Abstract
The production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion. Methods to improve the efficiency of the surface production of negative ions, without the use of low work function metals, are of interest for mitigating the complex engineering challenges these materials introduce. In this study we investigate the production of negative ions by doping diamond with nitrogen. Negatively biased (−20 V or −130 V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (helicon source operated in capacitive mode, 2 Pa, 26 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 suggest that nitrogen doping has little influence on the yield when the sample is biased at −130 V, but when a relatively small bias voltage of −20 V is applied the yield is increased by a factor of 2 above that of un-doped diamond when its temperature reaches 550 °C. The doping of diamond with nitrogen is a new method for controlling the surface production of negative ions, which continues to be of significant interest for a wide variety of practical applications.
Highlights
The production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion
Biased (−20 V or −130 V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma and negative ion energy distribution functions (NIEDFs) are measured via mass spectrometry with respect to the surface temperature (30◦C to 750◦C) and dopant concentration
The development of negative ion sources is of significant interest due to their applications in particle acceleration[1,2,3,4,5], neutron generation[6,7], mass spectrometry[8,9,10,11], spacecraft propulsion[12,13,14], nanoelectronics manufacturing[15], and neutral beam heating for magnetic confinement fusion (MCF)[16,17,18,19]
Summary
One application of particular interest is the creation of negative-ion beams suitable for MCF neutral beam injection, which has a proposed requirement of accelerating a 40 A current of deuterium negative ions to 1 MeV16. This primarily utilises negative ion surface production, as distinct from volume production, to increase the density of negative ions close to the extraction grid[20,21,22]. There exist some limitations with this approach, such as controlling the application of the caesium so that it condenses in the right locations and at a rate that is sufficient to maintain an optimum thickness at the extraction grid[25] This method introduces complex engineering challenges, eg.
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