In Fig. 3 b) an alternative is shown where the high temperature superconducting current leads 12, 13 and the high temperature superconducting magnet persistent current switch 8 are integrated components forming one long high temperature superconducting lead. This has the advantage that unnecessary contacts and welds are avoided.
  • Fig. 4 shows two flow charts of a method for controlling a ramp-up process shown in Fig. 4 a) and for controlling a ramp-down process shown in Fig. 4 b) of a superconducting magnet system 1 according to an embodiment of the invention. In order to energize e.g., a zero boil-off (ZBO) magnet current leads are inserted prior to the actual energize or discharge procedure and removed after it. In general, the energize procedure starts with the magnet persistent mode switch 8, MPCS: the superconducting persistent current loop has to be opened by heating a part of this closed loop. Now the magnet power supply 4 can apply a voltage to the self-inductance of the magnet coil 2, but the resistive part of the circuit (MPCS) is connected in parallel of this. The applied voltage leads to a current increase in this magnet self-inductance. A part of the current is running through the normal wire of the MPCS 8 and dissipates energy and this keeps the MPCS 8 at a sufficiently high temperature to keep it normal. The heating element 18 is usually not needed at this phase of the process. When the current has reached its final current, the current is kept constant (dissipation in MPCS 8 stops) and if the heating element 18 would still be needed, this is switched off. When the MPCS 8 is cooling down and is getting superconducting again, the magnet coil 2 is energized. The discharge process is similar, but at the start of this process the current from the magnet power supply 4 has to be at a similar level as the current in the magnet coil 2.
  • The method for controlling a ramp-up process for a superconducting magnet system 1 starts with step 400 by providing a superconducting magnet system 1 according to claim 1 of the invention. Fig. 4 a) describes the ramp-up process starting at zero current. In case the magnet whould have been discharged partially, e.g., for shimming the magnet, the energizing process looks a bit different. Then the current from the power supply 4 must be set at the right level before opening the MPCS 8, similar to the first part of the ramp-down process. In step 410 the contactor 15 of the circuit 14 arranged in parallel to the magnet persistent current switch 8, MPCS in the third temperature region 7, is opened. In step 420 the MPCS 8 is opened by switching on the heating element 18 of the MPCS 8. In step 430 the contactors 16, 17 of the first current lead 10 and the second current lead 11 are closed. Since at the start of the ramp-up process there is no current in the superconducting coil 2, the order of steps 410 to 430 does not matter. The contactors 16, 17 are implemented in the third temperature region 7. The third temperature region can be e.g., a third temperature region around 300 K. In an embodiment of the invention the contactors 16, 17 can be implemented at the second temperautre 6 region at 40K as well. The practical limitation is that the contactors 16, 17 are preferably placed in a low-field area and that is close to the outside of the cryostat. In step 440 the superconducting magnet coil 2 is energized until full current. In step 450 the contactor 15 of the circuit 14 arranged in parallel to the MPCS 8 in the third temperature region 7 is closed. In step 460 the MPSC 8 is closed by switching off the heating element 18 of the MPSC 8. In step 470 the current from the power supply 4 is set to zero. In step 480 the contactors 16, 17 in the third temperature region 7 of the first current lead 10 and the second current lead 11 are opened. The superconducting magnet system 1 is now ramped-up.
  • Fig. 4 b) shows a flow chart of a method for controlling a ramp-down process for a superconducting magnet system 1. The method starts in step 500 with providing a superconducting magnet system 1 according to claim 1 of the invention. In step 510 the contactors 16, 17 of the first current lead 10 and the second current lead 11 are closed. The contactors 16, 17 are implemented in the third temperature region 7. The third temperature region can be e.g., a temperature region around 300K. In an embodiment of the invention the contactors 16, 17 can be implemented at the second temperautre region at 40K as well. The practical limitation is that the contactors 16, 17 are preferably placed in a low-field area and that is close to the outside of the cryostat. In step 520 closing the contactor 15 of the circuit 14 arranged in parallel to the magnet persistent current switch 8, MPSC in the third temperature region 7 is closed. In step 530 a current from the power supply 4 is set to the same current as in the superconducting coil 2. In step 540 the MPSC 8 is opened by switching on the heating element 18 of the MPSC 8. In step 550 the superconducting magnet coil 2 is discharged until zero current. In step 560 the MPSC 8 is closed by switching off the heating element 18 of the MPSC 8. In step 570 the contactor 15 of the circuit 14 arranged in parallel to the MPSC 8 is closed. In step 580 opening the contactors 16, 17 in the third temperature region 7 of the first current lead 10 and the second current lead 11 are opened. Since the current is zero the order of steps 560 to 580 does not matter. The superconducting magnet system 1 is now ramped down.
