Radial inhomogenity of plasma parameters in a low-pressure inductive RF discharge

This work is devoted to systematic investigation into the radial dependence of the plasma parameters of a low-pressure inductive radio-frequency (RF) discharge on pressure within a wide range of 0.8–1 Torr. Experimental results that were obtained under the considered pressures make it possible to analyze the patterns of the changes in plasma parameters upon both a nonlocal mode of discharge and a transition from a nonlocal to local mode of the RF power input. Discharges in helium, neon, argon, and krypton were considered. Experimental data were compared to the results of the numerical simulation of the inductive RF discharge using the particle-in-cell (PIC) method.     

Generation of runaway electrons in a nonuniform electric field by applying nanosecond voltage pulses with a frequency of 100–1000 Hz

Generation of runaway electrons and X-ray radiation in helium and air under the action of a pulsed-periodic discharge in a nonuniform electric field is studied. Positive and negative voltage pulses with a repetition rate of up to 1 kHz, a duration on the order of 1 ns, and an incident wave amplitude of 12.5 kV are applied to a needle-plane gap. For both polarities of the main voltage pulse and a helium pressure from several Torr to several tens of Torr, the arrival of negative reflected voltage pulses at the gap is shown to be accompanied by an electron beam generation. X-ray radiation is detected in a wide range of pressure, including under normal pressure of helium and air.     

Optical characteristics of the plasma of a nanosecond atmospheric-pressure volume discharge in a nonuniform electric field

Optical characteristics of the plasma of nanosecond volume discharges in air, nitrogen, krypton, argon, neon, and Ar/N₂ and Ar/Xe mixtures at elevated pressures are investigated. The discharges are excited in a gap with a cathode of small curvature radius. The waveforms and spectra of plasma emission from discharges in different gases in the 230-to 600-nm spectral range are measured. Optical generation in an Ar/Xe mixture is achieved at an active length of 1.5 cm. A comparison is performed of the spectral characteristics of the emission from nitrogen, krypton, argon, and neon excited by a volume discharge in a nonuniform electric field, by a nanosecond electron beam, and by a pulsed volume discharge in a uniform electric field at a high initial voltage.     

Transients and efficient generation of electron beams in the pulsed wide-aperture glow discharge

The quasi-continuous wide-aperture glow discharge in helium at pressures from 1.2 to 6.0 Torr is studied. It is found that electron beam generation efficiency η is higher than 96% in the pressure range 1.2–3.0 Torr at voltages from 1.0 to 2.6 kV. The maximum power was ≈ 0 kW at 6 Torr and a voltage across the discharge gap of 2.6 kV. Under these conditions, the beam generation efficiency is about 80%. The pressure and voltage dependences of main parameters of the discharge are explained from the standpoint of its photoemission nature.     

Pulsed discharge in nitrogen and argon under an elevated pressure in a nonuniform electric field

The characteristics of a discharge and radiation in nitrogen and argon under pressures of 10–760 Torr and the discharge formation without pre-ionization of the gap from an auxiliary source are considered. A peak is detected on the pressure dependence of the radiation power of the second positive system of nitrogen for E ₀/p ～ 270 V/cm Torr and nitrogen pressure p ～ 70 Torr. In the pressure range 10–760 Torr and for a voltage pulse leading front duration of ～ 10 ns, an electron beam is formed behind the grid anode with various half-amplitude pulse durations. It is shown that, under the given conditions, the electron beam is formed at the voltage pulse front both in the case of a discharge gap breakdown and in the absence of a clearly manifested breakdown, as well as for a 10-ns delay of breakdown at the leading front of a discharge current pulse.     

Generation of dual pulses of the runaway electron beam current during the subnanosecond breakdown of atomic and molecular gases

With a diaphragm placed behind the anode foil, dual runaway electron beams have been provided in helium, hydrogen, nitrogen, and air under a pressure of several torrs to several dozen torrs and a high-voltage pulse amplitude of about 250 kV. These beams consist of two pulses with commensurable amplitudes with a time interval between them of several dozen picoseconds to several hundred picoseconds. It has been shown that the breakdown of the interelectrode gap at pressures from several torrs to several dozen torrs may occur in different regimes and dual pulses of the electron beam current are registered when the initial current through the gap is below 1 kA. It has been found that a supershort avalanche electron beam that consists of one pulse is generated when the delay of breakdown equals several hundred picoseconds. It has been shown that, when the gas pressure reaches several hundred Torr, including atmospheric pressure, the runaway electrons are detected behind the foil after the termination of the supershort avalanche electron beam pulse.     

