Plasma source of negative hydrogen ions

Theoretical and numerical investigations are reported concerning the physical processes within a plasma source of negative hydrogen ions; the source described was developed at the Physical Institute of the National Academy of Sciences, Ukraine. The source consists of a heated-cathode reflective discharge with extraction of the H{sup {minus}} ions across the magnetic field direction. The theoretical part of the work includes the calculation of gas-discharge plasma parameters for given values of near-cathode electron energy and current, gas pressure, and discharge gap geometry. The plasma parameters, including the H{sup {minus}} ion concentration, were determined by numerically solving the Boltzmann equation, taking account of the main collision processes in the plasma volume, as well as at the surface of the discharge chamber. The main theoretical and experimental results are as follows: (1) Efficient source operation can be ensured provided the H{sup {minus}} ions are not extracted from the discharge column, but rather from the drift plasma with an electron temperature of no more than 1 eV. (2) Increase in the specific power deposition W to the discharge volume up to 500 W/cm{sup 3} resulted in a H{sup {minus}} ion emission current growth. Further increase in the specific power deposition W leads tomore » current saturation due to the H{sup {minus}} ion losses in collisions with both hydrogen atoms and plasma electrons. (3) Cesium vapor doping resulted in an order of magnitude increase in the gaseous efficiency of the source. This phenomenon was not related to a growth of the H{sup {minus}} ion production efficiency in plasma. In fact, it could be accounted for by a considerable enhancement of the flow of H{sup {minus}} ions produced at the discharge anode cesium-activated surface due to charge exchange of fast positive ions and atoms. (4) Using a pulse repetitive source operation mode instead of the CW mode increased the plasma source efficiency.« less