Critical assessment of multistage pseudospark switches

In repetitive pulsed power there is a strong increasing demand for the development of high voltage pulse forming networks (PFN), which can be operated with high repetition rates and simultaneously with extremely long lifetime. Typical applications for such PFN's are modulators for the next generation of accelerators, pulse generators for flue gas cleaning with electrostatic precipitators, high power gas lasers, accelerators for medical radiography and drivers of high power microwaves. For instance, in the next generation linear collider the traditional thyratron/PFN modulators will no longer meet the new requirements. Multistage thyratrons are available for hold-off voltages up to 240 kV, but their handling is complicated, the reliability poor and the costs are high. To equip the 1600 modulators, which are planned for SLAC's next linear collider, with multistage thyratrons is unaffordable. Solid-state devices in combination with step-up transformers are under discussion, but their performance is not yet satisfactory. Especially the size of the switches will hamper their application in the near future. The pseudospark switch belongs, with the thyratron, to the class of high-voltage, high-current low-pressure gas switches. In contrast to the thyratron, the pseudospark switch is much simpler, consumes less stand-by power, has a compact size and better overall switching parameters. First this paper presents an overview about the most critical design criteria for a multistage pseudospark switch in comparison to conventional multigap thyratrons, mainly focussed on the coupling of the conduction path between the single stages. In the second part, several different approaches to achieve fast transient coupling between the gaps will be discussed with respect to minimizing the overall inductance of the switch. In former experiments, with different multistage switch designs, a two-stage device using a floating intermediate electrode on the one hand and a three-stage configuration, developed for the beam-dump system of LHC at CERN, on the other hand, were tested. Based on the experience with these switches, a 100 kV-switch, which is planned to be integrated into the 100 kV-PFN of the injection/extraction unit of the future GSI (Gesellschaft fuer Schwerionenforschung at Darmstadt) accelerator complex, will be built.