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

Summary form only given, as follows. 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 classical capacitor discharge type generators which, combined with a resistive free-wheel diode circuit, deliver a critically damped half-sine current with a rise-time of 25/spl mu/s. Each generator consists of two units of 25 kA, connected in parallel to a magnet via a low inductance transmission line. The maximum operating voltage is 12 kV, the minimum pulse repetition rate is 5 s and the dissipated energy per switch is about 150 J per discharge at 12 kV. Originally the capacitor discharge units were equipped with water-cooled high-density graphite anode ignitrons, type National NL488A. These ignitrons have a height of 50 cm, a diameter of 14 cm and a peak anode voltage of 25 kV. Unfortunately ignitron switches suffer sporadically from self firing. In addition they present a serious danger of environmental pollution in the 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 coaxial stack of four Fast High Current Thyristor devices with snubber capacitors, a voltage divider and a specially designed trigger transformer. Two switches have successfully operated in the SPS for more than a year. As a result all remaining ignitrons have been replaced recently. The paper gives a description of the circuit and the construction of the switches, together with simulation and operational results, which were found to be in good agreement.