Ts Tsankov

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.

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.

Two-dimensional fluid model of a two-chamber plasma source

This study presents a two-dimensional fluid-plasma model developed for describing the cw regime operation of a tandem plasma source, consisting of a driver and an expansion plasma volume of different sizes. The moderate pressure range considered (tens to hundreds of milliTorr) allows a description within the drift‐diffusion approximation, as employed in the model. Argon discharges maintained in a metal gas-discharge vessel are treated. The discussions stress charged-particle and electron-energy fluxes as well as the spatial distribution of their components. The main conclusions are for (i) different electron and ion fluxes resulting in a net current in the discharge; (ii) a radial ion flux prevailing over the axial one and an axial electron flux prevailing over the radial one; (iii) ion motion determined by the dc electric field and drift‐diffusion electron motion influenced by thermal diffusion; (iv) plasma maintenance in the expansion plasma chamber due to charged-particle and electron-energy fluxes from the driver; (v) importance of the convective flux in the electron-energy balance; (vi) electron-energy losses for sustaining the dc electric field in the expansion plasma volume strongly predominating over the losses through collisions and (vii) electron cooling accompanied by a strong drop in the plasma density and in the potential of the dc electric field, due to the plasma expansion in a bigger volume. In general, the results show that the gas pressure range usually considered to be governed by ambipolar diffusion shows up in a different regime: a regime with a dc current, when the discharge is in a metal chamber with different dimensions in the transverse and longitudinal directions.

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.

Expanding hydrogen plasmas: photodetachment-technique diagnostics

The laser photodetachment technique in its combination with probe measurements is applied for the determination of the electronegativity and the concentration of the negative hydrogen ions in the expansion plasma volume of an inductively driven two-chamber plasma source. Both the implementation of the method and the obtained results are discussed in the study. The variations of the gas pressure and the applied rf power in the experiments are in the ranges p = (0.3‐5)Pa and P = (200‐700)W, respectively. The results for an almostconstant—withthegaspressure—electronegativityanditsincreasewith the applied power are in agreement with theoretical estimations. The obtained nonmonotonicvariationoftheconcentrationofthenegativeionswithincreasing gas pressure correlates with the gas pressure dependence of the electron density in the expansion plasma region of the source. (Some figures in this article are in colour only in the electronic version)

EEDF measurements in plasma expansion region of a high-frequency driven hydrogen discharge

The production of negative hydrogen ions by volume processes depends on the electron energy in the expansion region of a plasma source. The study presents experimental results for the electron energy distribution function (EEDF) obtained by probe diagnostics in this region. Measurements are performed in axial and radial directions at low gas pressures. The results for the EEDF show that its shape changes along the chamber axis because the plasma density and the electron temperature decrease to a nearly constant value in the main part of the expansion chamber. The EEDF is Maxwellian-like with tail up to 40 eV close to the driver. The nonlocal formation of the EEDF is experimentally obtained in expanded plasma.

2D model of a hydrogen inductive discharge with a planar coil

Based on a two-dimensional (2D) fluid-plasma model of low-pressure hydrogen discharges sustained by rf power deposition of the type of inductive discharges with a planar coil, the study presents the spatial distribution of the plasma parameters and of the fluxes in the discharge and discussions outlining plasma maintenance in regions outside the rf power input.

2D fluid-model simulations of plasma expansion

The study presents results from a 2D fluid-plasma model developed for description of th e plasma production in two-chamber plasma sources (tandem-type of plasma sources). The model demonstrates the role of the particle and energy fluxes from the driver region for the plasma maintenance in the second chamber of the source. The influence of the applied power variation on the basic plasma parameters is outlined in the study.

2D fluid-plasma model of a tandem-type plasma source

A two-dimensional fluid-plasma model developed for describing the spatial distribution of the characteristics of plasma sources with complicated configurations is employed for analysis of the operation of a tandem plasma source with respect to the influence of the position of its driver.