A modular coaxial solid state closing switch has been developed for the physical replacement of a NL488A high density graphite anode, water cooled ignitron in the horizontal sweeper generator of the beam dump system of the CERN SPS accelerator. The main reasons for the development in this application were environmental, low maintenance and exclusion of erratic firings. Two prototype switches have been made for a hold-off voltage of 12 kV. They consist each of a stack of 4 asymmetrical Fast High Current Thyristors together with capacitive over-voltage protections and resistive dividers. A specially designed trigger transformer allows the reuse of the original ignitron trigger unit without modification. The switches conduct a 45 ms half sine wave peak current of 25 kA at an initial rate of 4 kA/ms. Their losses are about 150 Joules at 12 kV. The minimum repetition rate is 5 seconds. The two switches have successfully operated for one year and the series is now under installation.
A similar switch has been designed for the LHC beam extraction system to prevent erratic firings. It consists of 10 series connected asymmetrical FHCTs and its total stray inductance is less than 150 nH. It has a hold-off voltage of 30 kV and switches a peak current of 20 kA in less than 2.7 ms. The 10:1 trigger transformer has a 5 mH stray inductance, which allows, at 2kV primary trigger voltage, a peak gate current of 250 A with a rate of rise of 400 A/ms. This switch has been tested over several years and has already demonstrated some 200.000 shots without any break-down or erratic firing. The test repetition rate of the switch varies between 8 hours and 20 seconds.
For reliability and standardization reasons, this switch will also be used in the pulse generators for the horizontally and vertically deflecting diluter systems of the LHC beam dumping system. The particularity of the switch for the horizontal system is the fact that its current is an attenuated sinusoidal oscillation of 25 kA amplitude at a frequency of 14 kHz. Tests have shown that, due to the lifetime constant of the charge carriers of about 500 ms, no anti-parallel diodes are required for the reverse current in the FHCTs.
This paper gives a short description of the electrical circuits and test results for these switches.