Passive Quench Propagation Circuit for Superconducting

Magnets

Technical Field

The invention provides a quench propagation circuit for a superconducting magnet.

Technical Background

Superconducting magnets are used in a variety of

applications, for example as magnetic field generators in Magnetic Resonance Imaging (MRI) equipment. Coils of

superconducting wire are held at cryogenic temperatures, typically at about 4 Kelvin, the boiling temperature of helium. An ever-present risk in the use of superconductive coils is the occurrence of a quench. For a reason such as localised heating, the temperature of a region of the

superconducting wire rises above its critical temperature and superconductivity breaks down to the effect that the region becomes resitive. The large current flowing through the coils of the superconducting magnet continues to flow through the now resistive region generating more heat. This heat causes a larger region of the superconductor to become resistive thereby increasing the dissipated heat. Since the resistive region is initially small, the heat dissipation is

concentrated in a small volume. The temperature of this small volume may accordingly rise to such a temperature that the superconductive coil is damaged. When a known MRI

superconducting magnet quenches, an energy of the order of tens of MegaJoules must be dissipated in a time period of a few seconds. It is known to avoid such damage by providing quench 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 quenched state then a resistive or inductive voltage is built up across each coil. This

inductive or resistive voltage is applied 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. The 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. This process is called quench propagation and represents an emergency shutdown of the superconducting magnet. Since MRI devices are commonly run in persistent mode, there is no power supply attached to the superconducting magnet. When the magnet is commissioned, the installation engineer energises the magnet by means of an external power supply. After reaching the desired operating current, a superconducting switch is closed and the current circulates within the magnet. Accordingly a false detection of a quenching coil must be avoided. On the other hand, a quench propagation circuits used for detecting a quench in a superconducting coil must be very responsive to any voltage sensed at a sensing input of the quench propagation circuit in order to avoid a potentially damaging or even dangerous quench

scenario .

Quench propagation circuits may be passive or active. Passive quench propagation circuits are powered purely by the

resistive and inductive voltages generated by the quenching coils, i.e. passive quench propagation circuits divert magnetic energy from the quenching superconductive magnet to the quench heaters. Active quench propagation circuits, on the other hand, use external electronics to interpret voltages measured on the coils and then decide to release externally provided energy to the quench heaters. Active quench propagation circuits thus require an external power supply, however, this poses many design issues as the power needs to be provided through the cryogenic shield of the superconducting magnet and may cause electromagnetic

interference with the MRI apparatus.

Passive quench propagation circuits must efficiently deliver the energy delivered to them by the quenching coil to the quench heaters as fast as possible. One parameter that determines this efficiency is that the output impedance of the quenching magnet closely matches the input impedance of the quench propagation circuit. This has the undesirable effect of increasing the coupling of any voltage induced into the magnet coils by means other than quenching into the propagation circuit and quenching the magnet. Previous generations of superconducting magnets have not been

susceptible to this as the propagation circuits were not required to be anything like as sensitive. Also, the main source of voltages being induced into the magnet coils has been from the gradient coil which is situated in the warm bore of the magnet and thus well screened by the conductive cryo-shields and magnet formers.

As magnet designs are evolving, the quench propagation circuit must be more sensitive, gradient power and rise times are increasing and conductive magnet formers may be replaced by other structures. This leaves the magnet coils much more exposed to electromagnetic coupling from external sources such as gradient coil interaction.

Accordingly it is an object of the invention to provide a passive quench propagation circuit that quickly responds to the voltage delivered by a quenching coil but does not respond to voltages generated by electromagnetic

interference . Summary of the Invention For these reasons the invention provides a passive quench propagation circuit for a superconducting magnet comprising a plurality of superconducting magnet coils. The passive quench propagation circuit includes a sensing input connected or connectable across a first one of the superconducting magnet coils and a quench heater arrangement in heating contact with a second one of the superconducting magnet coils and adapted to heat the second superconducting magnet coil in response to a voltage present at the sensing input. According to the invention a low-pass filter is connected between the sensing input and the quench heater arrangement and adapted to filter the voltage present at the sensing input and to output the filtered voltage to the quench heater arrangement.

