GB2414598A

GB2414598A - A superconducting magnet with at least one quench heater and a series connected capacitor

Info

Publication number: GB2414598A
Application number: GB0506944A
Authority: GB
Inventors: Hugh Alexander Blakes
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2004-05-29
Filing date: 2005-04-06
Publication date: 2005-11-30
Anticipated expiration: 2025-04-06
Also published as: GB2414598B; GB0506944D0

Abstract

A superconductive quench assembly comprises: superconductive coils 10 and at least one quench heater 16 associated with the said coils. The transfer of energy from the said coils 10 to the heater(s) 16 includes a series connected capacitor 30, through which the transferred energy must pass. This arrangement means that the power to the heaters will relate to the square of the differential of voltage relative to time. This provides a rapid power input to the quench heater(s) when voltage fluctuations arise due to quench conditions in the coils 10. The quench heater(s) 16 may be connected across a subset of coils 10, possibly half the coils. The capacitor 30 may be connected between the intermediate nodes of a bridge rectifier. A large capacitor may be used to provide a high power input to the heater(s) 16 whilst a smaller capacitor may provide the same peak power for a shorter duration and may be used to avoid damaging the heater(s).

Description

CIRCUIT FOR EFF,CTIVF, QVE.NCI I I IEA rlN(i IN rSUPERCONl))C'l'IN(]

MA(NETS The present invention relates to superconducthig coils. More particularly it relates to apparatus i-or preventing damage to superconducting coils h1 the case of a quench. s

Xuperconducthig coils are used h1 a variety of applications, t'or example as rnabn?etic field generators in MRI (Magnetic Resonance Imaging,) or NMK (nuclear magnetic resonance) equipment. Coils of superconducting wire are held at cryogenic temperatures, typically at about 4K, the temperature of'boiling helium. So-called hip,h-temperature superconductors still u, require cryogenic temperature of the order of 1() 0K. An ever-present risk in the use of superconductive coils is the risk ova quench. For a reason such as a localised heating, the temperature ova region of the superconducting wire rises above its critical temperature. 'I'hat region becomes resistive. 'I'he current flowing through the coil c- >ntinues to flow through the resistive region, and heat is accordingly dissipated. 'his heat causes a larger region ol the - superconductor to become resistive, increashig the heat dissipated. Since the resistive region is initially small, the heat dissipation is concentrated in a small volume. 'I'he temperature of this small volume may accordingly rise to such a temperature that the superconductive coil is damaged. When a known Mel superconducting magnet cluenciles, an energy ol'the order of 8M.J must be dissipated in a time period of 2-3 seconds.

It is known to avoid such damage by providing quencl1 heaters. In response to a region becoming resistive, energy is diverted to electrical heaters placed adjacent to other regions of the superconductive coils. With one coil, or one part of a coil, in a tiacnched state then a resistive or inductive voltage is built Up across each coil.'I'his hductive or resistive voltage is as apr>lied to the heaters to induce a quench in the other coils. These heaters heat the corresponding parts of the superconductive coils above their critical temperature, and those regions also become resistive. 'I'he effect of this is that substantial regions of the coils become resistive, so that heat is dissipated over a much larger region of the coils, meaning that high temperatures are not reached, and the coils are not damaged. Typically, this is achieved by > supplying the voltage developed across the first coil to quench to small foil heaters, such as NiCr alloy foil coils placed on each superconducting coil. These heaters may each supply approxhilately 2W of heat power to the associated superconductive coil.

It is generally desired to minhnise the amount of superconducting wire employed in the coils.

s This mhimises the cost of the resultant system, and reduced the chances of a quench-inducing detect heing present h1 the coil. 'she crosssectional area of the wire may also be reduced h1 an attempt to save cost, weight and size of the resultant system. Such reduced area wires will be more susceptible to damage from overheathlg. However, il'the heaters are capahie of reacting very quickly to a quench, the coils may he placed in a resistive state quickly enough to avoid u' damage, even to superconducting wires ol' reduced cross-section.

l ig. I shows a circuit diagram of a known arrangement ot'quench heaters. A series connected set ol'superconducthg coils 1() are hidivittually labelled A to 11. A power source 12 may be comiecied to the coils to supply a current. One end ol' the series connection ol' coils is is grounded. L3ack-to-hack diodes 14 and a superconducting cryogenic switch 18 are placed between the ends of'the series connection of coils to provide a current path lor current through the superconducting coils when the power source 12 is removed. An array ol' heaters 16 is provided. Although illustrated as separated away from the coils 10 I'or clarity, the heaters 16 would he arranged in close physical proxhnity with the coils 1().

All superconducting magnets which are operated h1 the so-called persistent mode have a cryogenic switch. Essentially, it is a piece of superconductor wire, in series with the magnet coils 1(), with a heater attached to it. If the heater is on, the cryogenic switch 18 is normally conducting and is open. When the system is attached to an external power supply by leads 12, current will flow through tile superconducthig coils 10, with only a trickle running through the cryogenic switch IX. Once the magnet system has been 'ramped' to the required current, the heater is turned off; and the cryogenic switch 18 becomes superconducting: the cryogenic switch is closed. As the external power supply connected to leads 12 is ramped down, the current through the cryogenic switch IX will increase by the same amount as the decrease in > the current through the external power supply. ()nce the external power supply is ramped down completely, the current leads 12 may be removed, to limit heat leakage into the cryogenic magnet system.

