Properties of a low-pressure inductive RF discharge I: Experiment

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.     

Effect of an External Magnetic Field on the Absorption Efficiency of the RF Power in a Spatially Bounded Inductive Plasma Source

The measured dependences of the equivalent plasma resistance on the external magnetic field (0–50 G) in a 46-cm-diameter RF inductive plasma source operating at frequencies of 2, 4, and 13.56 MHz and a power of 100–500 W are presented. The experiments were carried out in argon at pressures of 0.1–30 mTorr. The presence of the external magnetic field leads to the appearance of resonance domains of efficient RF power absorption corresponding to the conditions of resonance excitation of helicons coupled with Trivelpiece–Gould modes. It is shown that RF power absorption at frequencies of 2 MHz can be optimized by applying an external magnetic field corresponding to the domains of resonance absorption. The effect is enhanced with increasing operating frequency.     

Spatial distribution of the RF power absorbed in a helicon plasma source

The spatial distributions of the RF power absorbed by plasma electrons in an ion source operating in the helicon mode (ω _ci < ω < ω _ce < ω _pe ) are studied numerically by using a simplified model of an RF plasma source in an external uniform magnetic field. The parameters of the source used in numerical simulations are determined by the necessity of the simultaneous excitation of two types of waves, helicons and Trivelpiece-Gould modes, for which the corresponding transparency diagrams are used. The numerical simulations are carried out for two values of the working gas (helium) pressure and two values of the discharge chamber length under the assumption that symmetric modes are excited. The parameters of the source correspond to those of the injector of the nuclear scanning microprobe operating at the Institute of Applied Physics, National Academy of Sciences of Ukraine. It is assumed that the mechanism of RF power absorption is based on the acceleration of plasma electrons in the field of a Trivelpiece-Gould mode, which is interrupted by pair collisions of plasma electrons with neutral atoms and ions of the working gas. The simulation results show that the total absorbed RF power at a fixed plasma density depends in a resonant manner on the magnetic field. The resonance is found to become smoother with increasing working gas pressure. The distributions of the absorbed RF power in the discharge chamber are presented. The achievable density of the extracted current is estimated using the Bohm criterion.     

Self-consistent model of an inductive RF plasma source in an external magnetic field

A theory is developed that makes it possible to calculate RF power absorption in an inductive plasma source. Conditions are determined under which most of the power is deposited in the plasma. It is shown that these conditions correspond to the excitation of spatial waves (an oblique Langmuir wave and a helicon wave). A simple self-consistent model of a plasma source is proposed that describes all of the experimentally observed distinctive properties of plasma sources well.     

Characteristics of molecular hydrogen and CH* radicals in a methane plasma in a magnetically enhanced capacitive RF discharge

The parameters of a methane-containing plasma in an asymmetric RF capacitive discharge in an external magnetic field were studied using optical emission spectroscopy. The power deposited in the discharge was 90 W and the gas pressure and magnetic field were varied in the ranges 1–5 Pa and 50–200 G, respectively. The vibrational and rotational temperatures of hydrogen molecules and CH* radicals were measured as functions of the magnetic field and methane pressure. The ratio between the densities of atomic and molecular hydrogen was estimated. The processes responsible for the excitation of molecular hydrogen and CH* radicals in a methane-containing plasma in an RF capacitive discharge are analyzed.     

Effect of an external RF field on the beam-plasma discharge in a magnetic field

The effect of an RF field on a steady-state beam-plasma discharge with a plane electrode placed parallel to a sheetlike electron beam is studied experimentally. The plasma parameters were measured by a single probe, and the electron distribution function was determined with the use of an electrostatic analyzer. The energy and current of the electron beam were E _B=2.5 keV and J _B=0.05–1.5 A, respectively. The working pressure was p=2×10^?5–10^?3 torr. The frequency of the external RF field was 13.56 MHz. Both the steady-state regimes in which the RF field had no effect on the plasma parameters and regimes with a pronounced effect of the RF field were observed. The experiments show that the regime of the discharge depends strongly on the plasma density and the magnetic field. The parametric instability is studied theoretically in the weak-turbulence approximation. It is shown that, due to the decay nature of the spectrum of plasma oscillations, the onset of instability is accompanied by the transfer of the energy of fluctuations over the spectrum, from the pump frequency toward its harmonics.     

Stimulated ionization scattering of a wave beam forming a discharge channel in a magnetic mirror trap

The stimulated scattering of a whistler wave beam forming an extended discharge channel in a magnetic mirror trap is discovered and investigated experimentally. It is shown that the beam is scattered by relaxaction oscillations of the lattice of plasma inhomogeneities excited by the beam field. The spectrum of the pump field in the RF discharge plasma is found to broaden considerably and to contain individual modulation peaks corresponding to lattice oscillations. The peaks are observed at working gas pressures at which the electron mean free path is close to the wavelength of the standing wave forming the discharge channel. A physical model describing the phenomena observed is developed.     

