Abstract
Graphite is a useful low-Z material for limiters and divertor plates. Hydrogen bombardment results in trapping or, at higher temperatures, in beam-enhanced detrapping. The amount trapped is a function of the fluence φ, target temperature θ, and energy of the impinging ions E0. By means of the usual sheath model and trapping data from the literature the particle re-emission coefficient, r, is determined as a function of the power density at the target, p, plasma temperature in front of the target, T, and time. Largely irrespective of p and T, r has a maximum value when the surface temperature is around 350 ° C. The maximum value of r rises as p and T. For T = 20 eV (normal incidence) and p = 3.5 MW/cm2, for instance, it is r(max)−1 = 3.6 × 10−3. Application of the results to ASDEX Upgrade indicates that the use of graphite target plates is compatible with density control requirements.
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