The heaters 16 are shown ha Fig. I as connected h1 two parallel branches, each branch including a series connection of heaters. Other connection arrangements of the heaters may of course be used, depending on factors such as the voltages developed across the coils during a quench, the current handling requirement ofthe heaters, and the resistance and power handlhig capacities ol' the heaters. From a consideration of this circuit arrangement, it is clear that the polarity of voltage V which is developed across the heaters h1 the case of a coil quencl1 will u' depend on which coil quenches first. In the illustrated example, if'one ol'coils A-D quenches first, then the voltage V will be negative with respect to the ground shown. On the other hand, if one of coils t2-ll quenches first, then the voltage V will be positive with respect to the ground shown. 'I'he heater arrangement must be capable ol' coping with either polarity of applied voltage V. 'I'hc heaters are linked to the coils through a back-to-back arrangement cuff Is diodes 2() at one end. 'I'he superconducting coils 10 are shown in Fig. I as connected in series, with the heater arrangement connected between a ground terminal 22 and a point halt'way along the series connection of coils. 'I'he ground terminal is also linked to one end of the series connection ofthe coils.

to The back-to-back diode arrangement 2() connecting the heaters 16 to the coils 10 provides a threshold voltage which must be exceeded before any current flows through the heaters 16.

This threshold voltage should be chosen such that current does not flow through the heaters during ramp up, that is, while current is being established in the supereonducthig coils, but such that the voltage drop across the diodes 20 is not so high that the effectiveness of the us heaters will sut'f r during a quench. In current systems, the threshold voltage is h1 the order ol' 5-l()V.

In known systems, the heaters 16 may each have a resistance of 120 Ohms. Typically, the heater may be required to provide a power of 2W under a mean applied voltage of 1 5V.

The heater arrangement 16 must be capable ol' withstanding the full quench voltage applied across it. This quench voltage may reach Sky, and the heaters must be capable of tolerate such voltage without hurnhg out. In known systems, this limitation has been approached by adding more heaters in series. This introduces further problems in that the heaters each take s longer to reach their 2W power output. There will be an increased delay before the coils will he quenched by the heaters. L)urhlO this delay, there is a risk of' damage to the superconductive coil, shice the quench will remain kcalised, and an excessive heat build-up may occur h1 that region.

lo 'I-'he curve h1 Fig. 2 shows the development of V, the voltage across the heater circuit 16, with time following the start of a quench. 'I'he voltage V hlitially rises to a higl1 value as the large currents circulate ha the superconducting coil pass through the resistive, quenched, part of' the coil. 't'his causes heathig of the superconductive coil, Ieadbig k, greater resistance and greater dissipation of energy. 'I'he increasing voltage V eventually levels out to a maximum is voltage Vmax, which may he ot'the order of 5k V. I'he voltage V reaches a peak value Vinyl< a certain thile thorax after the start ova quench. 'I'he time tnX may be of the order of' two seconds. 'I'he values Vh of voltage are the mhlimunl voltage required to heat the heaters sut't'iciently to cause quench in the heated coils. 'I'he to voltage V exceeds Vat between times to and to,,. This thee must be sut'ficiently long to ensure ef'f'ective quench of the heated coils. 'I'he dissipation of energy within the coils continues, and the voltage V across the heaters falls. 'I'he heat dissipated by a heater is proportional to the square of the voltage across it.

s One solution which has been proposed is to reduce the resistance ot'tile heaters 16 so that the required heat dissipation may be reached more quickly. One disadvantage of this is that the heaters themselves may he damaged by the quenching voltage. In the arrangement illustrated in Fig. I, a quench voltage of'5kV may attempt to dissipate dissipate 3kW in a 12()2 heater designed to dissipate a maximum of 4()0W. This could damage the heaters. 3)

The present invention accordingly provides apparatus as def'hied in the appended clahns.

The above, and further, objects characteristics and advantages of the present application will become more apparent from consideration of the tol10wing description of certain embodiments, given by way of' examples only, with reference to the accompanying drawings s wherein: F'ig.l shows a schematic diagram of' superconducting coils equipped with quench heaters,

accordhg: to the prior art;

1 ig.2 shows the development of voltage across the heater circuit in the system ol'Fig 1; Fig.3 shows a schematic diagram of an arrangement similar to that of l:ig.l, modified lo according lo the present invention; and Fig.4 shows the development ol'voltage across the heater circuit ol'Fig.3, and the square of the time differential ol'that voltage.

Accordhg to the present invention, the heaters 16 are capacitively coupled to the Is superconducting coils, rather than being DC coupled. Fig,. 3 shows an example of a circuit according to an enbodiment of the present invention, in which a capacitor 30 is placed in a diode bridge rectifier 34 linking the two hitermediate nodes in place of the DC compaction normally provided. This particular placement of the capacitor enables a polarised capacitor to be used, shice the voltage on the capacitor will always be of a certain polarity. In other > embodiments of'tile invention, a non-polarised capacitor is employed, and it may be placed at any position in series between the heaters and the coil, or ground, connections. In such embodiments, the bridge rectifier 34 is not required.

