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.

Automated summary

Superconducting magnet protection needs to address the issues of hot-spot temperatures and circuit voltages. The use of varistors can enable more energy extraction or reduce peak voltage, providing important protection for superconducting magnets. This article covers the principles of varistors and presents three case studies.