Surface-wave produced discharges in hydrogen: I. Self-consistent model of diffusion-controlled discharges

Abstract

A fluid-plasma model of diffusion-controlled hydrogen discharges sustained in the field of propagating surface waves is presented in this study. The self-consistent description of the discharge structure achieved provides results for the inter-related variations of the discharge characteristics: the electron concentration n e , the concentrations of the three ionic species (H + , H + 2 and H + 3 ), the concentrations of the two neutral gas components (H and H 2 ), the electron temperature, the power absorbed on average by an electron, the gas temperature, the wavenumber and the space damping rate α of the wave. Wave behaviour in radially inhomogeneous collisional plasmas is taken into account because it provides the proper description of diffusion-controlled discharges. The general mechanism of nonlocal heating of the electrons in the wave field is considered. The model is extended to comparatively low gas pressures (p 0.2 Torr) by introducing effective mobilities of the ions in which, besides the ion-neutral elastic collisions, the production and destruction of ions by collisions are included. Based on these effective mobilities, the ambipolar diffusion coefficients of the charged particles are specified. The most important reactions that contribute-under the gas-discharge conditions considered-to the production of charged particles and hydrogen atoms as well as to the electron-energy and gas-energy balances are involved in the model. It is shown that in hydrogen discharges, the ( -n e )-relation, which besides the (α-n e )-relation ensures a self-consistent description of the axial structure of surface-wave-sustained plasmas, in general, stems from the dependence of on the concentrations of the neutral gas components (H, H 2 ) and their relation to the concentrations of the ions (H + , H + 2 and H + 3 ). The results obtained using the model are discussed in the context of experiments showing a peculiar behaviour of the axial structure of hydrogen discharges compared to discharges in other gases.

Magnetic filter operation in hydrogen plasmas

“…The following processes in the plasma volume, usually considered [35][36][37][38] as being the main contributors to the production and destruction of charged and neutral-gas species, are taken into account in the particle balance equations (3) and (22) …”

“…0 = z The electron energy losses coll P in collisions taken into account are the same as in the model in Ref. [38], including: (i) losses for atom excitation and ionization, (ii) losses for excitation of vibrational and singlet states of the molecules as well as for their dissociation and ionization, and (iii) losses in elastic collisions with atoms and molecules.…”

“…[38]. The rate coefficients of all the electron impact processes both in the particle-and electron-energy-balance are taken from Janev et al [39], calculated there for a Maxwellian electron energy distribution.…”

Travelling-wave-sustained discharges

“…Operation of stationary and pulsed discharges, discharge maintenance without and in an external magnetic field as well as discharge production in different gases (argon, helium, helium-argon gas mixtures and hydrogen) are covered [16][17][18][24][25][26][27][28][29][30][31][32][33]. In all the cases, the basis of the discharge maintenance-the discharge self-consistency and the electron heating in the high-frequency field-are stressed.…”

Rotational and vibrational temperatures in a hydrogen discharge with a magnetic X-point

“…The source concept is designed to enhance the volume production mechanism of negative hydrogen ions (H 2 (v)+e → H − +H) through optimization of the production of vibrationally excited molecules H 2 (v). By providing suitable conditions for atom recombination (H + H wall → H 2 (v)) on the walls, known to play an important role in the hydrogen atom balance [3], it should be possible to increase the overall population of vibrationally excited molecules in the discharge. The used magnetic field configuration is an additional element, intended to enhance the overall performance of the source [4] by separating the power deposition region from the negative ion production zone and by additionally enhancing the plasma production at the low pressures required for such sources.…”