Effect of the vibrational excitation of CO molecules on the parameters of an RF discharge

A one-dimensional model of an RF discharge in CO-containing gas mixtures is developed. The model takes into account the effect of the degree of vibrational excitation of CO molecules on the structure of the discharge and on its parameters. Experimental data are presented from measurements of the voltage-power characteristics of RF discharges in gas mixtures with different CO contents in the pressure range of 10–100 torr. The model developed is used to calculate the dependence of the root-mean-square discharge voltage on the specific power deposition in an RF discharge under our experimental conditions. The experimental data are compared to the results of numerical simulations. For working gas pressures of about 100 torr, which are typical of the operation of slab CO lasers, the calculated voltage-power characteristics of an RF discharge agree satisfactorily with those obtained experimentally. The theoretical model predicts that the vibrational excitation of CO molecules leads to a redistribution of the RF field in the discharge gap and to an increase in the laser efficiency.     

Ordered dusty structures in the plasma of an RF electrodeless gas discharge

Results are presented from the experimental studies and numerical simulations of the behavior of dust grains in the plasma of an inductive RF discharge. The experiments were carried out with neon at a pressure of 25–500 Pa and with 1.87-μm melamine formaldehyde grains. The discharge was excited by a ring inductor supplied from a generator operating at a 100-MHz frequency. The effective dust-grain interaction potential used in numerical simulations involved the spatial dependence of the grain charge on the plasma floating potential, grain-interaction anisotropy resulting from the focusing of the drift ion current by the negatively charged grains, and specific features of the shielding of the dust grains by the plasma electrons and ions recombining both in the plasma bulk and on the grain surface. The results of Monte Carlo simulations show that the dust grains form specific filament structures observed experimentally in the plasma of an inductive electrodeless discharge. __________ Translated from Fizika Plazmy, Vol. 26, No. 5, 2000, pp. 445–454. Original Russian Text Copyright ? 2000 by Zobnin, Nefedov, Sinel’shchikov, Sinkevich, Usachev, Filinov, Fortov.     

Contactless method for studying the parameters of the electrode region of an RF discharge

A nonintrusive contactless method for studying the parameters of the electrode region of a capacitive low-pressure RF discharge is proposed. The method involves the measurements of dc and ac electric voltages at the elements of the discharge circuit with subsequent calculations of both the electrostatic potential drop across the electrode sheath and the sheath thickness by using relations derived in the paper. For a collisionless electrode sheath, the density of the positive-ion current onto the electrode and the charge density at the plasma boundary are determined. It is shown experimentally that the method can be successfully applied to studying capacitive RF discharges with inner or outer electrodes.     

Effect of the periodic ripple in an axial confining magnetic field on the Alfvén heating of a cylindrical plasma

In a uniform axial magnetic field, the structure of local Alfvén resonance and the resonant absorption of RF power are governed by collisions, finite ion Larmor radius effects, and electron inertia. It is shown that, in a cylindrical plasma in a constant, periodically rippled, axial magnetic field, the structure of Alfvén resonance and the absorption of RF power can strongly depend on the ripple amplitude. The conditions under which the effect in question is dominant are intrinsic, e.g., to the modular Wendelstein stellarators.     

Effect of Resonant Radiation Transport on the Parameters of RF and DC Discharges in a He-Ar-Xe Mixture

A study is made of the effect of the transport of Xe 147-nm resonant radiation on the parameters of a low-temperature plasma of DC and RF discharges in gas mixtures used as the working medium in lasers based on infrared transitions in xenon. RF discharges are treated in the planar geometry typical of slab lasers. DC discharges in tubes are treated in cylindrical geometry. The trapping of resonant radiation is described using different approximate models: the decay time approximation for a plasma slab (the Holstein approximation) and the effective lifetime approximation (the Biberman approximation). The transport equation for resonant radiation is solved numerically. The effect of the radiation transport on both the current-voltage characteristics of a discharge and the spatial distribution of the excited Xe atoms is investigated. The current-voltage characteristics calculated for a DC discharge with allowance for the resonant radiation transport agree well with the experimental characteristics. It is found that, for an RF discharge, the effective lifetime approximation overestimates the density of the excited Xe atoms near the electrodes by several times and underestimates this density at the midplane of the discharge gap.     

Investigation of the helicon discharge plasma parameters in a hybrid RF plasma system

Results of an experimental study of the helicon discharge plasma parameters in a prototype of a hybrid RF plasma system equipped with a solenoidal antenna are described. It is shown that an increase in the external magnetic field leads to the formation of a plasma column and a shift of the maximum ion current along the discharge axis toward the bottom flange of the system. The shape of the plasma column can be controlled via varying the configuration of the magnetic field.     

Plasma parameters and existence conditions of monolayer dust structures in the electrode sheath of an RF discharge

The plasma parameters in the electrode sheath of an RF discharge were studied experimentally under the conditions of dust monolayer levitation. A new method is proposed for determining the plasma parameters, such as the average electric field, ion density, and ion velocity. The screening parameter and the dust grain charge are estimated. The criteria of stable levitation of a dust monolayer are considered. The obtained results are compared with the available theoretical and numerical data, as well as with the results obtained using other diagnostic methods.     

