SUPERCONDUCTING MAGNET ASSEMBLY

Abstract

A superconducting magnet arrangement comprises a field coil assembly with coil windings that when in operation are electrically superconducting. The field coil assembly is circuited between connection ports for a voltage supply. A switching module switches a sub-section of the field coil assembly's coil windings between its electrical superconducting and electrical resistive states, said sub-section forming a switching coil circuited between the connection ports. In the operational state where both the switching coil and the field coil(s) are superconducting and carry a permanent electrical current, the field coil(s) and the switching coil together generate a stationary magnetic field. According to the invention the switch windings give a significant contribution to the magnetic field. The field coil assembly's coil windings that may be switched between it electrically superconducting and resistive states form the switching coil. That is, the switching coil forms part of the field coil assembly and contributes significantly to the magnetic field generated by the field coil assembly.

Claims

1. A superconducting magnet arrangement comprising a field coil assembly with (i) coil windings that when in operation are electrically superconducting and (ii) connection ports for voltage supply, the field coil assembly's coil windings being circuited between the connection ports; and a switching module to switch a sub-section of the field coil assembly's coil windings between its electrical superconducting and electrical resistive states, said sub-section forming a switching coil circuited between the connection ports.

2. The superconducting magnet arrangement claim 1, wherein inductance of the switching coil is smaller than the inductance of the field coil assembly.

3. The superconducting magnet arrangement of claim 1 further comprising a support structure which carries the field coil assembly, wherein a thermal isolation is provided between the switching coil and the support structure.

4. The superconducting magnet arrangement of claim 1, wherein, when in its superconducting state the switching coil is circuited in series with the field coil assembly.

5. The superconducting magnet arrangement of claim 1, wherein the field coil assembly includes multiple field coils arranged coaxially along a common longitudinal axis, while the field coils' windings as well as the switching coil's windings are azimuthally around the longitudinal axis.

6. The superconducting magnet arrangement of claim 2, wherein a ratio of the inductance of the switching coil to the inductance of the field coil assembly is in a range of 1/20 to 1/2.

7. The superconducting magnet arrangement of claim 1, wherein the field coil assembly's field coils and the switching coil include windings of a high-temperature superconducting material having its critical temperature in the range of 20-80K.

8. The superconducting magnet arrangement of claim 7, wherein the high-temperature superconducting material is MgB.sub.2.

9. The superconducting magnet arrangement of claim 8, wherein the switching coil includes a length of superconducting wire in the range of 1600-2400 m.

10. A method of ramping-up a field coil assembly of a superconducting magnet arrangement as claimed in claim 1, the method comprising the steps of: setting the switching coil to its resistive state and connect a voltage source to the connection ports; when the electrical current through the field coils has reached its pre-set end-value, setting the switching coil to its superconductive state and switching-off the voltage source.

11. A magnetic resonance imaging system comprising a superconducting magnet arrangement as claimed in claim 1.

12. The superconducting magnet arrangement of claim 6, wherein the a ratio of the inductance of the switching coil to the inductance of the field coil assembly is in the range 0.08 to 0.12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is an electric circuit diagram of an example of the superconducting magnet assembly of the invention with the power supply coupled to it;

[0028] FIGS. 2a and 2b are graphical representations of the dynamics of the electrical quantities during the ramp-up and ramp-down procedures, respectively, of the super conducting magnet assembly of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] FIG. 1 is an electric circuit diagram of an example of the superconducting magnet assembly 100 of the invention with the power supply coupled to it. The field coil windings 101 of self-inductance L1 are for simplicity shown her as a single unit, but in practice are arranged as an assembly of several field coils. The switching coil 102 of self-inductance L2 is shown as a single unit, but in practice the switching coil may be arranged as an assembly of several switching coil(s). The switching coil 102 and the field coil together form a superconducting loop closed in itself. There is no need for the coil sections forming the switching coil to be physically grouped together in the magnet, where the interconnecting wiring of the coil sections has to be such that all sections of the switching coil are connected directly in series, without elements of the non-switching category in between.

[0030] A power supply is connected across the switching coil by means of electrically and thermally detachable leads 103, which may be remotely operated relays. The field coil 101 and the switching coil 102 may be cooled to superconducting temperature by a cryogenic system (not shown). It is advantageous to provide a means to increase the thermal resistance and/or reduce the refrigeration power for the switching coils during the time they have to be in the resistive state. The controllable power source 105 is connected to the current leads 103. A diode or stack of diodes 104 may be circuited in parallel to the power source 105 to allow electrical current from the magnet to circulate even when the power supply is in its switched off state.

