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

To avoid the lowering of the holdoff voltage due to the electrode eros ion in one stage high current pseudospark switches (PSS), a two stage PSS with no axial aperture in the intermediate electrode was tested. F or 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 m u s of a weakly damped sine wave with 90% current reversal. In compari son with a one stage PSS the breakdown characteristic was shifted to h igher pressure. With a free floating intermediate electrode, the devic e could not be triggered, however, with additional capacities of a few nF between the three electrodes the discharge was ignited. The discha rge in the second gap is triggered by the pseudospark discharge in the cathode gap, discharging the auxiliary capacities. Simultaneously, ob servation of both gaps with fast shutter photography showed an indepen dent movement of the discharges in the two gaps. In the cathode gap as current increases, the discharge moves away from the center to the pl ane electrode surface as has been observed in the one stage PSS. Howev er, 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 kin d of a two stage PSS holdoff voltages exceeding 35 kV would be possibl e. 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 a round the central aperture in a one stage PSS led to a centering of th e discharge to this raised area around the apertures. With fast shutte r photographs of the discharge, the stability of the limitation of the discharge movement was proved; the erosion patterns are limited to th e terraced area on the electrodes. The forward voltage drop of the met al vapor are decreases with diminishing the are length. Therefore, the discharge burns on the shortest path as observed with fast shutter ph otography. 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 lo ng lifetime switches.