For development of the positive-ion-based NBI (P-NBI) RF ion source, the high ratio of atomic ion density to total ion density (nD+/nion) is favorable because the energy of molecular ions, D2+ and D3+, becomes 1/2 and 1/3 of D+ energy by dissociation during the neutralization process in the plasma heating region. In this study, the global model for hydrogen plasma was developed to obtain the high nH+/nion ratio hydrogen plasma. In the global model, it is observed that H+ and H2+ productions are sensitive to the electron density and electron temperature because of the related electron impact collision processes. Electron impact collisions such as dissociative ionization processes promote production of H+ ions. For H2+ production, hot electron population (or the tail part of electrons in the electron energy distribution function; EEDF) is crucial because H2+ ions are generated without intermediate successive processes such as dissociative processes. (Threshold energy: 15.4 eV) Noticeably, H3+ production depends on the heavy particle exchange reaction as H2 + H2+  H + H3+, which does not require the electron impact reaction process and it rather depends on the neutral pressure of hydrogen. It also reduces the H2+ population. Therefore, it is expected that high nH+/nion plasma production may be achieved by high electron density with low hot electron population of EEDF and the optimum pressure. Preliminary experiments with hydrogen plasma were carried out in inductively coupled plasma (ICP) to investigate the optimum pressure and RF power conditions for high nH+/nion plasma production. Operating pressure was from 1 to 20 mTorr and RF power was up to 2.5 kW. Hydrogen ion densities ratio with varying pressure and RF power was measured by the quadrupole mass spectroscopy (QMS), supplied by Hiden Analytical Limited. As expected from the model, two noticeable characteristics on change of hydrogen ion densities according to pressure and RF power were observed. One is that nH+/nion ratio has the maximum value on pressure variation and its pressure is about 5 mTorr. As H2 molecule density increases with pressure, the reactant H2+ decreases and the product H3+ density increases due to the heavy particle exchange reaction. At about 5 mTorr, the sum of H2+ and H3+ densities becomes to the minimum value, then nH+/nion ratio goes the maximum value. Another characteristic is that nH+/nion ratio increases with RF power at a given pressure. Electron density is proportional to RF power and electron promotes to produce H+ ions, resulting in an increase of nH+/nion ratio. Consequently, RF ion source with high nH+/nion ratio may be achieved at the optimum pressure and high RF power. However, the small population of hot electron is favorable. More details on data and analysis will be presented.

ICIS11_Abstract_SRHuh.pdf

ICIS11_Poster_SRHuh.pdf