M.I. Lomaev

Supershort Avalanche Electron Beams in Discharges in Air and Other Gases at High Pressure

In this paper, the generation of supershort avalanche electron beams (SAEBs) from the plasmas of nanosecond diffuse discharges in air and other gases at atmospheric pressure was investigated. It is shown that, in the mode of SAEB generation, the plasma in the discharge gap with an inhomogeneous electric field can be produced in a time no greater than 100 ps with the charged-particle density sufficient to force out the electric field from the discharge-gap region occupied by the dense plasma. It is demonstrated that decreasing the voltage pulsewidth in the discharge gap (to ~100-ps FWHM) reduces the optimum gap for SAEB generation. It is also found that the difference in electron path toward the foil center and the foil edge affects the SAEB current pulsewidth. For lower voltages across the gap (~25 kV), the SAEB pulsewidth at half maximum is ~200 ps.

High-pressure runaway-electron-preionized diffuse discharges in a nonuniform electric field

High-pressure nanosecond diffuse (volume) discharges in a nonuniform electric field are studied experimentally using a recording system with a ?100-ps time resolution. As the voltage pulse shrinks to a width of ≈100 ps, the initiation of a diffuse discharge without a source of additional ionization is facilitated; specifically, a runaway-electron-preionized diffuse discharge is ignited in atmospheric-pressure air in the case of short interelectrode gaps. It is found that a major energy deposit into the plasma of this discharge is from an abnormal glow discharge following a maximum of the gap voltage.

Effect of gas pressure on amplitude and duration of electron beam current in a gas-filled diode

The parameters of an electron beam generated in helium in the pressure range p = 10−4−12 atm are studied. Nanosecond high-voltage pulses are applied to a gap between a tubular cathode and planar anode, which is made of 45-μm-thick AlBe foil. Behind the anode, an electron beam is detected at a helium pressure of 12 atm. The pressure dependence of the beam current amplitude shows three peaks at p ≈ 0.01, ≈ 0.07, and ≈ 3 atm. The beam-induced glow of a luminescent film placed behind the foil and the discharge glow at different helium pressures in the gas-filled diode are photographed.

Theoretical simulation of the picosecond runaway-electron beam in coaxial diode filled with SF6 at atmospheric pressure

This paper presents detailed results of gas discharge theoretical simulation and the explanation of probabilistic mechanism of fast-electrons generation. Within the framework of a hybrid mathematical model, the hydrodynamic and the kinetic approaches are used simultaneously in order to describe the dynamics of different components of a low-temperature discharge plasma. The breakdown of a coaxial diode occurs in the form of a dense plasma region expanding from the cathode. On this background there is a formation of runaway electrons that are initiated by the ensemble of plasma electrons generated in the region of locally enhanced electric field within the front of the dense plasma. It is shown that the power spectrum of fast electrons in the discharge contains the group of electrons with the so-called anomalous energies. Comparison of the calculation results with the existent experimental data gives a good agreement for all major process parameters.

Emission characteristics of a pulsed discharge in xenon

Emission characteristics of a high-current pulsed discharge in xenon are studied experimentally. The study is aimed at developing a source of spontaneous UV radiation (with λ ≤ 250 nm) for controlling highvoltage crystalline diamond switches.

Blue and green jets in laboratory discharges initiated by runaway electrons

Spectral and amplitude-temporal characteristics of plasma radiation of nanosecond pulse-periodic discharge in air, nitrogen and argon in pressure range of 30-760 Torr were investigated. Discharge gap geometry was a point-to-plane. Voltage pulses of negative polarity (amplitude, FWHM and risetime was 13 kV, 10 ns and 4 ns, respectively) were applied to a pointed cathode made of different metals (stainless steel, aluminum and copper). Jets of different colour were observed near a cathode tip. They are formed due to explosive emission. Colour of jets depends on the cathode material. Intense lines of the atoms and ions of iron in the wavelength range of 200-600 nm, aluminum with λ = 394.4, 396.15 nm, including multiply charged ion Al VI with λ = 360.39 and 361.65 nm, copper with λ = 324.8, 327.3, 510.6, 515.3, 521.8, 522 nm were registered. The resonance energy transfer from metastable A3Π+u level of nitrogen molecule to 3d104p level of Cu I was found. As a result a luminescence duration of Cu I was about 1.5 μs at duration of discharge current of 1.5 ps. During constriction of the diffuse discharge the sputtering of material occurs in the direction perpendicular to the longitudinal axis of the discharge gap.

