The Design of a RapidDischarge Varistor System for the MICE Magnet Circuits

Abstract

MICE Note-208 The Design of a Rapid Discharge Varistor System For the MICE Magnet Circuits Michael A. Green Lawrence Berkeley Laboratory, Berkeley CA 94720, USA 23 July 2008* Abstract The need for a magnet circuit discharge system, in order to protect the magnet HTS leads during a power failure, has been discussed in recent MICE reports [1], [2]. In order to rapidly discharge a magnet, one has to put enough resistance across the lead. The resistance in this case is varistor that is put across the magnet in the event of a power outage. The resistance consists of several diodes, which act as constant voltage resistors and the resistance of the cables connecting the magnets in the circuit to each other and to the power supply. In order for the rapid discharge system to work without quenching the magnets, the voltage across the magnets must be low enough so that the diodes in the quench protection circuit don’t fire and cause the magnet current to bypass the superconducting coils. It is proposed that six rapid discharge varistors be installed across the three magnet circuits the power the tracker solenoids, which are connected in series. The focusing magnets, which are also connected in series would have three varisitors (one for each magnet). The coupling magnets would have a varistor for each magnet. The peak voltage that is allowed per varistor depends on the number of quench protection diodes that make up the quench protection circuit for each magnet coil circuit. It is proposed that the varistors be water cooled as the magnet circuits are being discharged through them. The water cooling circuit can be supplied with tap water. The tap water flows only when the varistor temperature reaches a temperature of 45 C. TABLE OF CONTENTS Abstract The Basic Design Parameters for the Varistor Circuits Rapid Discharge Varistor Systems for the MICE Tracker Magnets Rapid Discharge Varistor Systems for the MICE Coupling Magnets Rapid Discharge Varistor Systems for the MICE Focusing Magnets The effect of the Rapid Discharge System on the Magnet Power Cables Some Concluding Comments Acknowledgements References * Last revision 31 August 2008 Page