ELASTIC BACKSCATTERING OF HYDROGEN MOLECULES FROM A PLASMA

Neutral gas measurements in the divertors of DIII-D and JET revealed very high neutral pressures on the wall in the private flux region in excess of expectations from DEGAS simulation for DIII-D and D(alpha) intensity measured at the same position in JET. This problem may be solved, if one -takes into account an additional scattering process between H-2 molecules and plasma ions. This is caused by the polarization of molecules in the field of ions and becomes important at low temperature. Its consequences are: - Not all neutrals starting at the wall will be absorbed by the plasma. - Neutrals between plasma and wall will be heated and obtain an isotropic velocity distribution. Using a Morse-like potential, collision data for elastic scattering of H-2-H+ are calculated which can be used in Monte Carlo calculations, an example of which will be shown.