Emissivity of the pulsed capacitive discharge in helium-iodine and neon-iodine mixtures

The emission parameters of a pulsed capacitive discharge initiated in helium-iodine and neon-iodine mixtures are reported. The discharge plasma emits at wavelengths of 183.0 and 206.2 nm, which correspond to iodine atom spectral lines. The capacitive discharge is initiated in a cylindrical quartz tube with an electrode distance of 10 cm. The discharge radiation is optimized in exciting pulse repetition rate and helium and neon pressures in He(Ne)-I₂ mixtures. The optimal pressures of helium, neon, and iodine vapor fall into the ranges 0.8–2.0 kPa, 0.5–1.0 Pa, and ≤60 Pa, respectively.     

Effect of the parameters of the plasma channel produced by a high-current relativistic electron beam in dense gaseous media on the beam transportation stability

Results are presented from experimental studies of the time evolution of the plasma channel produced by a high-current electron beam (with an electron energy of E _e = 1.1 MeV, a beam current of I _b = 24 kA, and a pulse duration of t = 60 ns) in helium, nitrogen, neon, air, argon, krypton, xenon, and humid air (air: H₂O) at pressures from 1 to 760 Torr. It is shown that, in gases characterized by a small ratio of the collision frequency to the gas ionization rate u _i , the electron beam produces a broad high-conductivity plasma channel, such that R _b/R _p < 1, where R _b and R _p are the beam and channel radii, respectively. As a result, large-scale resistive hose instability is suppressed.     

Possible application of a volume avalanche discharge initiated by an electron beam for designing a krypton dimer laser

The emissive and amplifying properties of volume nanosecond discharge plasma formed at high pressures in krypton have been investigated theoretically and experimentally. It is theoretically shown that lasing can be obtained in this type of discharge at krypton pressures exceeding 6–7 atm. In the discharge gap with a cathode of small curvature radius, volume discharge without preliminary ionization is obtained at a high pressure and intense krypton dimer emission that peaks at a wavelength of 146 nm is detected. It is shown that, upon excitation by a volume avalanche discharge initiated by an electron beam, no less than 90% of the energy in the range 120–540 nm is emitted by krypton dimers. At a krypton pressure of 1.5 atm, an energy of ～30 mJ of spontaneous emission into the total solid angle and a radiation pulse width at half maximum of ～240 ns are obtained.     

Effect of a transverse magnetic field on the generation of electron beams in the gas-filled diode

The effect of a transverse magnetic field (0.080 and 0.016 T) on generation of an electron beam in the gas-filled diode is experimentally investigated. It is shown that, at voltage U = 25 kV across the diode and a low helium pressure (45 Torr), the transverse magnetic field influences the beam current amplitude behind a foil and its distribution over the foil cross section. At elevated pressures and under the conditions of ultrashort avalanche electron beam formation in helium, nitrogen, and air, the transverse magnetic field (0.080 and 0.016 T) has a minor effect on the amplitude and duration of the beam behind the foil. It is established that, when the voltage of the pulse generator reaches several hundreds of kilovolts, some runaway electrons (including the electrons from the discharge plasma near the cathode) are incident on the side walls of the diode.     

Direct observation of atomic diffusion in warm rubidium ensembles

We present a robust method for measuring diffusion coefficients of warm atoms in buffer gases. Using optical pumping, we manipulate the atomic spin in a thin cylinder inside the cell. Then, we observe the spatial spread of optically pumped atoms in time using a camera, which allows us to determine the diffusion coefficient. As an example, we demonstrate measurements of diffusion coefficients of rubidium in neon, krypton and xenon acting as buffer gases. We have determined the normalized (273 K, 760 Torr) diffusion coefficients to be 0.18 ± 0.03 cm²/s for neon, 0.07 ± 0.01 cm²/s for krypton and 0.052 ± 0.006 cm²/s for xenon.     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U _cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U _cr , pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.     

