EP4068312A1

EP4068312A1 - Switching a ramp current for super conducting windings of a superconducting magnet assembly switch assembly

Info

Publication number: EP4068312A1
Application number: EP21166640.9A
Authority: EP
Inventors: Peter Forthmann
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2021-04-01
Filing date: 2021-04-01
Publication date: 2022-10-05

Abstract

The invention relates to switching a ramp current for super conducting windings of a superconducting magnet assembly switch assembly. In this respect, according to the invention, a superconducting magnet assembly (1) is provided which comprises super conducting windings (2) and a ramp current switch assembly (3), wherein the ramp current switch assembly (3) comprises a frame (4), a first current lead (5) and a second current lead (6) for supplying or drawing electrical current to or from the superconducting magnet windings (2), respectively, a flexible electrically conductive braid (7), and an actuator rod (8), wherein the first current lead (5) and the second current lead (6) are fixedly mounted to the frame (4), and the braid (7), at a first end (71), is fixedly connected to the first current lead (5), and at a second end (72), is fixedly connected to a first end (81) of the actuator rod (8), wherein the actuator rod (8) comprises a heater (9) for heating the actuator rod (8), and wherein the actuator rod (8), at a second end (82), is fixedly connected to the frame (8), the frame (8) being a counter bearing for the actuator rod (8) in such a way that heating or cooling the actuator rod (8) with the heater (9), due to thermal expansion of the actuator rod (8), leads to a movement of the second end (82) of the actuator rod (8) towards or away from the second current lead (6), respectively, such that a galvanically conducting electrical contact between the second end (72) of the braid (7) which is connected to the actuator rod (8) and the second current lead (6) is closed or opened, respectively. In this way, an easy and efficient method for ramp-up and ramp-down of super conducting windings (2) with reduced heat influx to the super conducting windings (2) is achieved.

Description

FIELD OF THE INVENTION

The invention relates to a superconducting magnet assembly comprising super conducting windings and a ramp current switch assembly, and in particular to a magnetic resonance examination system with such a superconducting magnet assembly.

