Study of physical and chemical assisted physical sputtering of beryllium in the JET ITER-like wall

Abstract

Abstract:The effective sputtering yield of Be (Y tot Be ) was determined in-situ by emission spectroscopy of low ionising Be as function of the deuteron impact energy (E in = 25 − 175eV ) and Be surface temperature ( ). The later mechanism has been clearly identified by the appearance of BeD A-X emission and quantified in cause of a temperature dependence at which the BeD practically vanishes at highest observed Be limiter temperatures. The recorded T surf dependence, obtained in a series of 34 identical discharges with ratch-up of T surf by plasma impact and inertial cooling after the discharge, revealed that the reduction of BeD is correlated with an increase of D 2 emission. The release mechanism of deuterium in the Be interaction layer is exchanged under otherwise constant recycling flux conditions at the limiter. The reduction of Y chem Be with T surf is also correlated to the reduction of the Be content in the core plasma providing information on the total source strength and Be screening. The chemical assisted physical sputtering, always present at the nominal limiter pre-heating temperature of T surf = 200 • C, is associated with an additional sputtering channel with respect to ordinary physical sputtering which is surface temperature independent. These JET experiments in limiter configuration are used to benchmark the ERO code and verify ITER first wall erosion prediction. The ERO code overestimates the observed Be sputtering in JET by a factor of about 2.5 which can be transferred to ITER predictions and prolong the expected lifetime of first wall elements.

Plasma-surface interaction in the Be/W environment: Conclusions drawn from the JET-ILW for ITER

“…Post-mortem analysis of PFCs provides complementary information though campaign integrated over all limited and diverted plasmas, resulting in a 15 times lower fuel inventory with JET-ILW in comparison with JET-C [42] and confirming the gas-balance measurements. A difference in absolute retention rates exist as the long outgassing phase between the last experiment and the actual tile analysis of about one year impacts strongly on the absolute fuel content in comparison with gas-balance studies and an outgassing period of 1.5 h [30]. The distribution of the fuel can under the assumption of toroidal symmetry be described as follows: one third of the retained fuel is found in the main chamber PFCs (5:7 Â 10 22 D on the low-field side, 2:8 Â 10 22 D on the high-field side, 2:1 Â 10 22 D on the upper wall PFCs) and two third are in the divertor (2:9 Â 10 23 D in the inner divertor Fig.…”

“…4a is this gross erosion yield depicted as function of the local electron temperature (T e ) averaged over the observation spot. Y tot Be increases with T e from 4% to more than 100% due to beryllium self-sputtering at the highest T e or impact energies E in [30]. The beryllium sputtering measurements in JET are currently applied to benchmark the Monte-Carlo code ERO taking into account the local geometry, the impact angles, impact energies and two sets of Be sputtering yields obtained from MD calculations, TRIM calculations and laboratory experiments [31].…”

“…Be [30]. The appearance of Y chem Be is likely caused by the supersaturation of the beryllium surface by the impinging deuteron flux density, which induces a transient deuterium surface coverage of more than 50%, leading to both (a) the appearance of CAPS as additional channel with the release of BeD x with x ¼ 1 .…”

“…The value at highest surface temperatures represents the surface-temperature independent bare Y phys Be without any measured CAPS as well as low deuterium content in the interaction layer due to outgassing. Further details about CAPS in the JET-ILW, MD modelling, and PISCES-B are described in [30,33,34]. However, it needs to be stressed that both, physical and chemical assisted physical sputtering, have an energetic threshold that inhibits erosion at lowest impact energies.…”

Overview of the JET results

“…The initial assumption that pure physical sputtering by energetic charge exchange neutrals is solely determining the main chamber source turned out to be invalid-low energetic ions reaching the first wall also sputtered chemically the C in JET-C. This chemical erosion at lowest energies (10 eV) is absent due to energetic physical sputtering thresholds for Beeven if chemical assisted physical sputtering has been found for Be [43]. As consequence of the lower initial impurity source, the inner divertor is only partially covered by Be and indeed intact W surfaces are found.…”