Discharge controlled superconducting magnet

1. An MRI type superconducting magnet system comprising:

a coil winding for generating a magnetic field in a normal and in a superconducting state;

a power supply for charging and discharging the coil windings, wherein the power supply includes a resistor, a diode, or both a resistor and a diode to dissipate energy while discharging the coil winding;

a Persistent mode Current Switch (PCS) configured to form an electrically closed-loop with the coil windings while in a superconducting state; and

a combination of a diode and a controllable-voltage element or a diode and a controllable-resistance element connected across the coil winding terminals, wherein controlling the voltage across the controllable-voltage element or the resistance of the controllable-resistance element controls discharge voltage of the coil winding.

2. The system of claim 1 , wherein the superconducting magnet includes a cryocooler for its cooling.

3. The system of claim 1 , wherein the superconducting magnet is cooled through conduction, by a cryocooler.

4. The system of claim 1 , wherein the controllable-resistance element is a Discharge Superconducting Switch (DSS).

5. The system of claim 1 , wherein the controllable-voltage element or the controlled-resistance element is strategically located for heat transfer considerations.

6. The system of claim 1 , wherein the control of the voltage across the controllable-voltage element or the resistance of the controllable-resistance element includes heating or cooling of the controllable elements.

7. The system of claim 1 , wherein the power supply resistor, diode or, resistor and diode are strategically located for heat transfer considerations.

8. The system of claim 1 , wherein PCS is a non-inductive coil of superconducting wires.

9. A quench-controlled MRI type superconducting magnet machine comprising:

a coil winding for generating a magnetic field in a normal and in a superconducting state;

a power supply attached to coil winding terminals;

a resistor, a diode, or both a resistor and a diode disposed between the coil winding terminals configured to dissipate energy while discharging the coil winding;

a Persistent mode Current Switch (PCS) configured to form an electrically closed-loop with the coil winding while in a superconducting state; and

a series combination of a diode and a controllable-resistance component disposed across the coil winding terminals or across the power supply, wherein controlling the resistance of the controllable-resistance component controls the discharge voltage of the coil winding.

10. The machine of claim 9 , wherein superconducting magnet cooling system is cryogen-free.

11. The machine of claim 9 , wherein the superconducting magnet is cooled by a cryocooler.

12. The machine of claim 9 , wherein the controllable-resistance component is a Discharge Superconducting Switch (DSS).

13. The machine of claim 9 , wherein the controllable-resistance component is strategically located for heat transfer considerations.

14. The machine of claim 9 , wherein the control of the controllable-resistance component comprises heating or cooling of the controllable-resistance component.

15. The machine of claim 9 , wherein the power supply resistor, diode or, resistor and diode are strategically located for heat transfer considerations.

16. The machine of claim 9 , wherein the controllable-resistance component is configured to operate in a normal resistive state and a superconducting non-resistive state.

17. A superconducting-magnet discharging method, the method comprising:

charging a superconducting coil winding at a first voltage until a desired magnetic field is achieved;

electrically connecting coil winding terminals together by a superconducting material such that a closed-loop coil is formed and a persistent-mode is achieved;

monitoring indicators of potential of a quench situation;

opening the closed-loop coil upon sensing the potential of a quench situation;

discharging current generated by collapsing magnetic field through heat-generating elements; and

controllably limiting the voltage across the coil winding during the discharging process and maintaining a desired discharge voltage at a second voltage, wherein the second voltage is higher than the first voltage.

18. The method of claim 17 , wherein the controlled discharge voltage is continuous and is not step-wise, and wherein the controlled discharge voltage is higher than the charging voltage but lower than a critical voltage of the coil winding.

19. The method of claim 17 , wherein the controlling of the discharge voltage is comprises using a series combination of a diode and a superconducting material placed across the coil winding terminals.

20. The method of claim 17 , wherein controlling the voltage across the coil winding is performed by a Discharge Superconducting Switch (DSS).