SCR switch aids protection of superconducting coils

When a high field superconducting coil becomes resistive, degradation or failure of the coil may occur if the energy stored in the magnetic field is dissipated in the initial resistive portion of wire. By sensing the onset of the normal transition, it has been possible to disconnect the external power from the coil and transfer the coil energy to an external energy sink rapidly enough to reduce the likelihood of damage to the coil. A controlled rectifier static switching circuit has been employed, which is capable of interrupting 450 amperes dc and blocking 475 volts. The initiating signal for triggering the switch operation is obtained from a mutual inductor carrying the superconducting coil current. Obtaining a reliable initiating signal is complicated by "flux jumps" which occur in the superconducting coil. Although many different types of energy sinks can be employed to store the coil energy, a precharged capacitor has proved very effective for small size superconducting solenoids.