High power semiconductor switches in the 12 kV, 50 kA pulse generator of the SPS beam dump kicker system

Horizontal deflection of the beam in the dump kicker system of the CERN SPS accelerator is obtained with a series of fast pulsed magnets. The high current pulses of 50 kA per magnet are generated with capacitor discharge type generators which, combined with a resistive freewheel diode circuit, deliver a critically damped half-sine current with a rise-time of 25 /spl mu/s. Each generator consists of two 25 kA units, connected in parallel to a magnet via a low inductance transmission line. Originally the pulse generators were equipped with ignitron switches. Unfortunately ignitron switches suffer sporadically from self-firing. In addition these mercury filled devices present a serious danger of environmental pollution in case of an accident. Since the development of new power semiconductor devices now offers an alternative, replacement of the ignitron switches was decided. A suitable semiconductor switch was developed with the same dimensions as the ignitron and compatible with its trigger unit. It consists of a stack of four Fast High Current Thyristor (FHCT) devices with snubber capacitors, a voltage divider and a specially designed trigger transformer. This paper gives a description of the circuit and the construction of the switches, together with measurement results, which were found to be in good agreement with simulation results.