Negative ion production in low temperature hydrogen plasmas

The production of negative ions in low temperature plasmas presents significant opportunities and challenges for technological applications and fundamental plasma physics. Negative hydrogen ions are of particular interest for applications including materials surface processing, mass spectrometry, and neutral beams. Negative ions in hydrogen plasma are produced via both volume and surface processes. The surface production of negative ions in hydrogen plasmas is important for the development of next-generation neutral beam heating systems, and in particular for decreasing the reliance on caesium. To address this challenge, the production of negative ions using doped diamond is investigated. Continuously and pulsed bias (-20 V or -130 V at 5 kHz), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (2 Pa, 26 W or 2 Pa, 130 W) and negative ion energy distribution functions are measured via mass spectrometry with respect to the surface temperature (30°C to 750°C) and dopant concentration. The results show that the use of nitrogen doping can enhance negative ion yield from diamond above that observed under similar conditions for un-doped diamond. A relatively low yield is observed using a pulsed bias compared to a dc bias, which suggests that a certain ratio of defects to preserved diamond bonds on the films can promote optimum conditions for negative ion formation. The volume production of negative ions is investigated in the context of inductively coupled plasmas, which for high power densities - and surface fluxes - are subject to significant spatial gradients in the neutral gas temperature and density. To meet this challenge, we build upon established simulation work in the context of low power density capacitively coupled plasmas to simulate negative ion production via 2D fluid-kinetic simulations with the Hybrid Plasma Equipment Model, which enables self-consistent calculation of gas heating for all of the vibrational levels of the ground electronic state. The simulations, which show good agreement with previously published experimental data (6-20 Pa, 300 W), are used to investigate the impact of gradients on the macroscopic plasma properties and the negative ion production. The results demonstrate that when thermal gradients are present in the plasma, atomic hydrogen and the vibrational states diffuse down the gradients, affecting their distribution within the plasma. However, it is observed that thermal gradients have only a small influence on negative ion density despite observed changes in the densities of the vibrational states in the bulk of the plasma. The results of this work are expected to be of interest for the fundamental understanding of low-temperature hydrogen plasmas and the development of negative ion sources.