Surface production of negative ions from pulse-biased nitrogen doped diamond within a low-pressure deuterium plasma

TL;DR

This study investigates negative ion production from nitrogen-doped diamond in low-pressure deuterium plasma using pulsed biasing, revealing enhanced yields and insights into defect surface ratios for improved ion generation.

Contribution

It introduces pulsed biasing as a method to study negative ion surface production on nitrogen-doped diamond, highlighting its potential to improve ion yields without low work function metals.

Findings

Negative ion yield is approximately 20% higher for nitrogen-doped diamond.

Pulsed bias results in lower peak negative ion yield than continuous bias.

Optimal defect surface ratios can enhance negative ion production.

Abstract

The production of negative ions is of significant interest for applications including mass spectrometry, materials surface processing, and neutral beam injection for magnetic confined fusion. Neutral beam injection sources maximise negative ion production through the use of surface production processes and low work function metals, which introduce complex engineering. Investigating materials and techniques to avoid the use of low work function metals is of interest to broaden the application of negative ion sources and simplify future devices. In this study, we use pulsed sample biasing to investigate the surface production of negative ions from nitrogen doped diamond. The use of a pulsed bias allows for the study of insulating samples in a preserved surface state at temperatures between 150$^{\circ}$C and 700$^{\circ}$C in a 2 Pa, 130 W, (n$_e$ $\sim$ $10^9$ cm$^{-3}$, T$_e$ $\sim$ 0.6 eV) inductively coupled deuterium plasma. The negative ion yield during the application of a pulsed negative bias is measured using a mass spectrometer and found to be approximately 20% higher for nitrogen doped diamond compared to non-doped diamond. It is also shown that the pulsed sample bias has a lower peak negative ion yield compared to a continuous sample bias, which suggests that the formation of an optimum ratio of defects on its surface can be favourable for negative ion production.