Experimental Examples of Quench Protection With Varistors to Reduce Quench Voltages and Hot-Spot Peak Temperatures

Superconducting magnet protection must address two main areas of the magnet and circuit performance, namely conductor hot-spot temperatures and circuit voltages. The maximum hot-spot temperatures and voltages occur during the superconductor's transition called a quench and the subsequent energy extraction. As has been previously demonstrated, by using a varistor more energy can be extracted from the coil for the same maximum voltage, or the peak voltage can be reduced compared to a resistor for comparable extraction times. The principles of operation and theory behind the varistor are covered and three case studies are presented. Case 1 presents experimental data on energy extraction with varistors for the CERN Hi-Lumi MCBRD magnets under test at IMPCAS. Case 2 presents experimental data on the FECR sextupole magnet, also under test at IMPCAS. Case 3 covers simulated data for the CERN Hi-Lumi MBXF magnets, under test at KEK.