Ultraviolet And Infrared Lasers With High Efficiency

repetition rate and a pulse width of 10 ns. The input pulse propagates through a poLarizer P,, a Faraday rotator and a polarizer P,. The pulse then passes through the slab amplifier eight times and is extracted by using polarization rotation. The Nd: YAG slab (4 mm X 8 mm X 35 mm) is pumped from both sides by four quasiCW (200 k s ) 300W LD arrays at 50 Hz repetition rate. The LD arrays were fabricated by Hamamatsu Photonics K.K. The slab is cut for normal incidence to eliminate polarization dependent loss and the input faces (four in total) are AR coated at 1064 nm. The thermal birefringence in the slab is compensated by means of a 90 degree quartz rotator. Figure 2 shows energy extraction efficiency of the amplifier plotted as a function of the initial small signal gain. From Fig. 2, it is seen that the extraction efficiency increased after thermal birefringence compensation especially at higher small signal gain values. A maximum extraction efficiency of up to 58% was obtained at a small signal gain of 2.01. The stored energy was estimated to be 19 mJ while the output energy of the 8-pass amplifier was 13 mJ at the maximum energy of the LD arrays. The successful operation of this system demonstrates that it is applicable and scalable to the design of higher power laser systems with high efficiency and high beam quality. *Central Research Laboratory, Hamamatsu Photonics K.K. 5000, Hirakuchi, Hamakita, Shizuoka 434, JAPAN

The physical nature of electrons with “anomalous” energies in fast atmospheric discharges

This paper deals with the phenomenon of runaway electrons generation in fast discharges under atmospheric pressures. For the first time we explain the existence of runaway electrons having so-called “anomalous” energies (above the applied voltage value) according to physical kinetics principles. The model we consider provides comprehensive information on the formation and development of runaway electron beams with respect to different initial experimental conditions. The numerical results we obtain fit the existing experimental data for discharges in air.

Parameters of runaway electron beam generated during excitation by nanosecond voltage pulses in short gaps filled with nitrogen

Parameters of a supershort avalanche electron beam (SAEB) in nitrogen excited by the triangular-shaped voltage pulses with 45kV amplitude in incident wave and full width at half-maximum (FWHM) of ~ 1 ns were investigated. In experiments, cylindrical-shaped cathodes made of aluminum and stainless steel were used. An interelectrode distance in the gas-filled diode was 3, 5, and 8 mm. It was established that the highest values of SAEBs current were registered with an aluminum cathode. It was shown that, in contrast to the case of 8-mm gap length, when the interelectrode distances were 3 and 5 mm, the amplitude of SAEB current pulse in nitrogen began to decrease with high values of the pressure – 100 and 50 kPa, respectively.

Efficient gas lasers pumped by run-away electron preionized diffuse discharge

It was shown that run-away electron preionized volume (diffuse) discharge (REP DD) can be used as an excitation source of active gas mixtures at elevated pressures and can produce laser emission. We report experimental and calculated results of application of the REP DD for excitation of different active gas mixtures. It was shown that the REP DD allows to obtain efficient lasing stimulated radiation in the IR, visible and UV spectral ranges. Kinetic model of the REP DD in mixtures of nitrogen with SF6 is developed allowing to predict the radiation parameters of nitrogen laser at 337.1 nm. Promising prospects of REP DD employment for exciting a series of gas lasers was demonstrated. Lasing was obtained on molecules N2, HF, and DF with the efficiency close to the limiting value. It was established that the REP DD is most efficient for pumping lasers with the mixtures comprising electro-negative gases.