Fuel retention in impurity seeded discharges in JET after Be evaporation

Abstract

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 2.0g 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, impurity fluxes, and divertor radiation have been studied in ELMy H-mode plasmas (B t = 2.7T, I p = 2.5MA, 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 40 whereas the C flux decreased by ~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 1.94×10 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 for 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 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 exposures time, and both contributors causing an increase of 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.

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

“…This heat load reduction lasts for the following shots with no N injection, indicating the presence of substantial remaining N in the device. This N legacy effect was already observed in other C-wall devices such as JET [36]. A possible explanation for the N lasting for a long time in the vessel is surface dilution of the PFC material by nitrogen leading either to N-containing layers or to nitride formation [37].…”

First nitrogen-seeding experiments in JET with the ITER-like Wall

“…While visual spectroscopy indicates the formation of ND molecules, which in turn suggests the production also of ND 3 Table 1). The bottom graph shows the shot-integrated partial pressures for mass to charge ratios indicative of (deuterated) ammonia.…”

Basic considerations on the pump-down time in the dwell phase of a pulsed fusion DEMO

“…The resulting equation can be solved numerically. It is remarkable that in spite of the different material mix a single value for the decay index n of ≈0.75 was found for several carbon machines (early JET, ASDEX, TFTR) and for JET with the beryllium wall and carbon divertor [17], as well as, just recently, for JET with the ITER-like wall [18,19]. In order to validate the power law expression for tungsten outgassing, and, hence the applicability of the equation above, we analysed the outgassing after plasma shot #28653 in AUG, which has a full tungsten wall.…”