Fuel retention in impurity seeded discharges in JET after Be evaporation

TitleFuel retention in impurity seeded discharges in JET after Be evaporation
Publication TypeJournal Article
Year of Publication2011
AuthorsS. Brezinsek, T. Loarer, K. Krieger, S. Jachmich, M. Tsalas, I. Coffey, H.G Esser, T. Eich, W. Fundamenski, C. Giroud, S. Grunhagen, A. Huber, U. Kruezi, S. Knipe, G.P Maddison, K. McCormick, A.G Meigs, P. Morgan, V. Philipps, G. Sergienko, R. Stagg, M.F Stamp, F.L Tabares
JournalNuclear Fusion
Date PublishedJul
Type of ArticleArticle
ISBN Number0029-5515

Preparatory experiments for the ITER-Like Wall in JET were carried out to simulate the massive Be first wall by a thin Be layer, induced by evaporation of about 2.0 g 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 of 5 in the partial pressure owing to the evaporation. The evolution of wall conditions, impurity fluxes and divertor radiation have been studied in ELMy H-mode plasmas (B(t) = 2.7T, I(p) = 2.5 MA, P(aux) = 16MW) whereas a non-seeded reference discharge was executed prior to the evaporation. The in situ measured Be flux at the midplane increased by about a factor of 40 whereas the C flux decreased by similar to 50% in the limiter phase of the first discharge with respect to the reference, but 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 four discharges were employed for a gas balance analysis providing a D retention rate of 1.94 x 10(21) Ds(-1) which is comparable to rates with C walls. 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 is overestimated with respect to steady-state conditions like that of the ILW. Moreover, C was only moderately reduced and co-deposition of fuel with eroded Be and C occurs. The lower C content leads to a minor reduction in divertor radiation as the reference phase prior to seeding indicates. N adds to the radiation of D and the remaining C, and the N content rises due to the legacy effect which has been quantified by gas balance to be 30% of the injected N. C radiation increases with exposure time, and both contributors cause an increase in the radiated fraction in the divertor from 50% to 70%. The radiation pattern suggests that N dominates the increase in the first discharges though C is still the dominating radiator. Therefore, the validity of a proxy of the Be first wall by a thin Be layer is limited and restricted to plasma operation directly after the Be evaporation.







Alternate TitleNucl. Fusion

Go back one page.