  • Fig. 5 shows a diagram of the temperature and the current plotted against the time in seconds according to the state of the art. The magnet used in Fig. 5 is a sealed magnet, wherein there are two problems involved with the previously described sealed magnet with ΔT, wherein ΔT = TAutoDischarge-TEnergize is:
    The magnet first needs to be cooled to a pretty low temperature Tenergize where the cold head has little cooling power. This takes a lot of time to reach that temperature. See Fig. 5 as indicative behavior.
  • The critical temperature of the superconducting coil 2 at the end of the energization process must have a sufficiently large margin compared to the temperature at the end of the energize process (TAutoDischarge) in order to avoid a Loss of Field (quench). A higher critical temperature is achieved by adding more superconductor wire to the coils and this is expensive.
  • One could argue to add a pause during the energize process to allow the magnet to cool down again. This may reduce ΔT and therefore reduce the additional cost due to the additional superconductor. But pausing also increases the total time to energize the magnet. Usually a magnet is cooling down (at installation or e.g., after a Loss of Field (quench)) and one first need to wait until the magnet has been cooled down below a threshold temperature and then start energizing. This threshold temperature Tenergize has been chosen in such a way that the temperature at the end of the energize process remains below TAutoDischarge. There are several insights that are important for this invention:
    1. 1. The dissipation of the superconducting magnet coil 2 at the first temperature region 5 at around 4K is a function of the ramp speed (dl/dt). The dissipation gets lower at a lower ramp speed. The total accumulated dissipation is a function of the ramp speed and there may be an optimum dl/dt, wherein it could be optimal to ramp the magnet at field in shortest possible time and/or using a minimum of superconductor (allowing a lower TAutoDischarge).
    2. 2. A longer energize (or discharge) process increases the total dissipation at the second temperature region 6 of the cold head. A magnet and cold head can be designed that the thermal performance of the second temperature region 6 is sufficient to cool down the heat load at the second temperature region 6 for a very long time (preferably infinite time).
    3. 3. The critical temperature of the superconducting coil 2 is a function of the current that is running through it and a function of the magnetic field that is applied to it. In case NbTi is used, the critical current at 1=0 is about 10K. But the critical current drops to e.g., 5.2K at full current. This assumes a thermal margin of 1K when cooled at 4.2K.
    4. 4. The higher the temperature, the higher the cooling power of the cold head. Based on insight 4 one can conclude that the energize process preferably takes place at as high as possible temperature, i.e., where the cold head has the best thermal performance. Insight 3 suggests that the energize process at low current can take place at a temperature well above threshold used for full current (T> TAutoDischarge). Of course, at every temperature there must be a sufficient current margin compared to the critical current at that temperature. Therefore, a maximum current is defined at each temperature, which includes a sufficient margin to the actual critical current at that temperature. The actual current should always remain below this maximum allowed current. One could define a linear behavior between the full current at TAutoDischarge and the zero current at a temperature between TAutoDischarge and 10K, say at 7K.
  • The total time that the current leads carry current is much longer, but that is not a problem according to insight 2. Also, the dissipation due to low dl/dt is low (insight 1) leading to hardly noticeable delay in cooldown time, while being energized.
  • While cooling down, the energize may start when the temperature drops below 7K, i.e., where the maximum allowed current becomes >0. When the temperature is decreasing, the maximum allowed current increases further and the energize current can be increased to the maximum allowed value.
  • In that way, the majority of the energize procedure has happened above TAutoDischarge. The additional energize current is kind of "titrated" to the magnet, based on the actual temperature and the related maximum allowed current.
  • Fig. 6 shows a diagram of a critical current and a simplified curve for maximum allowed current as function of temperature according to an embodiment of the invention.