BACKGROUND OF THE INVENTION

Typically, cryogen-free superconducting magnet systems may be mechanically cooled down to 4.2 K eliminating the need for liquid cryogens and, hence, avoiding the issues associated with handling liquid cryogens. Therefore, cryogen-free superconducting magnets are advantageous since they provide for easier handling because no liquid cryogen has to be transferred and since the purchase of expensive liquid helium may be avoided. Further, the handling of cryogen-free system operation is easier and less maintenance is required.
Cryogen-free magnets, also called Helium-less magnets, where the coil is placed in the insulating vacuum of a cryostat need to have permanently installed current leads which cannot be cooled by bleeding gas from a liquid helium reservoir. Thus, these leads cause a relatively large heat load on the system's refrigerator. For such sealed magnets operating most of the time at constant field in persistent mode, such as magnetic resonance imaging (MRI) magnets, the heat load at zero current in the current leads is the most problematic.
Therefore, since for automatic ramp-up and ramp-down in such sealed magnets, it is mandatary to have the ramp current leads permanently connected, a simple copper connection cannot reconcile the antagonizing requirements of low static ambient heat input and low joule heating during ramp. One solution is a switch that interrupts the current lead connection from room temperature into the cryostat.
From JP 2010 192 253 A a thermal switch device is known for restraining heat intrusion from outside to a superconducting device. The thermal switch device is provided with a base body forming a heat-insulating room, and a first member arranged in the heat-insulating room, and a second member, arranged in the heat-insulating room with at least a part formed of a base material having a higher coefficient of thermal expansion than the first member. Accordingly, as the second member is cooled, a mode is changed into a first one in which a contact degree of the first member and the second member gets higher, directly or indirectly. As a temperature of the second member is raised, the mode is changed into a second one in which a contact degree of the first member and the second member becomes, directly or non-directly, lower or becomes non-contacting.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an easy and efficient method for ramp-up and ramp-down of super conducting windings with reduced heat influx to the super conducting windings.
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 dependent claims.
Therefore, according to the invention, a superconducting magnet assembly comprising super conducting windings and a ramp current switch assembly, wherein
the ramp current switch assembly comprises a frame, a first current lead and a second current lead for supplying or drawing electrical current to or from the superconducting magnet windings, respectively, a flexible electrically conductive braid, and an actuator rod, wherein
the first current lead and the second current lead are fixedly mounted to the frame, and the braid, at a first end, is fixedly connected to the first current lead, and at a second end, is fixedly connected to a first end of the actuator rod, wherein
the actuator rod comprises a heater for heating the actuator rod, and wherein
the actuator rod, at a second end, is fixedly connected to the frame, the frame being a counter bearing for the actuator rod in such a way that heating or cooling the actuator rod with the heater, due to thermal expansion of the actuator rod, leads to a movement of the second end of the actuator rod towards or away from the second current lead, respectively, such that a galvanically conducting electrical contact between the second end of the braid which is connected to the actuator rod and the second current lead is closed or opened, respectively.
Here, "cooling the actuator rod with the heater" does not necessarily mean that heater actively cools the actuator rod. Rather, as a preferred embodiment of the invention, it is encompassed by the invention that, after heating up the actuator rod, the heater is not activated any more and, hence, the actuator rod is not heated any more. Therefore, the rod is allowed to cool down, i.e. back to ambient temperature. Hence, the feature "cooling the actuator rod with the heater" shall also be understood to mean "allowing the actuator rod to cool by not operating the heater".
Further, in the context of the present invention, the term "braid" relates to any type of flexible well-conducting connection.
The design of the superconducting magnet assembly according to the invention is simple, with the motion of the moving parts highly reduced. When mechanically constrained, thermally expanding parts can exert great forces, which makes for low contact resistance. In the open state, the thermal conduction is limited to that of the frame, which can be made very low. Therefore, according to a preferred embodiment of the invention, the frame is made from a material the thermal conductivity of which is less than the material of the current leads and the braid, respectively. Especially, according to a preferred embodiment of the invention, the frame is made from a ceramic material.
In general, the braid and the current leads may be made from different electrically conductive materials, e.g. from metals or metal alloys. However, according to a preferred embodiment of the invention, the current leads and the braid are made from copper. This offers the advantage of very low electrical resistance and, therefore, low heating due to the electrical current flowing through current leads and the braid which reduces heat influx to the super conducting windings.
For the actuator rod, in general, different materials may be used. However, according to a preferred embodiment of the invention, the actuator rod is made from a material which exhibits a coefficient of thermal expansion α for which the following relation applies: 70 · 10^-6 K^-1 < α < 200 · 10^-6 K^-1. In this respect, according to a preferred embodiment of the invention, the actuator rod is made from polytetrafluoroethylene PTFE, polyethylene PE, polypropylene PP and/or poloxymethylene POM. As a good approximation, when heating the rod up by ΔT, the increase in length ΔL, wherein ΔL = L ₁ - L ₀ , L ₀ being the length of the actuator rod before heating and L ₁ being the length of the actuator rod when it is heated up, can be calculated by ΔL = α · L ₀ · ΔT. Therefore, in case of an actuator rod which is made from PTFE with α = 100 · 10^-6 K^-1 and which has a length of 20 cm, heating this rod up by 60 K leads to an increase in length by 1.2 mm. This is sufficient for closing or opening a respective gap between the second end of the actuator rod and the second current lead, respectively, such that a galvanically conducting electrical contact between the second end of the braid which is connected to the actuator rod and the second current lead is closed or opened, respectively.
The heater may be designed and arranged relative to the rod in different ways as long as the rod may be sufficiently heated by the heater in order to achieve the desired expansion of the rod. In this respect, according to a preferred embodiment of the invention, the heater has an elongated form and runs parallel to the longitudinal axis of the actuator rod. This allows for efficiently heating the rod. Further, in this respect, according to a preferred embodiment of the invention, the heater is arranged on or in the actuator rod. Furthermore, according to a preferred embodiment of the invention, the length of the heater in the direction parallel to the longitudinal axis of the actuator rod is at least 80 % of the length of the extension of the actuator rod within the frame, even more preferably 90 %. In this way, a very uniform heating of the actuator rod can be achieved. According to a preferred embodiment of the invention, the length of the actuator rod within the frame is at least 10 cm, more preferably at least 15 cm, even more preferably at least 20 cm.
The invention also relates to a magnetic resonance examination system with a superconducting magnet assembly as described above.
Further, the invention also relates to a method of switching between a ramp-up/ramp-down state and an idle state of super conducting windings of a superconducting magnet assembly according to any of claims to or a magnetic resonance examination system according to claim wherein in the ramp-up/ramp-down state the braid comes into galvanic contact with the second current lead and in the idle state the braid becomes galvanically isolated from the second current lead by moving away from the second current lead, the method comprising the following method steps:

thermally enlarging the expansion of the actuator rod in its longitudinal direction for switching from idle state to ramp-up/ramp-down state, and
thermally reducing the expansion of the actuator rod in its longitudinal direction for switching from ramp-up/ramp-down state to idle state.

