K. Frank

The fundamentals of the pseudospark and its applications

The pseudospark, an axially symmetric gas discharge at low pressures of typically 10 to 100 Pa and high voltages (of some hundred volts up to several hundreds of kilovolts), has become of interest in fast high-power switching, in producing well-pinched high intensity electron or ion beams, and as an X-ray source. The essential features of this type of discharge are described. The results of fast time-resolved spectroscopic investigations and breakdown delay statistics of the pseudospark are discussed. >

High-power pseudospark and BLT switches

A review is presented of recent developments in a new group of high-power hollow-electrode switches, including the pseudospark and the backlighted thyratron (BLT). Experiments demonstrate that for several key high-power switching performance factors, the pseudospark and BLT switches are superior to either high-pressure spark gap switches or thyratrons or, in some cases, both. High performance has been demonstrated in peak current (>100 kA), current rate of rise (>10/sup 12/ A/s), switching precision, trigger efficiency, current reversal (100%), and recovery time. Several electrical and optical trigger methods have been demonstrated and are described. >

Generation of intense pulsed electron beams by the pseudospark discharge

A low-pressure gas discharge is presented as a source of intense pulsed electron beams. The pseudospark discharge emits a short-duration pinched electron beam during the breakdown phase. At voltages of typically 20 kV, approximately 10-20% of the total discharge current appears as the electron-beam current of typically 20 ns in duration. According to the breakdown voltage in the beam, a power density on the order of 10/sup 9/ W/cm/sup 2/ is reached. Thus, this electron beam turns out to be a good tool for material processing, comparable to pulsed high-power lasers. Besides the drilling of holes into metals and insulators, an interesting application is the production of high-temperature superconducting thing YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. The electron beam is used to evaporate material from a stoichiometric 1-2-3 target. Experimental results concerning the propagation behavior in neutral gas, the electron energy distribution, and the interaction with matter are reported. >

Investigations of pulsed surface flashovers for the triggering of pseudospark high-power switches

Surface discharges over insulators in vacuum under rectangular high-voltage pulses are investigated. These discharges are among the methods used to trigger pseudospark switches. Low breakdown voltage and long lifetime are the requirements these insulators have to meet to be used as trigger materials. For this purpose the breakdown behavior of certain ceramics stressed by high-voltage pulses was investigated in an oil-free vacuum of 10/sup -5/ Pa. The erosion of the insulator by the discharge plasma was investigated with a scanning electron microscope. An evaluation of the measurement and a comparison of the properties of the investigated insulators show that SiN, Al/sub 2/O/sub 3/, and CaTiO/sub 3/ have low breakdown voltages and exhibit the lowest erosion rates. First results obtained with these materials as trigger insulators in pseudospark switches are promising with regard to the achieved lifetimes. A lifetime of four million triggered discharges under 90% switch current reversal was reached at a peak switch current of 25 kA without a serious reduction in trigger performance. >

Investigations about triggering of coaxial multichannel pseudospark switches

A fundamental problem of pseudospark switches is erosion in the borehole area. One way to reduce erosion is to distribute the current to several discharge channels. Essential for multichannel operation is a reliable ignition of all these channels. The aim of this work was to find out the requirements for a trigger for multichannel pseudospark switches and to develop a suitable trigger device. The investigations were made with a three channel pseudospark switch. The developed trigger is a pulsed hollow cathode discharge with a 3 mA dc-preionization. A trigger voltage of 4 kV results in a current of about 6 A in the hollow cathode of the trigger-section. This hollow cathode discharge causes a trigger current into the hollow cathodes of the pseudospark chambers. The trigger current which is necessary to ignite an equally distributed discharge has to be at least 3 mA into each main switch hollow cathode. A jitter of 2 ns was achieved for the coaxial multichannel pseudospark switch. >

Spatial and time characteristics of high current, high voltage pseudospark discharges