[0031] The invention is employed according to the following procedure. To achieve that the superconducting magnet assembly generates the specified magnetic field, the switches are closed, the field coil 101 cooled so that it is superconducting and the switching coil 102 arranged to be in its resistive state. To that end an adjustable heater 106 is provided to dissipate heat into the switching coil to increase its temperature above the superconductive critical temperature. Optionally, the cooling of the switching coil is reduced or interrupted. A DC voltage V.sub.0 is generated by the power supply, which causes an increasing electrical current through the superconducting field coils 101. This electrical current continues to increase until, after a time tramp it reaches is pre-set value I.sub.set at which the voltage from the power source is reduced to zero. During the ramp, the voltage across the resistive switching coil causes a electrical current V.sub.0/R.sub.switch through this coil, which causes a dissipation V.sub.0.sup.2/R.sub.switch. Because of this ramp-related dissipation, the power of the heater can be in order to limit the total amount of heat accumulated in the switching coil. After reaching the current set point, the switching coil is made superconducting, e.g. by switching off the heater 106 and enabling the cooling of the switching coil. The required wait time for the switch coil to become superconducting is equal to ratio of the amount of energy dissipated in the switch coil during the ramp, which is equal to (L.sub.1.I.sub.set).sup.2/t.sub.ramp.R.sub.switch, and the cooling capacity of the refrigeration system of the switching coil. Note that neither the mass nor the volume of the switch coil influence the required wait time. Using a relatively large coil as a switch is advantageous because this results in a large resistance R.sub.switch. After the switching coil has reached its superconducting state, the electrical current from the power source is reduced to zero. This causes the electrical current in the switch coil 102 to increase. Simultaneously, there is a reduction in the electrical current in the field coil 101. In the final state, the electrical currents in all coils 101 and 102 become equal and there is no longer a electrical current flowing to the power supply. The power source may then be safely disconnected. The field coil and switching coil are in series carrying equal electrical current and both contribute to the magnetic field of the superconducting magnet.

[0032] The proper value for the electrical current set point I.sub.set at the end of the ramp up which is needed for a final electrical current I.sub.0 can easily be calculated using the values of the inductances of the field coils 101 L.sub.1 and of the switching coil 102 L.sub.2 and the mutual inductance between these two coils M.sub.12. As soon as the switch coil reaches its superconducting state the total magnetic flux in the superconducting magnet circuit 101-102 becomes entrapped and remains constant for as long as the circuit is superconducting. At the end of the ramp, this entrapped flux is equal to L.sub.1.I.sub.set whereas in the final state it is equal to L.sub.total.I.sub.0 . Here L.sub.total is the total inductance of the superconducting loop, which is equal to L.sub.1+L.sub.2+2M.sub.12. Therefore, the current set point has to be I.sub.set=I.sub.0.(L.sub.total/L.sub.1). Since the inductances are very well known, either by simulation or by measurement, the required current overshoot is accurately predictable. The above formula also shows that as long as L.sub.2 and M.sub.12 are much smaller than L.sub.1 the magnitude of the required current overshoot is limited.

[0033] For the magnet to ramp down, the current lead switches 103 are closed, the power supply is switched on and its electrical current is ramped up such that the electrical current in the switch coil 102 is reduced to a small value. Then the heater 106 is switched on to raise the temperature of the switching coil above its superconducting critical temperature. The field coil 101 remains superconducting. Applying a negative voltage across the field coils 101 then causes the electrical current to reduce to zero.

[0034] FIG. 2a is a graphical representation of the dynamics of the electrical quantities during the ramp-up procedure of a typical example of the superconducting magnet assembly of the invention, with a field coil inductance L.sub.1=20 H, a switching coil inductance L.sub.2=2 H, a mutual inductance M.sub.12=0.1 H. At the start of the ramp-up at t=0, the heater 106 is on, so that the switching coil 102 is in its resistive state. The dashed line represents the voltage supplied by the power supply 105. After an initial transient, the voltage is kept at a constant value V.sub.0. The drawn curve shows the electrical current through the field coil 101. The electrical current increases approximately linear with time at a rate V.sub.0/L.sub.1. At time point t.sub.1 the electrical current through L.sub.1 reaches its set point I.sub.set and the voltage of the power source is set to zero. Then, while the electrical current through the field coil 101 remains at its constant level I.sub.set, the heater of the switch coil is switched off and the normal cooling of this coil is re-activated. The system has to be kept in this state until the switch coil has become superconducting. After sufficient waiting time to allow the switching coil to become fully superconducting, the electrical current from the power supply is ramped to zero. This causes an increase of the electrical current in the switching coil 102 and a decrease of the electrical current in the field coils 101, until the two electrical currents have become equal and the electrical current flowing to the power supply has become zero. During the redistribution of the electrical current, a negative inductive voltage is generated, which is associated with the decrease of the energy in the field coils and switching coils due the re-distribution of the electrical current.

[0035] FIG. 2b is a graphical representation of the dynamics of the electrical quantities during a controlled ramp-down of a typical magnet according to the invention. First a positive voltage is applied to the magnet, which causes the electrical current in the switching coil 102 to decrease. Simultaneously, the electrical current in the field coils 101 increases. When the electrical current from the power supply has reached the value I.sub.set previously used during the ramp up, the electrical current in the switching coil has become zero. At this point, the heater of the switching coil can be switched on and optionally the cooling of the switching coil can be reduced or disabled. Applying a negative voltage across the switching coil then causes the electrical current in the field coils to reduce to zero.