Abstract
JET is used as a test bed for ITER, to investigate beryllium migration which connects the lifetime of first-wall components under erosion with tokamak safety, in relation to long-term fuel retention. The (i) limiter and the (ii) divertor configurations have been studied in JET-ILW (JET with a Be first wall and W divertor), and compared with those for the former JET-C (JET with carbon-based plasma-facing components (PFCs)). (i) For the limiter configuration, the Be gross erosion at the contact point was determined in situ by spectroscopy as between 4% (Ein = 35 eV) and more than 100%, caused by Be self-sputtering (Ein = 200 eV). Chemically assisted physical sputtering via BeD release has been identified to contribute to the effective Be sputtering yield, i.e. at Ein = 75 eV, erosion was enhanced by about 1/3 with respect to the bare physical sputtering case. An effective gross yield of 10% is on average representative for limiter plasma conditions, whereas a factor of 2 difference between the gross erosion and net erosion, determined by post-mortem analysis, was found. The primary impurity source in the limiter configuration in JET-ILW is only 25% higher (in weight) than that for the JET-C case. The main fraction of eroded Be stays within the main chamber. (ii) For the divertor configuration, neutral Be and BeD from physically and chemically assisted physical sputtering by charge exchange neutrals and residual ion flux at the recessed wall enter the plasma, ionize and are transported by scrape-off layer flows towards the inner divertor where significant net deposition takes place. The amount of Be eroded at the first wall (21 g) and the Be amount deposited in the inner divertor (28 g) are in fair agreement, though the balancing is as yet incomplete due to the limited analysis of PFCs. The primary impurity source in the JET-ILW is a factor of 5.3 less in comparison with that for JET-C, resulting in lower divertor material deposition, by more than one order of magnitude. Within the divertor, Be performs far fewer re-erosion and transport steps than C due to an energetic threshold for Be sputtering, and inhibits as a result of this the transport to the divertor floor and the pump duct entrance. The target plates in the JET-ILW inner divertor represent at the strike line a permanent net erosion zone, in contrast to the net deposition zone in JET-C with thick carbon deposits on the CFC (carbon-fibre composite) plates. The Be migration identified is consistent with the observed low long-term fuel retention and dust production with the JET-ILW.
Highlights
The understanding of material migration, and the process cycle of material erosion, transport and deposition, is one of the key issues for a successful and safe operation of the ITER tokamak and a future fusion reactor
The current understanding of the material migration in the JET-ILW in the divertor configuration can be described as follows (figure 6(a)): neutral Be and BeD are eroded by physical sputtering and chemically assisted physical sputtering (CAPS), respectively, from the recessed main chamber wall equipped with Be plasma-facing components (PFCs)
JET equipped with a Be first wall is an ideal test bed for ITER, for studying beryllium erosion and migration paths as well as verifying plasma–surface interaction codes such as ERO and WallDYN
Summary
The understanding of material migration, and the process cycle of material erosion, transport and deposition, is one of the key issues for a successful and safe operation of the ITER tokamak and a future fusion reactor. Fusion 55 (2015) 063021 cycle is associated with the lifetime of the first-wall material components, the so-called plasma-facing components (PFCs), under erosion, and with the safety aspect in relation to longterm tritium retention The latter is an issue both for current fusion devices and for ITER, dominated by the co-deposition of tritium with eroded material [1]. The exchange of PFCs, as in ASDEX Upgrade from graphite to tungsten [6], and recently in JET from the carbonfibre composite to beryllium (Be main chamber) and tungsten (W divertor) [7] provides the ideal test bed on which to verify the physical assumptions Both devices demonstrated a reduction in fuel retention and transport to remote areas, which underlined the fact that with carbon, chemical erosion at low or thermal impact energy (ions and neutrals) dominated the material migration cycle. Brief conclusions drawn for ITER from JET and a summary (section 6) complete this contribution
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