Slow Current Discharges and Quenching of the MSU Superconducting Cyclotron Gas-Stopper Magnet

The Michigan State University cyclotron gas-stopper magnet generates a strong focusing cyclotron magnetic field that permits an ion beam of up to 100 MeV/u to enter the magnet and have most of its energy reduced by low-pressure helium gas. The magnet has two superconducting coil cryostats mounted within warm iron poles and a warm split iron return yoke. The coils are connected in series using room temperature cables. The peak magnet stored energy is 3.5 MJ. The magnet has been tested to its full operating current of 180 A. During tests, the magnet discharged inadvertently three times. Two low-current discharges caused the magnet current to decay through the power supply system. A discharge at 180 A quenched the magnet, because the quench protection system caused the magnet to discharge across a 1.25-Ω resistor. This fired the cold quench protection diodes within the cryostat, quenching both coils. The magnetic field in the gap was measured as the magnet discharged. For the two low-current discharges, the magnetic field followed the current in the external magnet circuit. During a quench, the magnetic field decay rate was much slower than the external circuit current decay rate.