Studies of the release mechanisms of helium and neon and tungsten limiters in TEXTOR-94

Plasma wall interaction is one of the key problems on the way to controlled nuclear fusion. The interaction of the plasma with the plasma facing components has a direct and significant influence on the penetration depth of the atoms. The recycling properties of helium as a fusion product of DT-fusion and of neon as a promising candidate within the concept of power exhaust from a radiating plasma mantle are of special interest. In this theses the velocity distribution of helium and neon atoms has been measured from the Doppler broadening of atomic line emission and the penetration depth of atoms into the plasma has been determined from the measurements of the radial distribution of emission line intensity. For the first time three different release mechanisms of helium and neon recycling at carbon and tungsten limiters could be distinguished in a Tokamak: thermal desorption, ion induced desorption, particle reflection. Concerning the interpretation of the experimental results, especially with respect to particle reflection, the angle of incidence of helium and neon ions at the surface of the plasma facing components has a large significance. The determination of this angle of incidence has been performed solving the equation of motion in the magnetic sheath and the Debye sheath numerically. (orig.)