Method and apparatus for shimming a superconducting magnet

1. An apparatus for shimming a superconducting magnet, the superconducting magnet comprising a number of magnet coils electrically connected in series between a first current connection and a second current connection and a superconducting switch connected in parallel with the magnet coils, between the first current connection and the second current connection, the first current connection being electrically connected to an external current lead, the apparatus for shimming comprising:

superconducting a first solid-state switching device electrically interposed between the external current lead and the magnet coils;

a number of superconducting shim coils electrically connected between the first current connection and the second current connection through a further solid-state switching device;

switches, each connected in parallel with a respective superconducting shim coil; and

each superconducting shim coil being electrically connected in series with a respective solid-state switching device, and each series combination of superconducting shim coil and respective solid-state switching device being connected between the first current and the second current connection.

2. An apparatus for shimming a superconducting magnet according to claim 1 , wherein a current sensing resistor is electrically in series with the further solid-state switching device.

3. An apparatus according to claim 2 wherein the superconducting shim coils are connected in series with the current sensing resistor and the solid-state switching device.

4. An apparatus according to claim 1 , wherein each series combination further comprises a current sensing resistor connected in series.

5. An apparatus according to claim 1 , further comprising inverse-parallel connected diodes, connected between the first current connection and the second current connection.

6. A method for shimming a superconducting magnet comprising a number of magnet coils electrically connected in series between a first current connection and a second current connection and a first superconducting switch connected in parallel with the magnet coils, between the first current connection and the second current connection, the first current connection being electrically connected to a fixed current connection, said method comprising:

electrically connecting a first solid-state switching device between the external current lead and the magnet coils;

electrically connecting a number of superconducting shim coils between the first current connection and the second current connection through a further solid-state switching device;

connecting each of further superconducting switches in parallel with a respective superconducting shim coil;

turning ON the first solid-state switching device and turning OFF the further solid-state switching device;

turning OFF the first superconducting switch;

introducing current into the magnet coils through the external current lead and the second current connection;

turning ON the first superconducting switch;

turning OFF the first solid-state switching device;

turning ON a selected further solid-state switching device;

turning OFF a selected one of said further superconducting switches, in parallel with a selected shim coil, said selected shim coil being in series with the selected further solid-state switching device;

introducing, through the external current lead and the second current connection, a desired current into the selected shim coil;

turning ON the selected further superconducting switch; and

repeating the preceding four steps for each of the superconducting shim coils.

7. A method for shimming a superconducting magnet according to claim 6 , further comprising monitoring current flowing in each shim coil as current is introduced, and closing the selected further superconducting switch when a desired current is reached for each superconducting shim.

8. A method for shimming a superconducting magnet comprising a number of magnet coils electrically connected in series between a first current connection and a second current connection and a first superconducting switch connected in parallel with the magnet coils, between the first current connection and the second current connection, the first current connection being electrically connected to a fixed current connection, said method comprising:

electrically connecting a first solid-state switching device between the external current lead and the magnet coils;

electrically connecting each of a number of superconducting shim coils between the first current connection and the second current connection through a respective further solid-state switching device;

connecting each of further superconducting switches in parallel with a respective superconducting shim coil;

turning ON the first solid-state switching device and turning OFF the further solid-state switching devices;

turning OFF the first superconducting switch;

introducing current into the magnet coils through the external current lead and the second current connection;

turning ON the first superconducting switch;

turning OFF the first solid-state switching device;

controlling the conductivity of one or more selected further solid-state switching devices which are in series with selected shim coils;

turning OFF further superconducting switches that are parallel with the selected shim coils;

introducing, through the external current lead and the second current connection, a desired current into the selected shim coils, said current being distributed between the selected shim coils according to the relative controlled conductivities of the corresponding selected further solid-state switching devices; and

turning ON the selected further superconducting switches.

9. A method for shimming a superconducting magnet according to claim 8 , further comprising monitoring the amount of current flowing in the or each selected shim coil while the associated further superconducting switches are turned OFF, and controlling the conductivity of one or more selected further solid-state switching devices to achieve a required level of current in each selected shim coil.

10. A method of shimming a superconducting magnet according to claim 1 further comprising the steps of measuring the homogeneity of a magnetic field within an imaging region of the superconducting magnet and iteratively adjusting the current flowing in each of the superconducting shim coils so as to achieve a desired homogeneity of the magnetic field in the imaging region.