A comparative study of H- and D- production on graphite surfaces in H2 and D2 plasmas

Continuing our previous studies concerning H- negative ion generation mechanisms on graphite surfaces in hydrogen plasma, we studied D- surface generation in deuterium plasma. We measured the surface produced D- ion distribution functions (IDF) and compared them with H- ones. D- formation mechanisms are identical to H- ones. The backscattering of an incident positive ion as a negative ion accounts for the D- IDF high energy part, while the sputtering of an adsorbed deuterium atom as a negative ion and the backscattering mechanism are the two superimposed mechanisms explaining the D- IDF main peak. D- IDF maximum energy is always lower than that of H-. This isotopic effect is attributed to the higher energy released on the surface in the case of deuterium positive ion impact on graphite. The variations in the D- relative production rate versus positive ion energy are similar to those for H-. In particular, the sputtering threshold is around 20 eV and its maximum is around 80 eV. When the positive ion energy is around 80 eV, the sputtering mechanism accounts for more than 75% of D- negative ion production.