Jan Stroh

PSEUDOSPARK SWITCH DEVELOPMENT AT SIEMENS - COMPACT HIGH POWER SWITCHES FOR GAS DISCHARGE LASERS AND

The development of high average power, high peak power pseudospark switches for use in TE gas discharge lasers and applications requiring similar ratings is reported. The results achieved as of today in metaceramic sealed-off experimental tubes of conventional pseudospark geometry are: hold-off voltage 32 kV; peak current 15 kA; pulse duration 300 ns; current rate of rise > 300 Wp; pulse repetition rate > 250 pps; tum-on delay 200 ns; temporal jitter < 10 ns. These are all simultaneous ratings, with limitations mainly imposed by the power supply and the pulse forming network, respectively. An attractive alternative to spark gaps, with their lifetime being limited to the order of 40 kC, also is the pseudospark switch, which incorporates the advantages of both thyratron and spark gap high dI/dt, high reverse current capability, high charge transfer capability, long lifetime, low jitter, cold-cathode operation without having the correspondmg disadvantages. At SIEMENS, a pseudospark switch the CAVATRON has been developed in order to replace a commercial spark gap in the SIEMENS LITHOSTAR kidneystone hsintegrator. Typical operating parameters (simultaneous ratings) are: hold-off voltage 22 kV; current rate of rise > 10 kA/p; peak current 10 kA; peak reverse current 5 kA; pulse duration 3.5 ps; pulse repetition rate 5 5 pps. The total lifetime amounts to 1.5*105 A*s of transfered charge, as compared to 4*1

The characteristics of pulsed hollow cathode discharges used for pseudospark switch triggering

As for comparable spark gaps. At a repetition rate of 2 pps and dc charging conditions, the prefiring rate of the three-electrode CAVATRON is around

The thermal behavior of tantalum carbide cathodes in pseudospark switches

The complex interaction between the trigger discharge and the main switch discharge in high-power gas discharge switches influences both the switching characteristics, and the switch and trigger lifetime. Any attempts to improve either of these parameters has to take into account the pressure and geometry dependence of a particular trigger geometry. Yet, although not very intensely investigated in detail for this particular purpose, pulsed hollow cathode discharges are commonly used for low-pressure gas discharge triggering as in pseudospark switches. Measurements of the electron current flowing to the cathode backplane of a pseudospark switch from the pulsed hollow cathode trigger discharge show that maximum current densities are peaked around the symmetry axis of the trigger electrode, an effect which is more pronounced at low pressures. Delayed (and slowed-down) increase of the current density at larger radii leads to increasing delay and jitter, provided the trigger coupling holes in the cathode backplane are located off-axis. The electron current density increases with decreasing diameter of the trigger electrode, and with increasing pressure of the working gas. In addition, it is shown that a preionization (keep-alive) current in the trigger electrode region shows a distinct influence on the trigger current distribution, proving that there exists an optimum keep-alive current depending on the geometry and gas pressure. >

Arc quenching arrangement for a gas flow circuit breaker

Pseudospark cathodes made of either tungsten, tungsten with an addition of rhenium (WRe26), or tungsten with inclusions of tantalum carbide grains (WRe3TaC30) were investigated by laser-induced fluorescence on metal vapors and by measurements of the cathode power loss. Excitation temperatures of tungsten vapor are considerably higher (T/sub ex//spl les/8300 K) for the purely metallic cathodes than for WRe3TaC30 (T/sub ex//spl les/4000 K). Furthermore, the increase of the excitation temperature with the current amplitude, dT/sub ex//di, is about ten times less for WRe3TaC30 than for the other materials. Similarly, the cathode surface temperature T/sub s/ estimated from a comparison of the ratio of vapor densities of tungsten and rhenium with the ratio of their equilibrium vapor pressures is much higher (T/sub s//spl ap/7000 K) for WRe26 than for WRe3TaC30 (T/sub s//spl ap/4500 K). Finally, the power loss to the cathode is about equal for the tungsten and WRe26 cathodes (e.g., 200 W at a repetition rate of 230 Hz and 9-kA peak current), but it is 30% less for the tantalum carbide cathode, irrespective of the pulse repetition rate. Consequently, due to their lower power dissipation, WRe3TaC30 cathodes can be used for much higher currents than the purely metallic cathodes. The physical reason for the favorable thermal behavior of the WRe3TaC30 cathode is attributed to the fact that the electrical conductivity of tantalum carbide increases with increasing temperature. >