  • Fig. 7 shows a flow chart of a method for energizing a superconducting magnet system 1 according to another embodiment of the invention. The method starts with step 700 by providing a superconducting magnet system 1 wherein the superconducting magnet system 1 comprises a superconducting coil 2 with at least a superconducting coil winding 3 for generating a magnetic field, wherein the superconducting coil 2 is in thermal contact with a first temperature region 5 having a temperature below a critical temperature of a low temperature superconductor, LTS, the system further comprising a power supply 4 and a magnet persistent current switch 8, MPCS, wherein the MPCS 8 is arranged to switch between an open state and a closed state of the superconducting magnet coil 2. The method for energizing a superconducting magnet system 1 can be used independently of the superconducting magnet system 1 as described in claim 1. In an embodiment of the invention, however, it may be provided that the superconducting magnet system 1 is a previously described superconducting magnet system 1 with a magnet persistent current switch (MPCS) 8 as claimed in claim 1. In step 710 a current generated by the power supply 4 is set to an initial current value. In step 720 the MPCS 8 is activated to its open position and connecting the superconducting magnet coil 2 and the power supply 4, when the actual temperate Tactual of the superconducting coil winding 3 falls below the superconducting critical temperature Tc(I) at the actual electrical current density minus a safety margin ΔT which is higher than the ultimate operating temperature Top. In step 730 the superconducting magnet coil 2 is energized with a current, wherein the current and the energizing speed dl/dt is adjusted according to the temperature decrease during cooling of the superconducting coil winding 3, while keeping the actual temperature within the safety margin. In an embodiment of the invention the step of energizing the superconducting magnet coil 2 with a current takes place at a higher temperature than the superconducting critical temperature Tc(I) at full current. In step 740 the MPCS 8 is activated to its close position when the target magnetic field strength is reached, thereby placing the superconducting magnet coil 2 in a closed circuit.
  • Fig. 8 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system 1 according to an embodiment of the invention. The ramp speed is now a function of the cool down speed and is in general much lower than based on according to the 5V output of the magnet power supply.
  • As a result:
    • The sealed magnet is much earlier on field than the original way of working.
    • The large TAutoDischarge can be eliminated, leading to a major cost reduction of the sealed 1.5T magnet. This idea enables a 3T sealed magnet with similar magnet design as for ZBO magnets.
  • Fig. 9 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system 1 with reduced TAutoDischarge according to an embodiment of the invention. Fig. 9 is similar to Fig. 8, but with a 1K lower TAutoDischarge. Even for this situation the magnet is faster on field than with the existing energize procedure and with the original TAutoDischarge. This is the case when less superconductor would be used. That is the big (financial) advantage of using this energize idea.
  • Fig. 10 shows a diagram of the temperature and current as function of time for a method for energizing a superconducting magnet system according to the state of the art compared with the temperature and current as function of time for a method for energizing superconducting magnet system 1 according to an embodiment of the invention. Advantages that can be seen in Fig. 10 are shorter shim iterations and a faster taking the magnet back to field after a turn off of the magnetic field by initiating an automated ramp-up, minimizing operational downtime. This is the situation of faster shimming (discharge and faster energize again). You do not have to wait with cooling down until Tenergize has been reached. Furthermore, a shorter installation time and major cost down on superconductors can be achieved.
  • While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. Further, for the sake of clearness, not all elements in the drawings may have been supplied with reference signs. REFERENCE SYMBOL LIST
    superconducting magnet system 1
    superconducting coil 2
    superconducting coil winding 3
    magnet power supply 4
    first temperature region 5
    second temperature region 6
    third temperature region 7
    magnet persistent current switch 8
    electrical current lead assembly 9
    first current lead 10
    second current lead 11
    first high temperature superconductor current lead 12
    second high temperature superconductor current lead 13
    parallel circuit 14
    contactor 15
    contactor of first current lead 16
    contactor of second current lead 17
    heating element 18
    vessel 19

Claims (18)

  1. A superconducting magnet system (1) the system comprising:
    a superconducting coil (2) with a least a superconducting coil winding (3) for generating a magnetic field, wherein the superconducting coil winding (3) is located in a first temperature region (5),
    an electrical current lead assembly (9) for transferring electrical current to the superconducting coil winding (3), a power supply (4) for providing an electrical current to the electrical current lead assembly (9), the electrical current lead assembly comprising:
    a first electrical current lead (10, 12) and a second electrical current lead (11, 13), wherein each current lead (10, 11) is electrically connected with the superconducting coil winding (3), the first electrical current lead (10) and the second electrical current lead (11) each comprising a contactor (16, 17) between the power supply (4) and the superconducting coil winding (3), the superconducting magnet system (1) further comprising a magnet persistent current switch (8), MPCS, wherein the MPCS is arranged between the first electrical current lead (10, 12) and the second electrical current lead (11, 13) and the MPCS (8) being adapted to switch between an open state and a closed state of the superconducting coil (2), the MPCS (8) comprising a heating element (18),
    wherein the MPCS (8) is made of a high temperature superconductor, HTS, and thermally connected to a second temperature region (6),
    wherein the MPCS (8) is connected to the superconducting coil winding (3) by the electrical current lead assembly (9), wherein the first current lead (12) and the second current lead (13) are made of a high temperature superconductor, HTS.
