Fuel Retention in Discharges with Impurity Seeding after Strong Be Evaporation in JET
Preparatory experiments for the ILW were carried out to simulate the massive Be first wall by a thin Be layer, induced by evaporation of 2.0g Be, and to study its impact on fuel retention and divertor radiation with reduced C content and N seeding. Residual gas analysis reveals a reduction of hydrocarbons by one order of magnitude and of O by a factor 5 in the partial pressure owing to the evaporation. The evolution of wall conditions and the increase of C radiation with constant N radiation have been studied in a series of ELMy H-mode plasmas (2.7T, 2.5MA, P=16MW) whereas a non-seeded reference discharge was executed prior to the evaporation to document the initial wall fluxes. The Be flux at the midplane, measured in-situ by spectroscopy, increased by a factor 20 whereas the C flux decreased by ~50% in the limiter phase of the first discharge. Erosion of the Be layer and partial coverage with C takes place quickly. To make best use of the protective Be layer, only the first 4 discharges were employed for a gas balance analysis of the D and N inventory. The major fraction of the recovered gas is D which leads to a retention rate of 1.94¥1021Ds-1 - comparable to rates with C walls under similar plasma conditions. But the Be evaporation provides a non-saturated surface with respect to D and short term retention is not negligible in the balance; the measured retention rate is overestimating retention with respect to steady-state conditions. Moreover, C was only moderately reduced and continuous co-deposition with eroded Be and C occurs. 3.0% of the gas recovered is N; representing a legacy equivalent to 30% of injected N. The lower C content leads only to a minor reduction in divertor radiation as the reference phase prior to seeding indicates. N adds to the radiation of D and remaining C and the N content rises due to the legacy effect. Also C radiation increases with exposures time, and both contributors causing an increase of the radiation fraction in divertor from 50% to 70%. The radiation pattern suggests that N dominates the increase in the first discharges.