Simulation of hydrogen and hydrocarbon release from W–Ta and W–C twin test limiters in TEXTOR edge plasmas

We have performed a Monte Carlo simulation of transport of atomic and molecular hydrogen and hydrocarbons released from W, Ta and C in edge plasmas of TEXTOR. The simulation derives thermal, dissociation and reflection components from the observed radial distribution of Dγ line intensity around W–Ta and W–C twin test limiters. The Dγ intensity in front of the limiter is dominated by thermal reemission of atomic hydrogen, whereas at the position well away from the limiter, dissociation of reemitted molecular hydrogen and reflection of incident hydrogen are important. The simulation allows us to estimate the surface temperature and material dependence of atomic and molecular fractions of hydrogen reemitted from the limiter. For Ta, hydrogen is reemitted as a mixture of atoms and molecules at 1200 K and the atomic fraction increases with increasing temperature (1400 K), whereas for W, the atomic release dominates the reemission process at the temperatures.