Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas

Gilles Cartry,Hocine Khemliche,Dmitry Kogut,Thomas Bieber,Alix Gicquel,Jean-Marc Layet,Philippe Roncin,Kostiantyn Achkasov,Thomas Farley,Alain Simonin,Ovidiu Brinza,Jocelyn Achard

doi:10.1088/1367-2630/aa5f1f

Abstract

This paper deals with a study of H−/D− negative ion surface production on diamond in low pressure H2/D2 plasmas. A sample placed in the plasma is negatively biased with respect to plasma potential. Upon positive ion impacts on the sample, some negative ions are formed and detected according to their mass and energy by a mass spectrometer placed in front of the sample. The experimental methods developed to study negative ion surface production and obtain negative ion energy and angle distribution functions are first presented. Different diamond materials ranging from nanocrystalline to single crystal layers, either doped with boron or intrinsic, are then investigated and compared with graphite. The negative ion yields obtained are presented as a function of different experimental parameters such as the exposure time, the sample bias which determines the positive ion impact energy and the sample surface temperature. It is concluded from these experiments that the electronic properties of diamond materials, among them the negative electron affinity, seem to be favourable for negative-ion surface production. However, the negative ion yield decreases with the plasma induced defect density.

Highlights

Negative-ion production on surfaces in low-pressure plasmas rely on two distinct mechanisms
Different diamond materials ranging from nanocrystalline to single crystal layers, either doped with boron or intrinsic, are investigated and compared with graphite
The negative ion yields obtained are presented as a function of different experimental parameters such as the exposure time, the sample bias which determines the positive ion impact energy and the sample surface temperature

Summary

Introduction

Negative-ion production on surfaces in low-pressure plasmas rely on two distinct mechanisms. Depending on where the ions are formed, one distinguishes volume production associated with dissociative attachment of electrons on molecules and surface production associated with the capture of one or two electrons by neutral atoms or ions impinging on the surface. Depending on the targeted application, either surface or volume production can be the most favourable process. Hydrogen negative-ion sources for fusion, high energy linear particle accelerators, neutron generation, Tandem accelerators and accelerator based mass spectrometry all use the principle of enhanced surface production by injection of caesium. Negative-oxygen-ion sources for Secondary Ion Mass Spectrometry (SIMS) operate by volume production. While plasma thrusters for space propulsion and microelectronics etching plasmas17,18,19,20,21,22currently rely on volume production, they may benefit in future from utilising surface production

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