A.V. Batrakov

The effect of pulsed electron-beam treatment of electrodes on vacuum breakdown

A method for preliminary treatment of electrodes by a microsecond low-energy intense electron beam is proposed. It has been demonstrated that such a beam melts off the electrode surface and cleans the surface layers from impurities and dissolved gases. In combination with subsequent conditioning of the vacuum gap with low-current pulsed discharges, high breakdown electric fields can be attained. >

Electrohydrodynamic phenomena on the explosive-emission liquid-metal cathode

The paper describes the results of a direct experimental observation of the development of an electrohydrodynamic instability at an explosive-emission liquid-metal cathode along with a theoretical analysis of the growth and destruction of the protrusion formed on the cathode surface. >

Increasing the electric strength of vacuum insulation by irradiating the electrodes with a low-energy high-current electron beam

The paper describes how irradiation of the electrode surface with a LEHCEB (low-energy high-current electron beam) affects the prebreakdown current and the electric strength of vacuum insulation. This study is an extension of a study described earlier. Experiments have been performed for 0.1 mm vacuum gaps formed by refractory metal electrodes and for millimeter vacuum gaps with a pulsed voltage of amplitude 250 kV and duration 30 to 100 ns, both in a high oil-free vacuum and in technical-grade oil vacuum. Based on the results obtained, it is stated that the LEHCEB irradiation of electrode surfaces is a promising technique for increasing the electric strength of vacuum insulation over a wide range of voltages and under varied vacuum conditions.

Formation of accelerated electron flows in a low-current pulsed vacuum discharge

Intense short-term electron flows formed under the conditions of a low-current vacuum discharge have been detected in the circuit of a collector placed aside from the discharge axis and being at a significant (up to 1 kV) electron-decelerating potential with respect to the cathode. It has been shown that the generation of these flows is related to the formation of nonstationary double layers between the near-cathode and near-anode plasma regions and to the rapid decrease in the potential of the near-anode plasma on repeated initiation of an arc.

Time resolved resonant laser diagnostics of the low current vacuum arc cathode spot

The resonant laser diagnostics combined with high speed IMACON 468 frame/streak camera was used as a technique for data acquisition. The experiments were performed with an arced vacuum gap formed by the capillary-type liquid-metal cathode and the plane anode in conditions of high oil-free vacuum. The evolution of observable cathode spot plasma is analysed in terms of cyclicity of the cathode spot operation.

Spectroscopy of single vacuum arc cathode spots with improved sensitivity

Recently, we reported on the optical emission of vacuum arc discharges with a single cathode spot. The discharges were initiated by rectangular voltage pulses of 100-ns length produced by a cable generator. In combination with the application of a liquid Ga-In alloy cathode and ultrahigh vacuum conditions, a high reproducibility was obtained. An improvement of the sensitivity and the signal-to-noise ratio was achieved by the replacement of the streak camera by an intensified charge-coupled device camera. Thus, the accumulation of calibrated spectra with high temporal, spatial, and wavelength resolution was possible. Distinct differences in the temporal development of the atomic and ionic line intensities were found depending on the spatial position, that is, comparing the spot core and the spot surrounding.

Dynamics of cathode spot plasma parameters in spark and arc stages of vacuum discharge

Modern notations do not foresee principle differences in cathode spot mechanisms in the breakdown stage as compared with steady burning arc. There is no reason to have doubts in this statement. Really, cathode spots generate similar craters regardless of discharge burning time under the condition of a cold cathode. Cathode spot plasma has rather stable parameters while gradual changes in plasma parameters can be convincingly explained in terms of cathode heating effects. The only difference is that the beginning stage gives a chance to trace a separate cycle of cathode spot operation and confidence that the initial moment of a cycle is a proper one. Unlike stable long-lasting arc, the first spot cycle is not hazy with prehistory; and spot behaviour looks different as a result. Having utilized a first spot cycle for diagnostics, the authors have revealed dynamics of atomic and ionic spectral lines in single spot radiation with high temporal resolution. Higher charge state ions start to emit light first followed by lower charge state ions and neutrals being latest. All the spectral lines start to be highly broadened. These data was successfully used to revise conclusions drawn earlier concerning experiments on resonant laser diagnostics of cathode spot plasma.

Effect of pulsed electron-beam treatment of electrodes on the electric strength of the vacuum insulation

A method for preliminary treatment of electrodes by a microsecond low-energy intense electron beam is proposed. It has been demonstrated that such a beam melts off the electrode surface and cleans the surface layers from impurities and dissolved gases. In combination with subsequent conditioning of the vacuum gap by low-current pulsed discharges, high breakdown electric fields can be attained.

High-speed spectrally-resolved imaging of the laser ablation plasma

Summary form only given. The paper is devoted to the experimental investigation of the laser ablation. Free surface of the liquid phase gallium-indium alloy in vacuum was served as the ablation target. For the ablation, compact Nd-YAG laser pumped by LED was used, generating the irradiation pulse with energy content of 0.28 mJ/pulse at the FWHM duration of 2.7 ns. Method of spectrally-resolved frame imaging was realized for the visualization the luminescence of different plasma species. 4-channel intensified frame camera HSFC-Pro with the minimum exposure time of 3 ns was used. Three channels of the camera were equipped with the narrow-band interference optical filters for the registration the line radiation originated from different gallium components: neutral atoms, single- and double-charged ions. Spectral-spatial structure of plasma glowing was analyzed as well as the dynamics of expansion of plasma including the expansion of differently-charged components. It was demonstrated that the expansion velocity of heavy plasma components increases with the charge state of particles. Quantitatively, the results are in a good agreement with the data of mass-energy analysis performed earlier.

Spectroscopic study of the anode flare formation during the initial stage of vacuum arc discharge

Summary form only given. Generation of an anode flare in the initial stage of vacuum discharge was studied. Discharge gap was formed by a liquid-metal gallium-indium point cathode and a flat anode of the same material, with an electrode gap distance of 0.8-1 mm. The discharge was fed by pulse generator with an impedance of 50 Ohm, generating a rectangular pulse with a duration of 200 ns. Estimates show that with a charging voltage of 15-25 kV and a current amplitude of 150-250 A we can expect that power density at the anode surface could be as high as 108 W/cm2.Using a spectrometer combined with a intensified CCD camera, spatial and spatio-spectral pattern of plasma emission from the discharge gap were registered with a temporal resolution of 10 ns. Generation of the anode flare begins with the intensive evaporation of anode material in the spark (high-voltage) stage of the discharge. In this period, only lines of neutral atoms are present in the spectrum measured at anode position although higher charge states can be found in the cathode spectra. Intensive ionization of anode flare atoms starts when the discharge changes from the spark to an arc (low-voltage) stage. Lines of single and double charged ions consistently appear in the spectrum (lines of ions with a higher charge state were not available for registration). At the subsequent expansion of anode flare, the highest expansion velocity was observed for double charged ion lines, and the lowest one was found for neutral atoms. Expansion velocity was increasing with increasing a voltage and current (enhancement of the input energy). The mechanisms of evaporation, ionization, ion acceleration, and the role of the anode flare in the discharge are discussed.