I2ig. 4 illustrates an advantage of capacitively coupling the heaters, according to the present 2S invention. The upper curve h1 Fig. 4 shows the development of V, the voltage across the heater circuit 32, with time following the start ol'a quench. 'I'he voltage V initially rises to a higl1 value as the large currents circulating in the superconducthig coil pass through the resistive, quenched part of the coil. 'I'his causes heating of the superconductive coil, leading to greater resistance and greater dissipation of energy. The increasing voltage V eventually so levels out to a maximum voltage Vmax, which may be of the order of 5kV. The dissipation ol' energy within the coils continues, and the voltage V across the heaters falls. The heat dissipated by a heater is proportional to the square of the voltage across it. The lower curve shows the development of the square of the rate of change of tile voltage V with thee t, | dV/dt | -, with thee t. Since, according to the present invention, the heaters are capacitively coupled to the coils, the power dissipated by each heater will be proportional to | dV/dt | . As s shown h1 Fig. 4, | dV/dt 12 reaches a peak value at thee it,,, which is rather sooner than the time tmax of the peak value ol' voltage V. The peak heathig output prom the heaters is accordingly reached at time t'i,,,. The fact that this occurs earlier than At',, in the case of' DC' coupled heaters (1 ig. 2) means that the heaters can quench the associated coils correspondingly earlier, and so reduce the likelihood of damage to the superconducting coils.

To induce a quench in a superconductive coil, it is typically sufficient to heat the coil at at least 2W for ().3 seconds. As illustrated by the curves h1 Fig. 4, heaters arranged according to the present Prevention reach this level of heating more rapidly than those ol'the prior art, leading to a more rapid quench.

Is Ir1 Fig.4, h represents the minimum level of | dV/dt 12 required to produce the heathig required to quench the heated coils. Siuf'ficient power is provided between time t', and t',. 'I'he thee period between points t', and It', must he sul'licient to ensure an effective quericil.

According to a benefit of the present invention, the thee t', occurs significantly earlier than the thee t'', in Fig.2. The heated coils will be quenched sooner than in the known system, leading to reduced likelihood of'damage to the superconductive coils.

The present invention allows all coils to be quenched sooner than with known arrangements which means that the required energy dissipation may be more effectively spread across all of the coils. Tllis he turn means that each coil need only be designed to tolerate a reduced as maximum energy dissipation. 'I'his allows thinner copper cladding to he provided around the superconducting conductor in each coil, in turn allowing for a cheaper, lighter and smaller magnet as a result.

The capacitor 3() used must be of a relatively high value' and be capable ol' withstanding relatively high voltages. For example, a capacitor of 47F capacitance with a voltage rating of 5kV may he suitable. Certain types of lihn capacitor may he suitable to t; lfil this role, and may tolerate operation at cryogenic temperatures. Electrolytic capacitors are available in appropriate capacitance and voltage ratings. I lowever, such capacitors may be unsuited to being located inside the superconducthig coil system, at cryogenic temperatures. An electrolytic capacitor may be provided for the purposes of the present invention on the outside s of the superconducting coil system, but would require careful provision of a high voltage connecting cable, and steps would need to be taken to avoid disconnection of the high voltage connecting cable.

I'he value of the capacitor 30 can be selected to suit the heater resistances and the energy required to be dissipated. When the capacitor has fully charged, that is, once the quench voltage is at its peak, the dissipation of the heaters returns to zero. For a capacitor of 50,uF charged to 4kV, the energy stored in the capacitor is q=/L'V9: 5()X 1()'6 X 20()()' - I ()()J.

The value of' the capacitor should be selected to provide the required performance. A larger Is capacitance will allow greater energy storage, required for providing enough heat to quench the coils, while a smaller capacitance will provide the same peak power dissipation but for a shorter time. 'I'his shorter thee may be advantageous in preventing damage to the heaters.

Claims

Cl,AIMS: 1. An assembly comprising,: - a number of superconductive coils,

s - at Ieast one quench heater arranged to heat the superconductive coil(s) in the event that at Ieast part ot'at Ieast one of the coils enters a quenched state; - means lor transl'erring energy from the coil(s) to the heater(s) in the event that at least part of at Ieast one ol'the coils enters a quenched state; wherein the means for transl'errbly energy from the coil(s) to the heater(s) includes a series no capacitance, through which the energy transferred must pass.
2. An assembly according to claim I wherein a plurality of superconductive coils are connected in series, and the heater(s) is/are connected across a subset of the coils.

Is
3. An assembly according to claim 2, wherein the heater(s) is/are connected across one hal i'of the coi Is.
4. An assembly according to any precedhg claim wherein the heater(s) is/are connected to the superconductive coils through a bridge rectifier' and wherein the series capacitance is so provided between intermediate nodes of the bridge rectifier in place of a DC connection.