Magnetic field control

1. An assembly for generating a stable magnetic field comprising a main power supply unit arranged to provide a main direct current to a superconducting magnetic circuit to generate a superconducting magnetic field, a magnetic field measurement device for measuring the generated magnetic field, and an auxiliary power supply unit in parallel with the main power supply unit and arranged to output an auxiliary direct current to the superconducting magnetic circuit based on the measured magnetic field.

2. An assembly according to claim 1 wherein the magnetic field measurement device produces an error signal based on the measured magnetic field value and a predetermined magnetic field value.

3. An assembly according to claim 2 wherein the error signal is proportional to the difference between the measured magnetic field value and the predetermined magnetic field value.

4. An assembly according to claim 1 wherein the field measurement device comprises a nuclear magnetic resonance probe and a detection circuit.

5. An assembly according to claim 1 wherein the field measurement device comprises a pick up coil which produces an error signal, dependent upon the rate of change of the magnetic field, indicating the difference between the measured magnetic field and a predetermined magnetic field value.

6. An assembly according to claim 1 wherein the field measurement device is a Hall effect sensor, GMR sensor, or SQUID sensor.

7. An assembly according to claim 1 wherein the error signal is conditioned by a PID function before it is input to the auxiliary power supply unit.

8. An assembly according to claim 1 wherein the auxiliary power supply unit has a maximum output current of about +/−0.01 to about 1% of the output current of the main current unit.

9. An assembly according to claim 1 wherein the auxiliary power supply unit has a stability of better than 25 PPM (parts per million) per hour of its maximum output current.

10. An assembly according to claim 1 wherein the superconducting magnetic circuit comprises a high temperature superconductor coil.

11. An assembly according to claim 1 wherein the assembly further comprises a DC current transducer to control the main power supply unit.

12. A method for generating a stable superconducting magnetic field, comprising:

providing a main direct current to a superconducting magnetic circuit to generate a magnetic field,

measuring the generated magnetic field,

providing an auxiliary direct current in parallel to the superconducting magnetic circuit and based on the measured magnetic field value.

13. A method according to claim 12 including measuring the generated magnetic field and producing an error signal based on the measured magnetic field value and a predetermined magnetic field value.

14. A method according to claim 12 including measuring the generated magnetic field and producing an error signal based on the measured magnetic field value and a predetermined magnetic field value proportional to the difference between the measured magnetic field value and the predetermined magnetic field value.

15. A method according to claim 14 including conditioning the error signal is by a PID function before providing an auxiliary current to the superconducting magnetic circuit based.

16. A method according to claim 12 wherein the superconducting magnetic circuit comprises a high temperature superconductor coil.

17. A method according to claim 12 including controlling the main power supply unit via a DC current transducer.

18. A method for stabilising a superconducting magnetic field comprising measuring a generated magnetic field value, comparing the measured magnetic field value with a predetermined magnetic field value and calculating an error signal, supplying the error signal to an auxiliary power supply unit connected in parallel with a main power supply unit to produce a stabilising direct current, feeding the stabilising direct current to the superconducting magnetic field generating circuit to thereby stabilise the generated magnetic field.