The quench propagation circuit of the invention responds quickly to what essentially is a relatively slow and

predictable unipolar voltage rise time delivered by a

quenching coil but does not respond to voltages that contain fast rise times or oscillate either side of zero but could still deliver enough energy to quench the magnet via the quench propagation circuit. The low-pass filter of the quench propagation circuit of the invention evens out fast transient voltages such that energy will not be provided to the quench heater (s) included in the quench heater arrangement. On the other hand, if a comparatively slow and steadily rising voltage appears at the sensings input, the low-pass filter will output this voltage to the quench heaters and a

controlled quenching of the superconducting magnet will ensue. The delay thus produced in response to real quench voltages is insignificant whilst interference voltages are attenuated effectively. Preferably the low-pass filter includes a capacitor connected between a first contact and a second contact of the sensing input. The low-pass filter may further include a first resistor connected between one of the first or second contact of the sensing input and an electrode of the capacitor. The cutoff frequency of the low-pass filter may be tuned by chosing appropriate values for the resistor and the

capacitor . The low-pass filter further includes a second resistor connected across the capacitor. This second resistor may function as a bleed resistor slowly discharging any charge transferred to the capacitor of the low-pass filter by interference voltages without passing this energy to the quench heaters. The second resistor should have a resistivity higher than that of the first resistor, for example five or ten times higher.

The low-pass filter has a cutoff frequency of less than 100 Hertz. Preferably the low-pass filter has a cutoff frequency of less than 50 or even less than 20 Hertz. These relatively low cutoff frequencies suffice to let the voltage of a quenching coil pass but reject all typical interference voltages .

The quench propagation circuit may further include at least one shunt diode connected in parallel to the quench heater arrangement. The at least one shunt diode serve to limit the voltage across the quench heaters and thus the heat generated by the quench heaters. This has an advantage in that the quenching process takes place at a predefined maximum speed limiting coolant vaporisation. For example, a maximum voltage across the quench heaters may be set to an arbitrary value, e.g. 100 Volts or less, by providing an appropriate number of series-connected shunt diodes in parallel to the quench heater arrangement. Furthermore, the quench propagation circuit may further include a rectifier connected between an output of the low- pass filter and the quench heater arrangement. The polarity of the voltage generated by a quenching coil depends on the location of the quenching coil in the coil arrangement of the superconducting magnet. If a rectifier is present, such a voltage of inverted polarity may be provided to the quench heaters in the same way as one of expected polarity. This is especially useful when shunt diodes are provided in parallel to the quench heater arrangement due to the polarity

sensitive nature of diodes.

The rectifier may include for each contact of the sensing input a corresponding pair of diodes. A first diode of the respective pair of diodes has a cathode connected to the contact and an anode connected to a first electrode of the quench heater arrangement. A second diode of the respective pair of diodes has an anode connected to the contact and a cathode connected to a second electrode of the quench heater arrangement.

The sensing input of the quench propagation circuit may include N contacts with N being a number greater than two. The quench propagation circuit may then include a number of N-l low-pass filters each of which having an input connected between a corresponding one of the N contacts and a remaining one of the N contacts. This constitutes a multi-tap quench protection circuit for monitoring a plurality of

superconducting coils at the same time.

The quench propagation circuit may further comprise a plurality of coupling capacitors. Each of the coupling capacitors is connected between a corresponding pair of outputs of the N-l low-pass filters. The coupling capacitors may provide a low-pass filtering function but can be present in addition to other capacitors in the low-pass filters. Brief Description of the Drawings

The invention will be better understood from the following drawings in which preferred embodiments of the invention will be illustrated by way of example. Throughout the drawings the same reference numerals refer to the same or similar items. In the drawings: Figure 1 shows a first embodiment of the invention;

Figure 2 shows a second embodiment of the invention;

Figure 3 shows a third embodiment of the invention;

Figure 4 shows a fourth embodiment of the invention; and Figure 5 shows a fifth embodiment of the invention.

Detailed Description of the Drawings

Figure 1 shows a first embodiment of the invention. A passive quench propagation circuit 1 is connected to a

superconducting magnet 2 by means of a sensing input 4. The superconducting magnet 2 comprises a plurality of

superconducting magnet coils 3 of which two are shown. The superconducting magnet 2 can comprise any number of

superconducting magnet coils 3. The superconducting magnet coils 3 are connected in a circular fashion by

superconducting switch 8 enabling the current through the coils 3 to persistently flow. In the superconducting state the magnet coils 3 expose no ohmic resistance. Since the current through the coils 3 is constant, there is no voltage present across each of the coils 3. If one of the coils 3 or a section thereof leaves the superconducting state, a voltage will build up. The quench propagation circuit 1 senses this voltage and diverts power from the magnet coils 3 to a quench heater arrangement 5 included in the quench propagation circuit 1. The quench heater arrangement 5 can comprise any number of quench heaters 6. The quench heaters 6 are in heat- conducting contact with the superconducting magnet coils 3 and serve to heat the coils 3 in the case that a quench occurs. The heating causes the coils 3 to raise above the critical temperature and to thus collectively return from the superconducting state.

