A. Shivarova

Magnetic filter operation in hydrogen plasmas

A fluid-plasma model description of the operation of a magnetic filter for electron cooling in gas-discharge plasmas is presented in the study. Directed to the use of weak magnetic fields in the sources of negative hydrogen ion beams for additional heating of fusion plasmas, hydrogen discharges have been considered. The numerical results obtained within a 2D-model are stressed. The 1D-model presented aims at showing the main trends whereas the results obtained within the 3D-model, also developed, confirm the 2D-model description. The models outline the importance of the transport phenomena: electron-energy and charged-particle fluxes. Reduction of the thermal flux across the magnetic field together with thermal diffusion and diffusion, acting in combination, is the basis of the electron cooling and of the spatial distribution of the electron density. Effects due to the (E × B)-drift and the diamagnetic drift form a fine spatial structure of the plasma-parameter variations.

Surface-wave-produced plasmas in a diffusion-controlled regime

A numerical model of surface-wave-produced plasmas in a diffusion-controlled regime is presented in this study. With the wide gas-pressure range (p=0.05–5 Torr) considered in a combination with a frequency (f=144 MHz) in the rf-range, the model covers discharge maintenance under different conditions with respect to mechanisms of electron heating and discharge self-consistency. The precise description of the wave dispersion properties and field distribution in inhomogeneous collisional plasmas included in the electrodynamical part of the model and the specification of the discharge kinetics in its gas-discharge part provide the quantitative description of the discharge production. The model results in the self-consistent structure of the discharge composed by interrelated variations, along the discharge, of wave number, damping rate, field distribution, power absorbed in average by an electron, plasma density, electron temperature, and Joule heating. The total absorbed power finally obtained in its relatio...

Analytical expressions for the axial structure of surface wave sustained plasmas under various regimes of charged particle loss

An analytical model for predicting the axial structure of weakly collisional plasma columns sustained by a propagating electromagnetic surface wave has been developed. It considers both the ambipolar diffusion and volume recombination regimes. The results are expressed as electrodynamical similarity relations and show the respective influence of the waves dispersion and damping of discharge conditions and of the charged particle loss mechanisms upon the axial gradient of electron density.

Slow ionising high-frequency electromagnetic wave along a thin plasma column

The problem of the propagation of a slow high-frequency ionising electromagnetic wave along a thin plasma cylinder surrounded by a dielectric tube is considered, assuming that the characteristic diffusion and thermal conductivity lengths are small in comparison with the discharge length. The wave phase characteristics as well as the radial and axial profiles of the wave field and of the plasma electron temperature and density are determined. The obtained results suggest an approach for explanation of the experimental data.

Influence of nonlinearly induced diffraction on spatial solitary waves

Spatial solitary waves carried by a TE mode in planar optical waveguides with Kerr-type of nonlinearity are investigated based on a modified nonlinear (1 + 1) Schrödinger equation. Looking for effects instigated by the spatial inhomogeneity of the nonlinear polarization, the nonzero divE-term is taken into account in the nonlinear guided wave equation together with the nonparaxial approach and vector model. It is shown that, at self-focusing of beams in a (1 + 1) geometry, the effect coming out from the divergence term predominates over modifications related to the nonparaxial vector model. This term is interpreted as nonlinearly induced diffraction which plays the role of an effective saturation. Analytical solutions supported by numerical results are found.

Modulation instability in pulsed surface-wave sustained discharges

Experimental results showing development of modulation instability and tendency to soliton formation in pulsed gas discharges sustained by surface waves are presented. The measurements are performed in argon discharges at gas pressure 13-1.3/spl middot/10/sup 3/ Pa sustained by pulsed microwave power at 2.45 GHz, The changes along the discharge length of the shape of the pulses of the total light emission, which is an indication for the plasma density, and of the wave electric field creating the discharge, are registrated. The cases when the modulation instability arises from the level of fluctuations in the plasma and when it is forced by the applied signal producing the discharge are demonstrated. The obtained results are discussed in terms of weak nonlinearity superimposed on the mechanism of strong ionization nonlinearity responsible for the discharge creation itself. A separation of a solitary-like wave front the leading edge of the pulse is also reported as an experimental result.

Nonlinearity: Basis for self‐consistent modeling of surface wave produced plasmas in diffusion controlled regimes

A fluid model of surface wave produced discharges is presented for diffusion controlled regimes taking into account simultaneously nonlinear contributions from stepwise ionization and volume recombination. The saturation of metastable densities with growing electron density reduces the effect of step ionization and allows recombination to become effective toward high electron densities. The charged particle continuity and energy balance equations linking electron density and electric field intensity yield the plasma permittivity under conditions of strong ionization nonlinearity. This permittivity is valid for high frequency discharges in general. In the second part of the modeling this nonlinear permittivity is introduced into the electrodynamical relations for discharges maintained by the field of traveling surface waves, and subsequently the self‐consistent behavior of plasma and wave characteristics along the discharge length is calculated. Both the main part of the discharge column produced by Joule ...

Plasma behaviour affected by a magnetic filter

Operation of magnetic filters is studied experimentally regarding their use for electron cooling in the sources of the negative hydrogen ion beams. Axial profiles of the plasma parameters-electron temperature and density-are measured by probe diagnostics in the expansion plasma region of an inductively driven tandem type of a plasma source. The obtained drop of the electron temperature down to values of about and below 1 eV required for efficient production of negative hydrogen ions is the result proving the proper operation of the filter. The mechanism of electron cooling is discussed based on thermal conductivity effects.

Self-organization of surface wave sustained discharges in the pressure range from 10 to 200 Torr

Experiments showing the dynamics in the self-organization of surface wave sustained discharges are presented. Microwave (2.4 GHz) discharges maintained in an argon gas in a continuous wave regime at a constant applied power and varying gas pressure are studied. The evolution of the discharge from a stationary plasma column at comparatively low pressure (p⩽10 Torr) to a plasma torch at atmospheric pressure passes through different stages of self-organization of the wave-field↔plasma nonlinear structure showing evidence of the general trends of behavior of nonequilibrium dissipative systems. The measurements are carried out at the stage of the discharge self-organization into a filamentary structure with an azimuthal rotation. Macroscopic characteristics (number, size, velocity of rotation) of the filaments and their dependence on the gas pressure and its time variation are given. The total light emission of the plasma considered as giving information about the plasma density is measured and different metho...

Negative hydrogen ion maintenance in small radius discharges: Two-dimensional modeling

The results from a two-dimensional model of hydrogen discharges sustained in a single-chamber small radius plasma source presented in this study show that when the plasma maintenance is nonlocal, the conditions ensuring high concentration of the negative ions are formed by the behavior of the entire discharge structure and, in particular, of the fluxes in the discharge. The traditionally accepted requirements for low-electron temperature and high-electron density formulated based on the locality of the discharge behavior can no longer be employed. The obtained results show strong accumulation of negative ions in the discharge center, which results from their flux in the dc electric field, not from local balance of the ions there.