Magnetic filter operation in hydrogen plasmas

Abstract

Abstract. 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 main trends whereas the results obtained within the 3D model, also developed, come to confirm the 2D-model description. The models outline 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 a combination, are in the basis of the electron cooling and of the spatial distribution of the electron density. Effects due to the ) ( B E × -drift and to the diamagnetic drift form the fine spatial structure of the plasma-parameter variations. IntroductionThe development of sources of negative ion beams for fusion plasma heating by neutral beam injection [1-4] is one of the stimuli motivating the active research on low-pressure hydrogen discharges. In general, the sources of negative hydrogen ions with volume-production based processes as well as the hybrid sources where surface production is employed are tandem-type sources with a construction ensuring space separation of regions of high and low electron temperature [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Thus, electron cooling in the discharge is needed and this is provided by a magnetic filter.The magnetic filter is a localized transverse magnetic field. Its effect for cooling the electrons has been proved both experimentally [3,6,7,13,[20][21][22][23][24][25][26], by probe diagnostics and measurements of the current density of the extracted negative ions, and theoretically [6][7][8][9][10][11][12]15,27] by modelling and discussions on the mechanisms governing the operation of the filter. The fluid-plasma models [6][7][8][9][10]15] of the magnetic filter involve importance of the transport processes. However, the different models stress different aspects as mechanisms of the filter operation: (i) reduction of the electron mobility and of the diffusion in magnetized plasmas acting in a combination with the temperature dependence of the Coulomb collision frequency, the latter considered as a factor ensuring lower diffusion of the hot electrons; (ii) thermal conductivity effects, again acting together with Coulomb collisions; (iii) importance of the Lorentz force showing evidence due to cancelled effects of the -) ( B E × and diamagnetic drifts; (iv) importance of the diamagnetic drift; (v) diffusion acting together with elastic electron-neutral collisions. Both 1D and 2D models have been developed, however, as it has been usually stressed, 2D models are needed for proper description of the problem. Due to the complexity of the description, simplifying assumptions, like neglecting of col...

Spatial distribution of the plasma parameters in the RF negative ion source prototype for fusion

“…The charged particles involved in the model are as in [8]: electrons and the three types of positive hydrogen ions. The negative hydrogen ions are not involved in the model since the source is based on a surface production and, therefore, they do not take part in the plasma bulk processes.…”

“…The boundary conditions for the continuity equations of the charged particles (1) and for the electron energy balance equation (3) are for the fluxes at the walls, as given in [8]. The plasma grid is biased with respect to the metal walls of the source.…”

“…Previously applied models (fluid [8,9] and PIC [10] models) showed that transport processes are in the basis of the electron cooling and of the spatial distribution of the electron density. Moreover, due to the E×B and diamagnetic drifts, as well as to the electron energy fluxes related to them, the spatial symmetry of the plasma parameters is broken.…”

“…The specific geometry of the IPP prototype RF source (cylindrical driver and rectangular expansion plasma volume) in combination with the three-dimensional topology of the magnetic filter field, calls for a 3D model description. However, this is a complex and time-consuming task, and the problem is, usually, treated as a 2D one [8][9][10].…”

“…A previous model of the interaction of a plasma with a transverse MF [8], developed within the fluid plasma approach, has been used. It is extended to the configuration and operational conditions of the IPP prototype RF source, accounting for the magnetic filter field with its actual value and location as well as for the plasma grid with the bias potential applied to it.…”