H - Formation by scattering of hydrogen atoms/ions on carbonaceous surface Y. Xiang, H. Khemliche, A.Momeni, P. Roncin Groupe E L’Institut Science Moléculaire d’Orsay (ISMO) Université Paris-Sud 11 7 Mars 2011 La journée de l’EDOM

Motivation  Charge transfer, neutralization and formation of ions Plasma-wall interaction Divertor physics Negative ion source  ITER ( International Thermonuclear Experimental Reactor ) Heat plasma~150 million °C Maintain kinetic energy

Why negative ions source? Neutral beam D° D+, D- Residual Ion dump Neutraliser Ion source Residual Ion deflection Accelerator Vacuum cell with Cryo pumps Shutter Insulating gate Vacuum pump ~10-30 m Plasma ITER Given or taken? E~1 MeV  Previous generation JET (100 keV capture) H+ -> H°  ITER 1 MeV H- ->H°

How could make an efficient negative ion source?  Caesiated surface Too expensive for all the reactor Poison the plasma- contamination  Metal surface—capture electron 2.1 eV Decrease work function Metal Isolant Potentiel image : V ~ -1/(4.R) Potentiel Coulombien : V ~ -1/(R) Métal HOPGCB Semi-metal (conductor) work-function ∼ 5 e Deep valence band Low density state at fermi level

17 detectors working in coincidence Å 20 meV < E  < 10 eV 200 < E 0 < eV

Production of H - on diamond Diamond CVD (chemical vapor deposition), naturelly hydrogenated - gap de  5.5 eV - very deep valence band - negative electron affinity (-1 eV), depending on H surface coverage Projectile E=1 keV Fraction of H - (%) H+H+ 2.5 ± 0.5 H°3.0 ± 0.8 H2+H ± 0.5 Conclusion : diamond CVD - resonant neutralization of H + - formation of H - by capturing electron from moved affinity level - H- survival thanks to the forbidden band Resultats of H 2 + agree well with the reference (Wurz P., Schletti R. and Aellig M.R., Surf. Sci 373, 56, 1997) BC gap H - (0.75eV) BV H° (13.6eV)

Production of H - on graphite Graphite HOPG - semi-metal (conductor) - work function  4.6 eV - deep valence band CB H - (0.75eV) VB H° (13.6eV) at the fixed incidence (  1.5 °), the rate of H- increase with total energy Both V ⊥ and V // are incresed

Conclusion  The first results from diamond are disappointing  Uncertainty level of hydrogenation (->temperature variations)  The trend of H- fraction for graphite is quite different  Results on electron emission to investigate the role band gap

Perspective  Extend our work on graphite and possibly on hydrogenated diamond Exploit energy loss data in coincidence with electron emission Go to larger incidence angles Investigate graphite with H and defects  Investigate other carbon based materials (C60…)  Inelastic Diffraction of neutral H° Momentum distribution of the quantum state H° + Exciton H° + electron Reorientation

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