Preferably, the length difference of the actuator rod due to the heating or cooling, respectively, is at least 0.8 mm, more preferably at least 1.2 mm.
According to a preferred embodiment of the invention, the method further comprises the following method steps:

heating the actuator rod of the ramp current switch assembly by a temperature difference of at least 40 K for switching from idle state to ramp-up/ramp-down state, and
allowing the actuator rod of the ramp current switch assembly to cool by a temperature difference of at least 40 K for switching from ramp-up/ramp-down state to idle state.

Though, in general, a temperature difference of 40 K may be sufficient for switching from idle state to ramp-up/ramp-down state, preferably, the temperature difference is at least 50 K, more preferably at least 60 K.
Furthermore, the invention also relates to a non-transitory computer-readable medium, comprising instructions stored thereon, that when executed on a processor induce a superconducting magnet assembly as described above or a magnetic resonance examination system as described above to perform the method described before.

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 magnetic resonance examination system according to a preferred embodiment of the invention,
Fig. 2 schematically depicts a ramp current switch assembly according to a preferred embodiment of the invention in an open state,
Fig. 3 schematically depicts the ramp current switch assembly according to the preferred embodiment of Fig. 2 in a closed state,
Fig. 4 schematically depicts an actuator rod with a heater according to a preferred embodiment of the invention, and
Fig. 5 schematically depicts an actuator rod with a heater according to another preferred embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically depicts a magnetic resonance examination system 100 according to a preferred embodiment of the invention, with a superconducting magnet assembly 1 which comprises super conducting windings 2 and a ramp current switch assembly 3. The ramp current switch assembly 3 is depicted in more detail in Figs. 2 and 3, i.e. in an open and in a closed state, respectively. The ramp current switch assembly 3 comprises a frame 4, a first current lead 5 and a second current lead 6 for supplying or drawing electrical current to or from the superconducting magnet windings 2, respectively, a flexible electrically conductive braid 7, and an actuator rod 8. Heating of the actuator rod allows for switching from the open to the closed position as explained in more detail in the following. For this switching function, the first current lead 5 and the second current lead 6 are fixedly mounted to the frame 4, and the braid 7, at a first end 71, is fixedly connected to the first current lead 5, and at a second end 72, is fixedly connected to a first end 81 of the actuator rod 8. This design is complemented in that the actuator rod 8 comprises a heater 9 for heating the actuator rod 8, wherein the actuator rod 8, at a second end 82, is fixedly connected to the frame 8.
Here, the frame 8 acts as a counter bearing for the actuator rod 8 in such a way that heating or cooling the actuator rod 8 with the heater 9 leads to a movement of the second end 82 of the actuator rod 8 due to thermal expansion of the actuator rod 8. This movement of the actuator rod 8 is towards or away from the second current lead 6, respectively, depending on the fact whether the heater 9 is operated or not. In this way, a galvanically conducting electrical contact between the second end 72 of the braid 7 which is connected to the actuator rod 8 and the second current lead 6 may closed or opened, respectively.
According to the preferred embodiment of the invention described here, the frame 4 is made from a ceramic material which reduces heat influx to the super conducting windings 2 via the frame 4. Further, the current leads 5, 6 and the braid 7 are made from copper in order to reduce the electrical resistance. In this way, only little heating due to the electrical current flowing through current leads 5, 6 and the braid 7 occurs which also reduces heat influx to the super conducting windings 2.
The actuator rod 8 is made from polytetrafluoroethylene (PTFE) with α = 100 · 10^-6 K^-1 and has a length of 20 cm. Heating this actuator rod 8 up by 60 K leads to an increase in length by 1.2 mm. This is sufficient for closing or opening the gap between the second end 82 of the actuator rod 8 and the second current lead 6, respectively, such that the galvanically conducting electrical contact is closed or opened, respectively. Due to the high pressure the actuator rod can exert, a contact resistance of 50 mΩ or even less may be achieved.
As depicted in more detail in Fig. 4, according to the preferred embodiment of the invention described here, the heater 9 has an elongated form and runs parallel to the longitudinal axis 10 of the actuator rod 8. The length of the heater 9 in the direction parallel to the longitudinal axis 10 of the actuator rod 8 is more than 90 % of the length of the extension of the actuator rod 8 within the frame 4. In this way, a very uniform heating of the actuator rod 8 along its length can be achieved. Fig. 4 shows an embodiment where the heater 9 is arranged on the outside of the actuator rod 8. However, as depicted in Fig. 5, according to another embodiment it is also possible to arrange the heater 9 inside the actuator rod 8. This provides for an even better heat transfer from the heater 9 to the actuator rod 8.

