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 ∘C and 700 ∘C in a 2 Pa, 130 W, (ne ∼ 109 cm−3, Te ∼ 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.
Highlights
It is 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
The authors believe that these observations suggest that an optimum surface state for negative ion yield possibly exists on diamond that is dependent on the pulsed bias frequency, doping, positive ion energy and temperature of the samples
The pulsed negative bias is applied in a square waveform pulse at 5 kHz, with a duty cycle of 3% to preserve the surface of the samples and allow for measurement of negative ion yield in the absence of sample conductivity
Summary
Negative ions play an important role in applications including particle acceleration [1,2,3,4,5], neutron generation [6, 7], mass spectrometry [8,9,10,11], spacecraft propulsion [12,13,14], microprocessor manufacturing [15] and neutral beam heating for magnetic confinement fusion (MCF) [16,17,18,19]. The magnitude of a continuous negative bias has been shown to influence the negative ion yield [29] This has previously been attributed to the formation of defects on the surface which alters its electronic properties [23, 29, 30]. It is of significant interest to investigate the negative ion yield from micro crystalline nitrogen doped diamond (MCNDD) with a preserved sample surface and at temperatures where the material is non-conductive. The negative ion yield is compared between un-doped micro crystalline diamond (MCD) and MCNDD films This comparison is carried out over a temperature range between 150 ◦C and 700 ◦C to develop the understanding of negative ion production from diamond at temperatures below 450 ◦C where MCNDD is non-conductive [41, 43, 44].
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