Dynamics of plasma−dust structures formed in a trap created in the narrowing of a current channel in a magnetic field

The geometry and dynamics of plasma?dust structures in a longitudinal magnetic field is studied experimentally. The structures are formed in a glow-discharge trap created in the double electric layer produced as a result of discharge narrowing by means of a dielectric insert introduced in the discharge tube. Studies of structures formed in the new type of glow-discharge trap are of interest from the standpoint of future experiments with complex plasmas in superstrong magnetic fields in which the dust component is magnetized. Different types of dielectric inserts were used: conical and plane ones with symmetric and asymmetric apertures. Conditions for the existence of stable dust structures are determined for dust grains of different density and different dispersity. According to the experimental results, the angular velocity of dust rotation is ≥10 s^–1, which is the fastest type of dust motion for all types of discharges in a magnetic field. The rotation is interpreted by analyzing the dynamics of individual dust grains.     

Low-current moderate-pressure RF discharge with secondary electron photoemission

A model is proposed for a low-current RF discharge with secondary electron photoemission from the electrode surface caused by photons originating in the electrode sheath. The low-current state of RF discharges at moderate pressures is peculiar in that the electrons and ions produced during the preceding periods of the RF field promote the development of the discharge during subsequent periods. Since the ion space charge is induced during many periods of the RF field, even comparatively moderate fields in the electrode sheath are sufficient to ensure the conditions under which the current is self-sustaining, in which case the electron photoemission dominates over the remaining secondary processes at the electrode surface. In a low-current RF discharge, the ion-electron emission has essentially no impact on the formation of the electrode sheath because the half-period of the RF field is much shorter than the ion transit time through the sheath. The sheath results from the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface. The sheath parameters are determined by the conditions under which the current in the sheath is selfsustaining due to the secondary electron photoemission from the electrode surface. The capacitive susceptance of the electrode sheath is substantially higher than its electrical conductance. Low-current RF discharges can only exist when the time required for the ions to drift through the sheath and reach the electrode is much longer than the half-period of the RF field.     

Atmospheric-pressure electric discharge as an instrument of chemical activation of water solutions

Results of experimental studies and numerical simulations of physicochemical characteristics of plasmas generated in different types of atmospheric-pressure discharges (pulsed streamer corona, gliding electric arc, dielectric barrier discharge, glow-discharge electrolysis, diaphragmatic discharge, and dc glow discharge) used to initiate various chemical processes in water solutions are analyzed. Typical reactor designs are considered. Data on the power supply characteristics, plasma electron parameters, gas temperatures, and densities of active particles in different types of discharges excited in different gases and their dependences on the external parameters of discharges are presented. The chemical composition of active particles formed in water is described. Possible mechanisms of production and loss of plasma particles are discussed.     

Helicon plasma in a nonuniform magnetic field

The distributions of the electron density in a plasma produced by helicon waves and the correspond-ing wave amplitudes and phases are studied experimentally. The measurements were carried out in an argon plasma at a pressure of 3 mtorr and at an input RF power of up to 600 W. The magnetic field was caried in the range from 0 to 200 G. The efficiency of plasma production in both uniform and nonuniform fields is investigated. It is shown that, in a nonuniform magnetic field, the electron density can be substantially increased (up to 5×10¹² cm^?3) by placing an antenna in the region in which the magnetic field is weaker than in the main plasma.     

Characteristic properties of the frame-antenna-produced RF discharge evolution in the Uragan-3M torsatron

In the ? = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ? ω _ci ). To this end, a frame antenna with a broad spectrum of generated parallel wavenumbers is used. The RF discharge evolution is studied experimentally at different values of the RF power fed to the antenna (the anode voltage of the oscillator and the antenna current) and the initial pressure of the fueling gas. It is shown that, depending on the antenna current and hydrogen pressure, the discharge can operate in two regimes differing in the plasma density, temperature, and particle loss. The change in the discharge regime with increasing anode voltage is steplike in character. The particular values of the anode voltage and pressure at which the change occurs are affected by RF preionization or breakdown stabilization by a microwave discharge. The obtained results will be used in future experiments to choose the optimal regimes of the frame-antenna-produced RF discharge as a target for the production and heating of a denser plasma by another, shorter wavelength three-half-turn antenna.     

Near field of a loop antenna operating in plasma in the whistler frequency range

The structure of the RF magnetic field in the vicinity of a loop antenna operating in the whistler frequency range has been studied experimentally and theoretically. The experiments were performed over a wide frequency range at different values of the plasma density, electron temperature, and ambient magnetic field strength. It is shown that, when a loop antenna is smaller than the wavelength of a quasi-longitudinal whistler, the structure of the magnetic field of such an antenna is nearly the same as that of the field of a current-carrying loop in vacuum; otherwise, the RF field is localized near the antenna wire. The results of numerical calculations agree with the measured field distributions. The antenna field is calculated by expanding it in the eigenmodes of a magnetized plasma with allowance for not only propagating but also nonpropagating (exponentially decaying) waves, which make the main contribution to the near field. An analytic estimate of the depth to which the RF magnetic field of a loop antenna penetrates into the plasma is obtained.