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 assembly	1
super conducting windings	2
ramp current switch assembly	3
frame	4
first current lead	5
second current lead	6
braid	7
first end of the braid	71
second end of the braid	72
actuator rod	8
first end of the actuator rod	81
second end of the actuator rod	82
heater	9
the longitudinal axis of the actuator rod	10
magnetic resonance examination system	100

Claims

A superconducting magnet assembly (1) comprising super conducting windings (2) and a ramp current switch assembly (3), wherein
the ramp current switch assembly (3) comprises a frame (4), a first current lead (5) and a second current lead (6) for supplying or drawing electrical current to or from the superconducting magnet windings (2), respectively, a flexible electrically conductive braid (7), and an actuator rod (8), wherein
the first current lead (5) and the second current lead (6) are fixedly mounted to the frame (4), and the braid (7), at a first end (71), is fixedly connected to the first current lead (5), and at a second end (72), is fixedly connected to a first end (81) of the actuator rod (8), wherein
the actuator rod (8) comprises a heater (9) for heating the actuator rod (8), and wherein
the actuator rod (8), at a second end (82), is fixedly connected to the frame (8), the frame (8) being a counter bearing for the actuator rod (8) in such a way that heating or cooling the actuator rod (8) with the heater (9), due to thermal expansion of the actuator rod (8), leads to a movement of the second end (82) of the actuator rod (8) towards or away from the second current lead (6), respectively, such that a galvanically conducting electrical contact between the second end (72) of the braid (7) which is connected to the actuator rod (8) and the second current lead (6) is closed or opened, respectively.
The superconducting magnet assembly (1) according to claim 1, wherein the frame (4) is made from a material the thermal conductivity of which is less than the material of the current leads (5, 6) and the braid (7), respectively.
The superconducting magnet assembly (1) according to claim 2, wherein the frame (4) is made from a ceramic material.
The superconducting magnet assembly (1) according to any of the previous claims, wherein the current leads (5, 6) and the braid (7) are made from copper.
The superconducting magnet assembly according to any of the previous claims, wherein the actuator rod (8) is made from a material which exhibits a coefficient of thermal expansion α for which the following relation applies: 70 · 10^-6 K^-1 < α < 200 · 10^-6 K^-1.
The superconducting magnet assembly (1) according to claim 5, wherein the actuator rod (8) is made from polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP) and/or poloxymethylene (POM).
The superconducting magnet assembly (1) according to any of the previous claims, wherein the heater (9) has an elongated form and runs parallel to the longitudinal axis (10) of the actuator rod (8).
The superconducting magnet assembly (1) according to claim 7, wherein the length of the heater (9) in the direction parallel to the longitudinal axis (10) of the actuator rod (8) is at least 80 % of the length of the extension of the actuator rod (8) within the frame (4).
The superconducting magnet assembly (1) according to any of the previous claims, wherein the length of the actuator rod (8) within the frame is at least 10 cm.
A magnetic resonance examination system (100) with a superconducting magnet assembly (1) according to any of claims 1 to 8.
A method of switching between a ramp-up/ramp-down state and an idle state of super conducting windings (2) of a superconducting magnet assembly (1) according to any of claims 1 to 9 or a magnetic resonance examination system (100) according to claim 10 wherein in the ramp-up/ramp-down state the braid (7) comes into galvanic contact with the second current lead (6) and in the idle state the braid (7) becomes galvanically isolated from the second current lead (6) by moving away from the second current lead (6), the method comprising the following method steps:
thermally enlarging the expansion of the actuator rod (8) in its longitudinal direction for switching from idle state to ramp-up/ramp-down state, and

thermally reducing the expansion of the actuator rod (8) in its longitudinal direction for switching from ramp-up/ramp-down state to idle state.
The method according to claim 11, further comprising the following method steps:
heating the actuator rod (8) of the ramp current switch assembly (3) by a temperature difference of at least 40 K for switching from idle state to ramp-up/ramp-down state, and

allowing the actuator rod (8) of the ramp current switch assembly (3) to cool by a temperature difference of at least 40 K for switching from ramp-up/ramp-down state to idle state.
A non-transitory computer-readable medium, comprising instructions stored thereon, that when executed on a processor induce a superconducting magnet assembly (1) according to any of claims 1 to 9 or a magnetic resonance examination system (100) according to claim 10 to perform the method according to claim 11 or 12.