Coaxial triggered vacum gap : switching and delay characteristics

Abstract

Vacuum is an excellent insulator, albeit has a poorly defined intrinsic electric breakdown strength. Because of this, the ordinary vacuum gap with only two electrodes is unsuitable for switching applications. The electric breakdown depends on many parameters, foremost among them being the surface condition of the electrodes and the presence of the dielectric between the electrodes. The triggered vacuum gap (TVG) is a three?electrode switching device enclosed in a vacuum envelope, in which the breakdown can be precisely initiated at will by applying a commanding trigger pulse to the trigger electrode. Large currents of few tens of kiloamperes, over a wide range of voltages (100 V – 100 kV) may easily be switched. The TVG makes use of the very high breakdown strength of vacuum (though ill?defined) to hold off voltages over a wide range (1 – 100 kV) while exploiting the reduction of the breakdown voltage that occurs in the presence of insulating material by placing this insulating material in the auxiliary gap. Thus highly controlled, sensitive triggering may be obtained. Another great asset is the rapid recovery (ns) of the dielectric strength of the vacuum gap after current interruption, which is unobtainable in the case of gaseous gaps (µs). The exact mechanisms of operation of the TVG are not well understood. Nevertheless, briefly stated, a trigger pulse applied to the trigger electrode starts a discharge over the dielectric surface insulating the trigger electrode from the main cathode and the resulting plasma spreads across to the main gap, thus causing it to break down. TVGs of varying degrees of complexity and sophistication have been described in literature. At the High Voltage Engineering Department of this Institute, investigations have been carried out over the past decade to develop a simple TVG requiring no special materials or processing. So far TVGs having planar type of electrodes have been studied. This thesis contains the investigations on a new type of TVG, having coaxial electrodes, developed by the author and a comparison of its performances (with respect to reliability of firing, sensitivity and other parameters) with the planar type. Studies have been conducted at moderately high voltages (1–20 kV). The past work here shows that the triggering voltage depends on the permittivity of the insulating material used in the auxiliary gap. This has been examined in good detail, taking into consideration the electric field enhancement in the vicinity of the metal/vacuum/dielectric junction. The exceptional behaviour of silicon carbide as compared to the other materials is also discussed, taking into account its special non?ohmic and non?dielectric characteristics. A very important parameter for a switching device is its delay time in turning on. By analogy with gaseous breakdown, this has been analysed into two parts, the statistical and the formative time lags. Using Laue’s technique, the two time lags have been separated. To the best of the knowledge of the author, this has been done for the first time with regard to TVGs. The new coaxial TVG is superior to the planar TVG as regards the trigger sensitivity and the trigger delay. The coaxial TVG enables the application of an axial magnetic field (crossed electric and magnetic fields) for future work. The new TVG constitutes an original contribution to the family of switching devices.