Pseudospark Switches (pss) for Pulsed Power Applications

Basing on the pseudospark discharge, a low pressure gas discharge in a special geometry, fast closing switches for different pulsed power applications have been designed. Medium power PSS (< 30 kA peak current) were used in laser circuits whereas high current PSS are tested successfully in high current pulsed power applications (< 200 kA). For currents of a few kA the discharge is supported by cathode spots on the cold cathode surface. For higher currents anode activity is observed too. Inserting semiconductor material seems not only to suppress high erosive spot formation but support diffuse large-area electrode emission. A different approach to solve the problem of lowering the erosion rate is the multichannel PSS (MUPS). In order to distribute the discharge current to more than one single channel three or more discharge channels are radial or coaxial arranged. With regard to high voltage applications the maximum hold-off voltage was increased by adding an intermediate electrode. INTRODUCTION The pseudospark discharge is a low pressure gas discharge on the left hand side of a breakdown characteristic comparable to the paschen curve [1]. The pseudospark is based on a special geometry shown in figure 1. In this basic set-up electrodes with a central aperture are separated a few millimetres by an insulator. The back of the electrodes are arranged as hollow electrodes. The PSS can be run in the self breakdown or triggered mode. In the last one the discharge is initiated by the injection of electrons into the hollow cathode due to a surface flashover [2] or a pulsed glow discharge behind the hollow cathode [3], but also other trigger methods like ferroelectric trigger [4] or triggering with an UV-light pulse [5] are used. The reliable trigger ability with a low jitter, the fast current rise up to 10 A/s and the 100% current reversal led to the application as high power switch. Different discharge modes must be distinguished during the discharge development [6]. In the selfbreakdown mode the discharge starts as a Townsend-discharge on the axis of the discharge chamber. With increasing current a positive space charge builds up near the cathode bore hole which leads to field enhancement in the hollow cathode. At a critical positive space charge density the breakdown of the hollow cathode is initiated. In the triggered mode the critical space charge density develops due to the trigger discharge. As soon as the hollow cathode discharge breaks down the voltage over the gap drops for a first time and the current rises up to several hundreds of ampere. After this phase a further drop of the anode voltage can be observed depending on the discharge parameters. In this phase the discharge is located at the inside of the cathode bore hole. The mechanism of this mode is still under discussion [7,8]. After this the discharge turns to the high current mode correlated with a further decrease of the voltage