Optimized H− extraction in an argon-magnesium seeded magnetized sheet plasma

The enhancement and optimization of H− extraction through argon and magnesium seeding of hydrogen discharges in a magnetized sheet plasma source are reported. The paper first presents the modification of the production chamber into a hexapole multicusp configuration resulting in decreased power requirements, improved plasma confinement and longer filament lifetime. By this, a wider choice of discharge currents for sustained quiescent plasmas is made possible. Second, the method of adding argon to the hydrogen plasma similar to the scheme in Abate and Ramos [Y. Abate, H. Ramos, Rev. Sci. Instr. 71 (10) (2000) 3689] was performed to find the optimum conditions for H− formation and extraction. Using an E × B probe, H− yields were investigated at varied argon-hydrogen admixtures, different discharge currents and spatial points relative to the core plasma. The optimum H− current density extracted at 3.0 cm from the plasma core using 3.0 A plasma current with 10% argon seeding increased by a factor of 2.42 (0.63 A/m2) compared to the measurement of Abate and Ramos [Y. Abate, H. Ramos, Rev. Sci. Instr. 71 (10) (2000) 3689]. Third, the argon-hydrogen plasma at the extraction chamber is seeded with magnesium. Mg disk with an effective area of 22 cm2 is placed at the extraction region's anode biased 175 V with respect to the cathode. With Mg seeding, the optimum H− current density at the same site and discharge conditions increased by 4.9 times (3.09 A/m2). The enhancement effects were analyzed vis-à-vis information gathered from the usual Langmuir probe (electron temperature and density), electron energy distribution function (EEDF) and the ensuing dissociative attachment (DA) reaction rates at different spatial points for various plasma discharges and gas ratios. Investigations on the changes in the effective electron temperature and electron density indicate that the enhancement is due to increased density of low-energy electrons in the volume, conducive for DA reactions. With Mg, the density of electrons with electron temperature of about 3 eV increased 3 orders of magnitude from 2.76 × 1012 m−3 to 2.90 × 1015 m−3.