  2. The superconducting magnet system according to claim 1, wherein the first temperature region (5) having a temperature below a critical temperature of a low temperature superconductor, LTS.
  3. The superconducting magnet system according to any preceding claim, wherein the second temperature region (6) having a temperature below a critical temperature of a high temperature superconductor, HTS.
  4. The superconducting magnet system according to any preceding claim, wherein at least a part of the high temperature superconducting, HTS material of the magnet persistent current switch (8), MPCS and/or the high temperature superconducting current leads (12, 13) connecting the MPCS (8) with the superconducting coil winding (3) comprises additional electrically conductive material preferred a metal, more preferred copper, wherein the additional electrically conductive material is cladded to the HTS material or embedded as filaments into the HTS material.
  5. The superconducting magnet system according to any preceding claim, wherein the superconducting magnet system comprises a circuit (14) with a contactor (15), wherein the circuit (14) is arranged in parallel to the MPCS (8) between the first electrical current lead (10) and the second electrical current lead (11) in a third temperature region (7).
  6. The superconducting magnet system according to claim 5, wherein the third temperature region (7) has a normal temperature around room temperature or has the same temperature as the first temperature region (5) or as the second temperature region (6)
  7. The superconducting magnet system according to any preceding claim, wherein the high temperature superconducting material, HTS of the magnet persistent current switch (8), MPCS has a high resistance in normal state.
  8. The superconducting magnet system according to claim 7, wherein the high temperature superconducting material, HTS of the magnet persistent current switch (8), MPCS is made of a ceramic material.
  9. The superconducting magnet system according to any preceding claim, wherein the high temperature superconducting, HTS current leads (12, 13) and the high temperature superconducting, HTS magnet persistent current switch (8), MPCS are separate components.
  10. The superconducting magnet system according to claims 1 to 8, wherein the high temperature superconducting, HTS current leads (12,13) and the high temperature superconducting, HTS magnet persistent current switch (8), MPCS are integrated components forming one long high temperature superconducting, HTS lead.
  11. A magnetic resonance examination system, the magnetic resonance examination system comprising a superconducting magnet system (1) according to any of the preceding claims.
  12. A method for controlling a ramp-up process for a superconducting magnet system, the method comprising the following steps:
    - Providing a superconducting magnet system (1) according to claim 1, wherein the superconducting magnet system (1) further comprises a circuit (14) with a contactor (15), wherein the circuit (14) is arranged in parallel to the MPCS (8) between the first electrical current lead (10) and the second electrical current lead (11) in a third temperature region (7),
    - Opening the contactor (15) of the circuit (14) arranged in parallel to the magnet persistent current switch (8), MPCS in the third temperature region (7), opening the MPCS (8) by switching on the heating element (18) of the MPCS (8), closing the contactors (16, 17) in the third temperature region (7) of the first current lead (10) and the second current lead (11), where the steps can be performed in any order,
    - Energizing the superconducting magnet coil (2) until full current,
    - Closing the contactor (15) of the circuit (14) arranged in parallel to the MPCS (8) in the third temperature region (7),
    - Closing the MPSC (8) by switching off the heating element (18) of the MPSC (8),
    - Setting the current from the power supply (4) to zero,
    - Opening the contactors (16, 17) in the third temperature region (7) of the first current lead (10) and the second current lead (11).
