St. Lishev

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.

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.

Small surface wave discharge at atmospheric pressure

A small surface wave driven source produces plasma at atmospheric pressure. Microwave power at frequency 2.45 GHz is coupled with the source and a discharge is ignited at power levels below 10 W. The coaxial exciter of the surface waves has a length of 10 mm because its dielectric is a high permittivity discharge tube. The plasma source operates as a plasma jet in the case of plasma columns longer than the tube length. The source maintains stable plasma columns over a wide range of neutral gas flow and applied power in continuous and pulse regimes. An additional advantage of this source is the discharge self-ignition. An electron temperature of Te ~ 1.9 eV and a density of ne ~ 3.9 × 1014 cm−3 are estimated by the probe diagnostics method. The emission spectra in the wavelength range 200–1000 nm under different experimental conditions are analysed and they prove the applicability of the source for analytical spectroscopy. The dependences of column length, reflected power and plasma parameters on the gas flow and the input power are discussed.

Matrix of small-radius radio-frequency discharges as a volume-production based source of negative hydrogen ions

Based on experience from a work--both theoretical and experimental one--on negative hydrogen ion beam sources studied regarding fusion applications, a novel design of a rf source with volume production of the ions is proposed. The suggestion is for a source constructed as a matrix of small-radius tandem discharges (with magnetic filters largely extended over the discharge length), inductively driven (by a single coil, for the whole matrix) and with a single aperture extraction from each of them.

On the inertia term in the momentum equation in the free-fall regime of discharge maintenance

The study, being on two-dimensional modelling of low pressure discharges, suggests an approach to the nonlinear inertia term in the momentum equation of the positive ions needed to be accounted for in the free-fall regime of the discharge maintenance. On the basis of conclusions that the inertia term acts in the wall sheath, where the ions fly perpendicularly to the walls, it is shown that (i) the parallel – to the walls – velocity component can be neglected, and (ii) the rest of the convective derivative can be determined by using the energy conservation law in the collisionless case. In a way, the inertia term acting as a retarding force is joined to the momentum loss term by introducing effective collision frequencies. The validity of the procedure is proved in a model of a low pressure argon discharge by comparison with the exact solutions for the two-dimensional spatial distribution of the discharge characteristics (ion velocity, electron density and temperature and DC electric field and its potential). The conclusion is that (i) ignoring the velocity component that is parallel to the walls does not cause deviation from the exact solution, and (ii) the approximation of using the energy conservation law in the collisionless case is good enough.

Diagnostics in a single element of a matrix source of negative hydrogen ionsa)

A small radius discharge in hydrogen inductively driven by a planar coil is studied experimentally regarding development of a matrix source of negative hydrogen ions: a matrix of small radius discharges, each of them completed with a magnetic filter and a single aperture extraction device. Probe and laser photodetachment diagnostics are the methods employed. Results for the axial structure of the discharge show the influence of the absorbed rf power, of the magnetic filter (and its position), and of the bias applied to the first electrode of the extraction device on the spatial distribution of the plasma parameters and on the discharge modes.

On the two modes of operation of planar-coil-driven inductive discharges in hydrogen

Langmuir probe and laser photodetachment technique diagnostics as well as optical emission spectroscopy and phase-resolved optical emission spectroscopy are used to study a planar-coil-driven inductive discharge in hydrogen sustained in the first chamber of a two-chamber plasma source. The discharge is operated in the power range 50?400?W at 27?MHz. The gas-pressure range studied is p?=?20?60?mTorr. The results obtained over the first half of the discharge length, that is starting from the position of the coil, outline discharge maintenance in the two modes of the inductive discharge. The inductive mode sustained at a high rf power appears with rf power deposition by ring-shaped rf electric field intensity close to the coil and high electron density there, followed by its strong drop in the remote plasma region. The capacitive mode sustaining low-density plasmas also appears with two regions specified by different mechanisms of rf power deposition: plasma heating by an electron beam acceleration in the wall sheath during its expansion and Joule heating in the plasma bulk. The similarity?at high and low rf power?in the axial structure of the discharge over the second half of its length, that is touching the transition between the two chambers of the source, and the appearance of a second maximum of the dc potential there are related to the configuration of the source. The obtained axial variation of the negative ion density obeys that of the potential of the dc electric field in the discharge.

Basis of the discharge maintenance in a matrix source of negative hydrogen ionsa)

The presented three-dimensional (3D) self-consistent model of a matrix source completed by small-radius planar-coil driven rf discharges in hydrogen is in the scope of the recent research on a new design of the rf sources of negative hydrogen ions for fusion applications. The analysis of the results for the spatial distribution of basic discharge characteristics (electron density and temperature, rf current density and rf field intensity) in the three configurations of the matrix considered aims at conclusions about a manner of rf inductive-mode driving proper for ensuring the same discharge behavior in the gas-discharge tubes of the matrix.

Spatial distribution of the plasma parameters in a radio-frequency driven negative ion sourcea)

Results from initial stage of modeling of the SPIDER source of negative hydrogen/deuterium ions currently under development in Consorzio RFX (Padova) regarding ITER are presented. A 2D model developed within the fluid plasma theory for low-pressure discharges (free-fall regime maintenance) is applied to the gas-discharge conditions planned and required for the SPIDER source: gas pressure of 0.3 Pa and radio-frequency (rf) power of 100 kW absorbed in a single driver. The results are for the spatial distribution of the plasma characteristics (charged particle densities, electron temperature and electron energy flux, plasma potential, and dc electric field) with conclusions for the role of the electron energy flux in the formation of the discharge structure.

Negative ion density measurements in an inductively driven hydrogen discharge

The study presents experiments on the determination of the electronegativity and its axial variation in an inductively driven hydrogen discharge, performed by using the laser photodetachment technique. The results showing high electronegativity with nonmonotonic axial variations and a maximum next to the rf power deposition region provide an experimental indication that a design of an efficient source of negative hydrogen ions based on a single-chamber discharge might be possible.