Design and Projected Performance of the Westinghouse µSMES Unit

Abstract

The Westinghouse design of a 5 MJ, 2 MW, 2 second discharge µSMES unit is described in this report. The magnet consists of concentric, potted coil modules, each with a support structure designed to limit eddy currents. It operates in a bath of liquid helium. The individual modules are wound with a superconducting Rutherford cable which operates at 1500 amperes in a peak field of 3.5 T. The coil modules are layer wound and potted with the support structure. The magnet is designed so that it will not quench during discharge and the subsequent charge. This performance is calculated by determining the losses due to hysteresis and eddy currents and the resulting temperature rise. The magnet current is compared to the temperature and field dependent critical current to determine that the superconductor has adequate stability margin to prevent a quench. Losses are reduced by using a Rutherford cable and by controlling the interstrand resistance. When fully charged, adequate margin ensures superconducting operation of the magnet where the conductor margin is found by comparing the operating point to the critical surface. The quench current and field are determined from the intersection of the magnet load line and the superconductor critical surface. Magnetic field calculations are performed with the finite element code MAFCO, and the superconductor critical surface is defined by an empirical relationship. Although no quenches are expected during normal operation, a quench detection circuit and protection resistor are provided so that the temperature rise, estimated from the MIITS generated at a selected discharge voltage, is small ensuring no magnet damage. Joint design, operation, and margin are also described in this paper.