The development of a large-area RF source for negative hydrogen ions, an official EFDA task agreement, is aiming at demonstrating ITER-relevant ion source parameters. This implies a current density of 20 mA/cm 2 accelerated Dions at a source filling pressure of ≤ 0.3 Pa and an electron to ion ratio of ≤ 1 from a PINI-size extraction area for pulse lengths of up to 1 hour. The work is progressing along three lines in parallel: (i) optimisation of current densities at low pressure and electron/ion ratio, utilising small extraction areas (< 100 cm 2) and short pulses (< 10 s); (ii); investigation of extended extraction areas (< 300 cm 2) and pulse lengths of up to 3600 s; (iii) investigation of a size-scaling on a half-size ITER plasma source. Three different testbeds are being used to carry out those investigations in parallel. An extensive diagnostic and modelling programme accompanies the activities. The paper contains the recent achievements and the status of preparations in those four areas of development
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...
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