Optimum extracted negative-ion current densities from tandem high-density systems

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Abstract
A tandem high-density hydrogen negative-ion-source system is optimized for the purpose of identifying the maximum possible extracted ion current densities. In the first chamber, vibrational excitation occurs by high-energy electron excitation (E-V process); in the second chamber negative ions are formed by dissociative attachment. The electron, atom, and molecular densities are varied together with the length of the second chamber. Electron excitation cross sections to the B1∑u and C1∏u states are calculated as a function of vibrational excitation. The vibrational excitation approaches an asymptotic value as the first-chamber electron density increases. For a system with scale length R=10 cm the optimum extracted current densities occur for gas densities near 1015 mol cm−3 for first-chamber electron densities equal to 1013 cm−3, and for second-chamber electron densities in the 4–6×1012 cm−3 range. For atomic densities equal to one-tenth the molecular density and for second-chamber scale lengths as short as Z/R=0.2, extracted current densities are approximately 30 mA cm−2. As Z/R decreases toward 0.1, the current density is doubled. If the atomic density is increased to one-fifth the molecular density, these current densities are halved; for atomic densities equal to the molecular density, these current densities are reduced by a factor of 6. Applying the system scaling law and varying R from 10 cm toward 1 cm, the extracted current densities tend toward several hundred mA cm−2.