Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas

TL;DR

This study investigates how positive ion energy affects negative ion production on graphite and diamond surfaces in deuterium plasma, revealing contrasting behaviors linked to sputtering and material properties.

Contribution

It provides new insights into the energy-dependent negative ion yields on different carbon surfaces, highlighting the roles of sputtering and material electronic properties.

Findings

Negative-ion yield decreases with energy on diamond surfaces.

Negative-ion yield increases with energy on graphite surfaces.

Sputtering mechanisms enhance negative-ion production at certain energies.

Abstract

This work focuses on the production of negative-ions on graphite and diamond surfaces bombarded by positive ions in a low pressure (2 Pa) low power (20 W) capacitively coupled deuterium plasma. A sample is placed opposite a mass spectrometer and negatively biased so that surface produced negative ions can be self-extracted from the plasma and measured by the mass spectrometer. The ratio between negative-ion counts at mass spectrometer and positive ion current at sample surface defines a relative negative-ion yield. Changes in negative-ion production yields versus positive ion energy in the range 10-60 eV are analysed. While the negative-ion production yield is decreasing for diamond surfaces when increasing the positive ion impact energy, it is strongly increasing for graphite. This increase is attributed to the onset of the sputtering mechanisms between 20 and 40 eV which creates negative ions at rather low energy that are efficiently collected by the mass spectrometer. The same mechanism occurs for diamond but is mitigated by a strong decrease of the ionization probability due to defect creation and loss of diamond electronic properties.