During the early phase of the discharge (ignition), fast ionization waves are observed propagating with a velocity of 10/sup 6/ m/s from cathode to anode. During this transient phase, a first peak of an energetic electron beam develops. Simultaneously, a moderate radial expansion of the axially concentrated background plasma (produced from beam electrons) is observed, but the plasma parameter remains still smaller than the borehole diameter (equal to 3 mm). The transition into the high current phase is characterized by further continuous radial expansion of background plasma, which is interrupted by a sudden and rapid radial expansion of plasma into the last two or three gaps in front of the anode. One reasonable explanation is based upon a kind of plasma blow-up by the field of the space charge accumulated there. Part of the beam electrons, extracted from the hollow cathode and adjacent gaps, are apparently deflected or even reflected in this high local electric field. Parallel with increasing total current, the internal resistance of the system drops dramatically, synonymous with the energy of the beam electrons, too. Characteristic for the development of the hollow-cathode plasma is a stepwise expansion. The plasma itself develops a hollow structure, and the diameter of it is still larger than the borehole diameter. During the high-current phase, the diameter of this characteristic hollow structure increases rapidly to the wall, indicating the end of the first current half-wave.

Long pulse switching properties of pseudospark switches

Increasing demands on high voltage, high power switches have led to the development of a new kind of low pressure hollow electrode switches called Pseudospark Switches (PSS, electrical triggering methods) and the Back-of-the-cathode Lighted Thyratron (BLT, optical triggering). For high power excimer laser applications, a small series of brazed ceramic metal switches has been manufactured and tested for lifetime and performance. in this contribution, results of tests at very long pulse durations are reported. The very encouraging results suggest that repetitive multi-kilojoule switching with single-channel PSSs is possible at a multi-kiloampere level. Repetition rates of up to 1 kHz seem to be feasible from the experimental results. This makes the PSS a promising candidate for flashlamp pumped high average power solid state lasers for material processing. Switching of high-power flashlamps by PSS seems to be even more interesting due to the observed phenomenon of complete switch recovery during current zero after conducting a full sine wave of 560 /spl mu/sec duration and /spl tilde/6 kA in amplitude.

Investigations of carbide electrodes in high-current pseudospark switches

Semiconducting carbides, SiC and BC, were tested as electrode material in pseudospark switches. Typical parameters of the test circuit were a charging voltage of 20 kV, a peak current up to 120 kA and a pulse length of 5 /spl mu/s (weakly damped sinusoidal pulse shape). In comparison to metal electrodes no differences in the electrical signals were detected. Optical investigations with a fast shutter assumed that the discharge starts on the axis. At later times the discharge plasma is homogeneously distributed over the total carbide surfaces of the electrodes. No cathode spots were observed. The erosion rate is low in comparison to molybdenum electrodes. The lifetime of the switch is enlarged as first tests show, but the holdoff voltage is limited due to field enhancement at the triple point metal, carbide, and gas.

Triggered low-pressure pseudospark-based high power switch

Investigations and operational results of a pseudospark-based high-power switch are reported. The switch is designed for hold-off voltages and peak currents of 40 kV and 25 kA, respectively. From results obtained with various laboratory-type O-ring sealed switches, an improved, fully brazed metal-ceramic version was developed and investigated. The results show the capability of switching pulse energies of up to 40 J at a current rise rate of 8*10/sup 11/ A/s and a current reversal of up to 95%. After several 10/sup 6/ discharges there was no significant change in the operational data of the switch. The long-term behavior of the pseudospark switch is currently under investigation under conditions similar to those in excimer laser circuits. >

An investigation of the temporal development of the pseudospark discharge

A study of the different discharge phases of the pseudospark discharge is presented. During the temporal development several modes of the pseudospark discharge are observed leading to a transient, high current, low pressure gas discharge with current densities of the order of 10/sup 4/A/cm/sup 2/ and a forward voltage drop of about 10/sup 2/ volts. Therefore measurements of the discharge current, the total charge transported, the forward voltage drop and the influence of the gas pressure and LRC circuit, from initial phase of discharge until the transient phase to the superdense glow, are reported. Through comparison of experimental results with literature the different types of discharges are characterized. Optical studies of the radially expanding plasma column enable the temporal and spatial location of the discharge plasma. In the high current phase estimations of the temperature on the cathode surface and the solution of the Schottky-Equation show that a constriction of the ion current at microscopic surface irregularities is necessary to sustain the discharge. >