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A sheet plasma of several-millimeter thickness and an area of ~600 cm2 is produced by a combination of a pair of strong dipole magnets (~1.5 kG on the surface) with opposing fields and a pair of Helmholtz coils producing a magnetic mirror field. The dipole magnets are located at a perpendicular distance of 10 cm away from the sheet plasma core, and the Helmholtz coils are positioned at each end of the extraction chamber, which are separated at 40 cm. H- is formed in the extraction chamber via the excitation of hydrogen molecules by energetic electrons and the subsequent dissociative attachment of the molecules with cold electrons. The ions produced at different neutral gas pressures and discharge conditions were extracted at various probe distances relative to the sheet plasma core. The effects of a noble gas, argon, and sputtered magnesium to H- ion yield were determined. The optimum H- current density extracted at 3.0 cm from the plasma core and 3-A plasma current increased by 4.2 times to 0.63 A/m2 with 10% argon seeding. Argon-hydrogen plasma was then seeded with magnesium (Mg). A Mg disk with an effective area of 22 cm2 is placed at the anode of the extraction region and subjected to sputtering. The optimum H- current density at 3.0 cm from the core plasma is further increased by 4.9 times to 3.09 A/m2. The increase in electron density (from 2.76 times 1012 m-3 to 2.90 times 1015 m-3) of cold electrons with an effective electron temperature of about 3 eV is conducive for DA reactions, leading to the enhanced yield.