According to the invention a low-pass filter 7 is provided between the sensing input 4 and the quench heater arrangement 5. The low-pass filter 7 filters any voltages present at the sensing input 4. In this way fast transient voltages such as interference voltages can be filtered away while

comparatively slowly rising voltages may still pass through the filter 7. Hence, only a slowly rising voltage will cause a current to flow through the quench heater arrangement 5 and to cause the controlled quenching of the superconducting magnet 2.

Figure 2 shows a second embodiment of the invention. As in the example of Figure 1, a quench propagation circuit 1 is connected to a superconducting magnet 2. In the case of a quench occurring in one coil of the superconducting magnet 2 the quench propagation circuit 1 will provide heat 10 to the magnet coils 3 by means of a quench heater arrangement 5.

The quench propagation circuit 1 of Figure 2 is similar to that of Figure 1 but comprises a rectifier 9 connected between an output of the low-pass filter 7 and the quench heater arrangement 5. The rectifier 9 decouples the low-pass filter 7 from the quench heater arrangement 5 and makes the quench propagation circuit insensitive to the polarity of the voltage present at its sensing input 4. Figure 3 shows a third embodiment of the invention. The quench propagation circuit 1 of the third embodiment is similar to that of Figure 2 but comprises a shunt diode arrangement 11 connected in parallel to the quench heater arrangement 5. The shunt diode arrangement 11 limits the voltage that may appear across the quench heater arrangement 5 and thus the amount of heat 10 provided to the

superconducting magnet 2. Accordingly the quenching process can be conducted at a controlled speed. Since the shunt diodes respond to voltage of a specific polarity, the

rectifier 9 serves to invert any voltages of opposite

polarity that may be present at the sensing input 4 and in consequence at the output of the low-pass filter 7.

Figure 4 shows a fourth embodiment of the invention. The low- pass filter 7 of the fourth embodiment of the quench

propagation circuit 1 includes a capacitor 12. The capacitor 12 passes high frequencies and thus directs such voltages back to the superconducting magnet 2. Only low frequencies are allowed to pass to the quench heater arrangement 5. The low-pass filter 7 further comprises a resistor 13 connected in series with the capacitor 12. The resistor 13 and the capacitor 12 determine a cutoff frequency of the low-pass filter 7.

While only a first order low-pass filter 7 is shown in Figure 4, filters of higher order may be used in all embodiments of the invention. An optional bleed resistor 14 may be provided in parallel with the capacitor 12 in order to slowly dispose of any voltage built up on the capacitor 12 in response to interference voltages.

The rectifier 9 of the fourth embodiment of the invention comprises a pair of diodes 16, 17 for each contact of the sensing input (or output contact of the low-pass filter 7) . As already shown in the embodiment of Figure 3, shunt diodes 15 are provided in parallel with the quench heaters 6. The quench heaters 6 are series-connected but any arrangement may be used including mixes of series and parallel connections.

Figure 5 shows a fifth embodiment of the invention. The fifth embodiment of the quench propagation circuit 1 constitutes a multi-tap circuit that may be used to monitor a plurality of magnet coils 3. Each resistor 13 forms a respective low-pass filter together with the corresponding one of the capacitors 12. In addition second capacitors 18 are provided which are referred to as coupling capacitors because they are connected between neighbouring signal lines. The coupling capacitors 18 may be provided together with the capacitors 12.

Alternatively only either the coupling capacitors 18 or the capacitors 12 may be present. As with the embodiment of

Figure 4, bleed resistors may be provided in parallel to the capacitors 12.

For each output of the multi-tap low-pass filter 7 a

corresponding pair of diodes 16, 17 is provided in the rectifier 9. The quench heater arrangement 5 and the shunt diodes 11 correspond to those of the embodiment of Figure 4.

Although the invention has been shown and described with respect to exemplary embodiments thereof, various other changes, omissions, and additions in form and detail thereof may be made therein without departing from the spirit and scope of the invention.

While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives,

modifications, and equivalents as may be included within the scope of the invention as defined by the appended claims.