The Regularities of the Dynamic Stark Effect in Rare Gases

The regularities of the dynamic Stark effect in a high-frequency discharge in rare gases as well as the regularities observed under shift and splitting of spectral lines in the electric field for helium, neon, argon, and krypton are studied theoretically. The Schrodinger equation is solved by the diagonalization of the atomic-energy matrix in the electric field within the rotating-wave approximation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 3–11, September, 2005.     

Electron beam formation in an anomalous glow discharge

Scaling parameters that account for the efficient formation of electron beams in sources based on an anomalous glow discharge are considered, and the processes leading to the violation of scaling laws are analyzed. Estimates show that the working pressure range in such sources can be increased significantly. The results obtained can be used to optimize discharge regimes with required electron beam parameters. It is demonstrated that sources with plane-parallel continuous electrodes can operate at pressures as high as 100 Torr (which is one to two orders of magnitude higher than that in similar existing electron beam sources), provided that parasitic spark breakdowns at the insulator near the discharge cathode are suppressed. The sources designed can be used for direct excitation of laser working media or their preionization by soft X-rays.     

Experimental Study of Ion-Acoustic Instability in Stratified Krypton and Xenon Plasma at High Pressures

The ion-acoustic instability in gaseous discharge plasma in krypton and xenon at the pressures 0.4 to 2.5 Torr was found and studied. The necessary conditions for the instability existence were satisfied only in the ionisation wave region where electron density was about an order larger than average one. The detected noise amplitude and frequency dependence on the glow discharge plasma parameters was also investigated.     

Formation of ball streamers at a subnanosecond breakdown of gases at a high pressure in a nonuniform electric field

The formation of a diffuse discharge plasma at a subnanosecond breakdown of a “cone–plane” gap filled with air, nitrogen, methane, hydrogen, argon, neon, and helium at various pressures has been studied. Nanosecond negative and positive voltage pulses have been applied to the conical electrode. The experimental data on the dynamics of plasma glow at the stage of formation and propagation of a streamer have been obtained with intensified charge-coupled device and streak cameras. It has been found that the formation of ball streamers is observed in all gases and at both polarities. A supershort avalanche electron beam has been detected behind the flat foil electrode in a wide range of pressures in the case of a negatively charged conical electrode. A mechanism of the formation of streamers at breakdown of various gases at high overvoltages has been discussed.     

Generation of runaway electron subnanosecond pulses in nitrogen and helium at a voltage of 25 kV across the gap

The formation of a runaway electron beam in helium and nitrogen at a generator voltage of 25 kV is studied experimentally. At low generator voltages, an ultrashort avalanche electron beam (UAEB) is shown to form at the flat top of the voltage pulse and its delay time relative to the leading edge of the pulse may attain several tens of nanoseconds. The conditions of runaway electron beam generation depend on the pressure in the gas-filled diode. The FWHM of the beam current varies from 200 ps to several nanoseconds. Beam electron energy distributions at different pressures are obtained. It is found that, if the gap is preionized by an additional source, the UAEB generation conditions break.     

Townsend coefficient and efficiency of the formation of runaway electrons in neon

Basic ionization and drift properties are simulated for neon by the method of multiparticle dynamics. This calculation revealed that, in neon—in just the same way as in other gases that were studied previously—the Townsend ionization regime is realized even in strong fields if the distance between electrodes is rather large. The dependences of basic ionization and drift properties on the reduced electric-field strength are obtained. The results agree with available experimental data. The escape curve separating the region of efficient electron multiplication from the region in which electrons leave the discharge gap without undergoing multiplication is found for neon. The efficiency of the formation of a runaway-electron beam in helium and neon is simulated.     

Initiation of a volume discharge in an elevated-pressure gaseous mixture of CO2 lasers

Initiation of a volume discharge in the gaseous mixture of CO₂ lasers at above-atmospheric pressures for a discharge gap 5 cm long is considered. It is shown that preionization by soft X rays allows initiation of the volume discharge when ratio E/p (p = 5 atm) is below a static breakdown value.     

Dependence of electron beam generation in an open discharge on the discharge gap configuration and gas pressure

Experimental investigation into an electron beam generator based on an open continuous discharge is carried out at different pressures of air and helium. The current-voltage characteristics of the discharge are taken at different geometrical parameters of the discharge gap.