PURPOSE: To restrict the end voltage across a superconducting coil to within a specific value and attenuate a coil current quickly by connecting the contacts of thyristor switches and resistors via electrostatic capacitors and connecting a pulse generation circuit to control the thyristor switches according to the voltage of the superconducting coil.

CONSTITUTION: The quenching time of a superconducting coil 1 is inspected, a signal is transmitted to a pulse generation circuit 7, and at the same time a DC circuit breaker 4 is opened. A pulse is supplied from the pulse generation circuit 7 to a first thyristor TH1 to pass a current therethrough. A superconducting coil current (i) is passed through a first resistor R1 and the thyristor TH1, generates terminal voltage V equal to iR1, and attenuates with a certain time constant. The voltage generated at that time charges a first electrostatic capacitor C1 through a second resistor R2 and generates positive and negative poles. When the voltage of the superconducting coil becomes lower then specified, a pulse signal is transmitted to a second thyristor TH2 to pass a current therethrough. In this time, a discharge current i1 is passed from the electrostatic capacitor C1 to a circuit of the first and second thyristor TH1 and TH2, cancels the current of the thyristor TH1, and prevents a current from being passed therethrough.