Multigap pseudospark switch for fair

Abstract

At the GSI Helmholtzzentrum fuer Schwerionenforschung GmbH a new accelerator complex, called Facility for Antiproton and Ion Research (FAIR), is under construction. Its main components are the SIS 100 and SIS300 heavy ion synchrotrons. To operate their injection/extraction kicker magnet systems, modulators with pulse-forming networks (PFNs) are necessary. The PFNs will be charged to a high voltage up to 70 kV and discharged via a high-voltage switch. The switch has to handle currents up to 6 kA, pulse durations up to 7 microseconds with an overall lifetime exceeding 108 shots. The repetition rate is about 4 Hz and a current rise rate of at least 4*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> A/s is required. The only commercially available switch in this parameter range is actually a multi-gap thyratron. As an alternative, a three-gap pseudospark switch is under development at GSI. It combines the major advantages of the thyratron with its low stand-by power as a cold-cathode device, as well as its insensitivity to large current reversal. Like for the thyratron, the maximum hold-off voltage of a single gap pseudospark switch is limited to about 35 kV. For a reliable hold-off voltage of 70 kV, a three-gap system was designed. Test results with a first prototype switch of this design are reported. The prototype has demonstrated a voltage hold-off capability of more than 80 kV. The circuit of capacitive and resistive voltage dividers was optimized to improve the switch control, the delay and the jitter values. As trigger unit, a conventional high-dielectric trigger is used. With such a trigger unit a crucial issue for minimum delay breakdown still remains the plasma coupling between the different gaps by drift spaces. Those drift spaces have to be designed carefully in order to minimize the internal delay of breakdown. An additional major issue is that the switch suffers from losses, which principally limit the lifetime of low-pressure gas discharge switches. A common way to minimize losses by anode dissipation is to integrate a so-called anode inductor. To see whether this technique can be used with a cold-cathode switch at low repetition rates and relatively large pulse lengths, preliminary tests with an anode inductor were performed.