Characterization of Hydrogen Plasma in an ECR based Large Volume Plasma Chamber

Abstract

Hydrogen plasma characterization was carried out in a large volume (dia∼ 1.0 m, h∼ 1.0 m) plasma chamber to evaluate the efficacy of production of uniform, large-area H^- beam for fusion applications. Up to seven Compact ECR Plasma Sources (CEPS; Indian Patent #301583, Patentee: IIT Delhi) can be mounted on the top dome of the chamber (one in centre and six on a ∼60 cm dia circle). Axially poled permanent ring magnets provide the magnetic field for each CEPS and the total field in the chamber is the combined field of all CEPS. Separate experiments were conducted with: (i) six CEPS with opposite polarity on adjacent magnets, (ii) seven CEPS with identical polarity on all the magnets, and (iii) a single CEPS at the center. In case (ii) the field lines repel while for (i) a cusp field is formed between adjacent sources. For case (iii), one obtains a uniform plasma density over 35 cm radius (n ∼ 4 ϗ 10¹⁰ cm^-3, T_e ∼1-2 eV), ∼ 60 cm downstream (from central source mouth) with 400W power at ∼ 1-3 mTorr pressure, yielding ideal conditions for volume mode H^- production over a large area. For case (ii), however, plasma from each source flows along its individual field lines, without forming uniform plasma down-tream. Whereas for case (i), because of cross-talk between adjacent sources, the system became unstable, giving oscillations in plasma formation and microwave reflected power. Hence, it appears that the single CEPS configuration is the most efficacious for large area H^- generation.