Production of negative ions at graphite surface in hydrogen plasmas

Production of hydrogen negative ions is of great interest for controlled fusion. Indeed neutralized beams of negative ions are used to heat plasma in fusion reactors. Up to now, negative ion sources use cesium surfaces but an important research effort is undertaken to go towards cesium free sources. Dissociative attachment on vibrationnally excited hydrogen molecules is known to be the main H$^{-}$ formation mechanism. Graphite surfaces are expected to give an important vibrationnal excitation through H recombination. Therefore graphite is a good candidate for next generation negative ion sources. In this work we show that under positive ion bombardment, a huge number of negative ions are produced on graphite surface. Our goal is to understand this unexpected negative ion production mechanism. In this aim, we put a graphite sample (HOPG) in a helicon reactor, in front of a EQP300 mass spectrometer. The sample is negatively biased with respect to plasma and negative ions energy distribution functions are recorded and analysed versus sample bias, plasma power and pressure, percentage of hydrogen in H$_{2}$-Ar mixtures.