INVESTIGATIONS OF 2-STAGE-PSEUDOSPARK SWITCHES FOR HIGH-CURRENT APPLICATIONS

To avoid the lowering of the holdoff voltage due to the electrode erosion in one stage high current pseudospark switches (PSS), a two stage PSS with no axial aperture in the intermediate electrode was tested. For investigations a pulse generator was used generating peak currents up to 120 kA at a maximum voltage of 30 kV with a period length of 5 mu s of a weakly damped sine wave with 90% current reversal. In comparison with a one stage PSS the breakdown characteristic was shifted to higher pressure. With a free floating intermediate electrode, the device could not be triggered, however, with additional capacities of a few nF between the three electrodes the discharge was ignited. The discharge in the second gap is triggered by the pseudospark discharge in the cathode gap, discharging the auxiliary capacities. Simultaneously, observation of both gaps with fast shutter photography showed an independent movement of the discharges in the two gaps. In the cathode gap as current increases, the discharge moves away from the center to the plane electrode surface as has been observed in the one stage PSS. However, in the anode gap the discharge moves away from the center after a contraction to the center. The two discharges are transitted to metal vapor are type discharges as the erosion patterns prove. With this kind of a two stage PSS holdoff voltages exceeding 35 kV would be possible. The characteristic switch data, i.e., delay and jitter, are nearly equal to a one stage PSS. Steps of 1 mm height on the two electrodes around the central aperture in a one stage PSS led to a centering of the discharge to this raised area around the apertures. With fast shutter photographs of the discharge, the stability of the limitation of the discharge movement was proved; the erosion patterns are limited to the terraced area on the electrodes. The forward voltage drop of the metal vapor are decreases with diminishing the are length. Therefore, the discharge burns on the shortest path as observed with fast shutter photography. With this electrode design the movement of the discharge to the insulator wall could be avoided. Therefore, the protection of the insulator is better and shielding structures can be simplified for long lifetime switches.