Generation of negative ions in tandem high-density hydrogen discharges

An optimized tandem two-chamber negative-ion source system is discussed. In the first chamber high-energy (E>20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm−2 and gas densities N2R in the range 1014–1015 molecules cm−2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm−2. The relative negative-ion yields from single-chamber and tandem two-chamber systems are compared. Estimates are made for the rates of polar dissociation of H2 molecules and H+3 ions, and these rates are compared with the dissociative attachment rates.