R. Tkotz

Pseudospark switches-technological aspects and application

We report results of the development of fast closing switches, so-called pseudospark switches, at Erlangen University. Two different parameter regimes are under investigation: medium power switches (32 kV anode voltage, 30 kA anode current and 0.02 C charge transfer per shot) for pulsed gas discharge lasers and high power switches (30 kV anode voltage, 400 kA anode current and 3.4 C charge transfer per shot) for high current applications. The lifetime of these switches is determined by erosion of the cathode. The total charge transfer of devices with one discharge channel is about 220 kC for the medium and 27 kC for the high power switch. At currents exceeding 45 kA a sudden increase in erosion rate was observed. Multichannel devices are suited to increase lifetime as the current per channel can be reduced. Successful experiments with radial and coaxial arrangements of the discharge channels were performed. In these systems the discharge channels move due to magnetic forces. A skilful use of this phenomena will result in a considerably increase of switch lifetime. Multigap devices enable an increase of anode voltage. A three gap switch has run reliably at an anode voltage of 70 kV. >

Investigation of the different discharge mechanisms in pseudospark discharges

The intention of this paper is to give an overview of recent experiments explaining the development and transition of the discharge phases in a pseudospark. The reported experiments include single gap pseudospark discharges in ultra-high-vacuum systems with hydrogen as working gas, as well as multigap pseudospark discharges in argon. Temporally and spatially resolved framing photography, spectrometry, raster electron microscopy and time resolved electrical measurements are presented. The experiments comprise a current range of some hundreds of amps to 60 kA. The results are used to specify the four characteristic phases of the pseudospark: Townsend-, hollow cathode-, high current- and metal vapor arc phase. >

Trigger devices for pseudospark switches

Effective triggering of pseudospark switches with long lifetime and low jitter remains an important problem. This paper presents results of investigations of trigger methods for pseudospark switches. based upon pulsed glow discharges in planar and hollow electrode geometry for charge injection. The influence of different wiring and geometries of the electrodes for preionization is investigated. The effect of additional blocking potentials in the hollow cathode to improve different trigger systems was measured. Calculations of the static potential in the hollow cathode with or without blocking potential are compared with parameters of the discharge. >

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.

Z-pinch current enhancement by the inverse skin effect

The dynamics of a linear z-pinch discharge are studied. The magnetic field distribution inside the discharge tube was measured by small magnetic probes. It was found that after the maximum contraction of the plasma column, current loops are generated in the vessel by which the pinch current is largely enhanced. This effect can be attributed to the inverse skin effect. The purpose of these experiments is to utilize the azimuthal magnetic field of z-pinch discharge to focus (plasma lens) high-energy antiprotons at CERN. >

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. >

Triggering of radial multichannel pseudospark switches by a pulsed hollow cathode discharge

We report on a special trigger discharge for pulsed high-power pseudospark switches. The switch used is a radial three-channel pseudospark switch. For triggering, a cylindrical trigger electrode is inserted into the hollow cathode of the main gap. This electrode acts as a hollow cathode for the dc preionization, while the hollow cathode of the main gap is the anode. A negative high-voltage pulse supplied to the trigger electrode ignites the main discharge. We report the temporal evolution of the trigger discharge observed with a fast camera. This trigger method gives an excellent current distribution among the discharge channels, as can be proven by fast photography. The switch has a delay of 220 ns and a jitter of 15 ns.

Electrode phenomena and measurements of the erosion rate in high-current pseudospark-switches

For microsecond pulses and peak currents up to 105 kA, the erosion rate, electrode profile, and electrode surface of molybdenum electrodes were investigated. The direction of electrode material transport was verified. The discharge behavior of molybdenum electrodes was studied by streak photography. A contraction of the plasma column occurs at the cathode for peak currents above 20 kA. A bright luminous anode flare appears at a peak current of about 40 kA, correlated with an evident anode melting. A further constriction of the anode flare was observed for currents exceeding 90 kA. Opposite to lower currents, an increase in the weight of the cathode was noticed as well.

The plasma in high-current pseudospark switches

The discharge behavior and the erosion rate of pseudospark switches for high currents (50-150 kA) and pulse lengths of several microseconds were investigated for different electrode materials. A capacitor discharge (3.3 /spl mu/F) without any load was used at a maximum voltage of 30 kV. Side-on optical investigations were performed either with a streak camera or a fast shutter camera. Using metal electrodes, the discharge ignites on axis, then widens up radially and burns homogeneously at the edge of the central apertures. After about 500 ns, a stable anode spot is observed on the plane electrode surface (at currents exceeding 45 kA), the location of which is statistical. The discharge is transformed to a metal vapor are discharge and the erosion rate increases by more than one order of magnitude. With semiconductor electrodes (i.e., silicon carbide), a different discharge behavior is observed, After ignition on axis, the discharge burns homogeneously on the whole carbide surface. No contraction to a small area occurs in comparison to metal electrodes. The reignition of later current half cycles starts at the triple point metal-carbide-gas. Then the discharge again spreads homogeneously over the total carbide surface. The erosion rate is about two magnitudes lower in comparison to metals. We assume that the current is conducted in a thin surface sheath which is heated to more than 2000 K.