Varistors: A Useful Tool for Superconducting Magnet Quench Protection

Abstract

A perfect varistor is a constant voltage resistor as a function of current through the resistor. A perfect varistor can't exist, because its resistance would be infinite at zero current. In electronic devices, varistors are often used as surge protectors. A varistor can be used in place of an ordinary resistor as the discharge resistance across superconducting coil sections. When used this way, the integral of I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> dt during the discharge can be reduced by almost one third. When a varistor is used for quench protection of coils that are coupled to shorted secondary circuits the integral of I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> dt can be reduced much more. This can be a key factor when coil current densities and stored energies are high as in many potential HTS magnet systems. This paper discusses the types of varistors that are commercially available and their characteristics.