  13. A method for controlling a ramp-down process for a superconducting magnet system (1), the method comprising the following steps:
    - Providing a superconducting magnet system (1) according to claim 1, wherein the superconducting magnet system (1) further comprises a circuit (14) with a contactor (15), wherein the circuit (14) is arranged in parallel to the MPCS (8) between the first electrical current lead (10) and the second electrical current lead (11) in a third temperature region (7),
    - Closing the contactors (16, 17) in the third temperature region (7) of the first current lead (10) and the second current lead (11),
    - Opening the contactor (15) of the circuit (14) arranged in parallel to the magnet persistent current switch (8), MPSC in the third temperature region (7),
    - Setting a current from the power supply (4) to the same current as in the superconducting coil,
    - Opening the MPSC (8) by switching on the heating element (18) of the MPCS (8),
    - Discharging the superconducting magnet coil (2) until zero current,
    - Closing the MPSC (8) by switching off the heating element (18) of the MPSC (8), closing the contactor (15) of the circuit (14) arranged in parallel to the MPSC (8), opening the contactors (16, 17) in the third temperature region (7) of the first current lead (10) and the second current lead (11), where the steps can be performed in any order.
  14. A method for energizing a superconducting magnet system the method comprising the following steps:
    - Providing a superconducting magnet system (1) wherein the superconducting magnet system comprises a superconducting coil (2) with at least a superconducting coil winding (3) for generating a magnetic field, wherein the superconducting coil (2) is in thermal contact with a first temperature region (5) having a temperature below a critical temperature of a low temperature superconductor, LTS, the system further comprising a power supply (4) and a magnet persistent current switch (8), MPCS, wherein the MPCS (8) is arranged to switch between an open state and a closed state of the superconducting magnet coil (2),
    - Setting a current generated by the power supply (4) to an initial current value,
    - Activating the MPCS (8) to its open position and connecting the superconducting magnet coil (2) and the power supply (4), wherein the activation starts at a temperature above the critical temperature of the superconducting magnet coil (2) at its full current temperature Tc(Ifull),
    - Energizing the superconducting magnet coil (2), while the superconducting magnet coil (2) is above its critical temperature at its full current Tc(Ifull) during a part of the energization process, where the full current is the current at the target field,
    - Activating the MPCS (8) to its closed position when the target magnetic field strength is reached, thereby placing the superconducting magnet coil (2) in a closed circuit.
  15. The method according to claim 14, wherein the current I remains below a max I(T) including a margin to Ic(T), wherein Ic(T) is the maximum current that can be carried by the superconducting magnet coil (2) without generating a Loss of Field.
  16. The method according to claim 15, wherein an energizing speed dI/dT is determined by the speed of cooling down of the superconducting magnet coil (2).
  17. A method according to claim 14 to 16, wherein the step of providing a superconducting magnet system (1) comprises the step of providing a superconducting magnet system (1), wherein the MPCS (8) is made of a high temperature superconductor, HTS.
  18. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of claim 12 to 17.
EP21150247.1A 2021-01-05 2021-01-05 Superconducting magnet system and method for operating the system Withdrawn EP4024415A1 (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2023144247A1 (en) 2022-01-27 2023-08-03 Koninklijke Philips N.V. Persistent current switch for a superconducting electromagnet

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0645830A1 (en) * 1993-09-20 1995-03-29 Hitachi, Ltd. Permanent current switch and superconducting magnet system
WO1998000848A1 (en) * 1996-07-02 1998-01-08 American Superconductor Corporation Improved superconducting magnets and power supplies for superconducting devices
WO2017178560A1 (en) * 2016-04-12 2017-10-19 Koninklijke Philips N.V. Lead and thermal disconnect for ramping of an mri or other superconducting magnet
WO2020254158A1 (en) * 2019-06-20 2020-12-24 Koninklijke Philips N.V. Quench protection for high temperature superconducting (hts) leads

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0645830A1 (en) * 1993-09-20 1995-03-29 Hitachi, Ltd. Permanent current switch and superconducting magnet system
WO1998000848A1 (en) * 1996-07-02 1998-01-08 American Superconductor Corporation Improved superconducting magnets and power supplies for superconducting devices
WO2017178560A1 (en) * 2016-04-12 2017-10-19 Koninklijke Philips N.V. Lead and thermal disconnect for ramping of an mri or other superconducting magnet
WO2020254158A1 (en) * 2019-06-20 2020-12-24 Koninklijke Philips N.V. Quench protection for high temperature superconducting (hts) leads

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2023144247A1 (en) 2022-01-27 2023-08-03 Koninklijke Philips N.V. Persistent current switch for a superconducting electromagnet

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