Negative ions in low-pressure plasma sources are created either in the plasma volume by dissociative attachment or, at the plasma surface interaction due to surface ionization of backscattered or sputtered particles. Negative-ions formed on surfaces are accelerated towards the plasma by the sheath. They can influence the plasma kinetics through collisions with plasma species, or are self-extracted from the plasma thanks to the energy acquired in the sheath. Self-extraction of negative-ions can affect processes like sputtering, where the negative-ions formed on the cathode bombard the layer being deposited. In applications such as negative-ion sources for accelerator or fusion devices, it is taken advantage of negative-ion surface production. A low work-function material (usually caesium-covered metals) is in contact with the plasma and greatly enhances negative-ion production because of the low energy required to extract an electron from the surface. However, caesium free negative-ion sources would be greatly valuable for fusion applications because of the strong maintenance constraints induced by caesium injection.

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Boron-doped polycrystalline diamond (BDD) and highly oriented pyrolytic graphite (HOPG) surfaces were exposed to low pressure hydrogen plasma. The relative yields of surface-produced H− ions were measured by an energy analyser quadrupole mass spectrometer. The highest H− yield was obtained at 400 °C for a BDD surface and at room temperature for an HOPG surface. At low ion bombardment energy, the maximum yield on a BDD surface is about 5 times higher than that on an HOPG surface, which has been the best carbon material so far for surface production of H− ions in caesium-free plasma. Raman measurements revealed surface modifications after plasma exposure.

In previous works, surface-produced negative-ion distribution-functions have been measured in H 2 and D 2 plasmas using graphite surfaces (HOPG). In the present paper we use the SRIM software to interpret the measured negative-ion distribution-functions. For this purpose the distribution-functions of backscattered and sputtered atoms arising due to the impact of hydrogen ions on a-CH and a-CD surfaces are calculated. The SRIM calculations confirm the experimental deduction that backscattering and sputtering are the mechanisms of the origin of the creation of negative ions at the surface. It is shown that the SRIM calculations compare well with the experiments regarding the maximum energy of the negative ions and reproduce the experimentally observed isotopic effect. A discrepancy between calculations and measurements is found concerning the yields for backscattering and sputtering. An explanation is proposed based on a study of the emitted-particle angulardistributions as calculated by SRIM. A-IntroductionThe ITER project and its successor DEMO (first nuclear-fusion based power plant prototype) aim at demonstrating power production by magnetic confinement nuclear fusion. Plasma start-up and continuous operation in the case of a tokamak reactor require very efficient heating and non-inductive current drive systems. Neutral beam injection (NBI) which uses high power beams of fast neutral atoms of hydrogen isotopes to heat the plasma is a promising candidate. In most existing NBI systems on contemporary fusion experiments a beam of positive ions is extracted from a conventional ion source, accelerated to energies of the order of 100 keV and neutralized via charge exchange in a background of neutral hydrogen. Due to the much larger physical dimensions the NBI systems for both ITER and DEMO require neutral beam energies in the 1 to 2 MeV range where the neutralization of positive ions becomes very inefficient and only the negative ions can be neutralized with sufficient yield. Hence the development of high current negative ion sources (50A D-beam for ITER) is crucial to the development of the ITER and DEMO NBI systems. The highcurrent-density negative-ion source for ITER 1 is currently the subject of intense research