Progress in enhanced triggering and increasing of hold-off voltage capability with pseudospark plasma switches

Precise triggering is one of the most important issues with high power switches. The most prominent and characteristic feature of pseudospark plasma switches is their hollow-cathode geometry into which all different trigger units are integrated. This offers the unique advantage to have easy access to the trigger and to have the system outside the main discharge volume. As consequence the trigger is protected against damage from impact by the discharge plasma. The basic mode of triggering in this configurations relies on the production of a dense trigger plasma by the hollow-cathode effect. This plasma provides a sufficient dense electron flux through the cathode bore into the main gap. This paper reports on two novel trigger techniques for pseudosparks switches. One is a trigger unit, based upon a modified surface flashover; the other one works by electron emission from high-dielectric materials such as PLZT perovskite ceramics or related materials of similar high dielectric constant. With this trigger scheme pseudospark plasma switches have been operated at pulse repetition rates (PRR) up to 20 Hz and more. A PRR of 1 kHz in burst mode is possible. The cumulative number of trigger discharges is above 10(7) without any significant performance degradation. The surface flashover unit consists of a cylindrical ceramic rod with two contact electrodes. Depending on trigger voltage polarity one electrode turns positive to the grounded hollow cavity. In other words, the cavity starts playing a part of a hollow-anode with respect to this electrode. As consequence a manifold of mini-are discharges directed towards the hollow-anode appear inside the cavity. The hollow-anode plasma itself serves now as an electron source for the main gap. The main advantage of both triggering schemes lie in their simultaneous simplicity and reliability allowing high repetition rates, long lifetime, and the complete absence of keep-alive electrodes and stand-by power consumption. In order to improve hold-off voltage capability two-stage pseudospark switches with an intermediate electrode are under test. The apertures of this electrode are not aligned with the boreholes of the main electrodes. By this slight modification the hold-off voltage reliability could be extended beyond 35 kV compared to 22 kV for a one-stage device.