Overview of material re-deposition and fuel retention studies at JET with the Gas Box divertor

Abstract

In the period 1998—2001 the JET tokamak was operated with the MkII Gas Box divertor. On two occasions during that period a number of limiter and divertor tiles were retrieved from the torus and then examined ex situ with surface sensitive techniques. Erosion and deposition patterns were determined in order to assess the material erosion, material migration and fuel inventory on plasma facing components. Tracer techniques, e.g. injection of 13C labelled methane and tiles coated with a low-Z and high-Z marker layer, were used to enhance the volume of information on the material transport. The results show significant asymmetry in the distribution of fuel and plasma impurity species between the inner (net deposition area) and the outer (net erosion) divertor channels. No significant formation of highly hydrogenated carbon films has been found in the Gas Box structure. The important processes for material migration, and the influence of operation scenarios on the morphology of the deposits are discussed. Comparison is also made with results obtained following previous divertor campaigns.

Material mixing on plasma-facing components: Compound formation

“…Surface morphology was recorded with high-resolution scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscope (EDS) used for qualitative examination of deposits. The composition was determined quantitatively with accelerator-based ion beam analysis (IBA) using a 3 He + (0.7-1.5 MeV) beam for deuterium and a H + beam (0.6-1.75 MeV) or 4 He + (1.5 MeV) for boron, carbon, oxygen, silicon and tungsten and other metals (e.g. copper).…”

“…This will be the case in ITER where several elements are proposed for plasma-facing components (PFC) [1]. As a result of material erosion, migration and redeposition, so-called mixed material layers are formed [2][3][4]. The major issues in studies of material mixing are: (a) the impact on fuel retention, (b) the modification of PFC surface composition (chemical and crystallographic structure) and, as a consequence, (c) the degradation of thermo-mechanical properties which are decisive for behaviour of PFCs under high power loads.…”

“…In the latter case, the formation of low melting point and/or brittle phases is of particular importance because their presence would limit the lifetime of components. While the fuel retention in mixed layers has been studied in detail at several laboratories [2][3][4][5][6], the data on phase formation on real components from tokamaks are rather sparse. The systems of particular interest for ITER are: (i) tungsten compounds with carbon, beryllium, oxygen and (ii) beryllium with carbon and oxygen [7].…”

Dynamics of erosion and deposition in tokamaks

“…For MkII-GB, the programme comprised a considerable fraction of configurations with the SPs at the vertical targets. However, the septum, separating the inner and outer divertor legs and hindering particles to cross the PFR, may be the reason there was no significant deposition in the outer louvre [15]. It was the MkII-SRP divertor, for which for the first time a substantial amount of deposition was observed in the outer louvre [16].…”

“…Results of post-mortem observations and refined experimental techniques, such as 13 CH 4 tracer injection, in ASDEX Upgrade [10,11,12], DIII-D [13,14], JET [4,5,15,16] and JT-60U [17,18] have led to a general understanding about the major pathways of material in divertor tokamaks. Protruding elements of the main chamber wall have been identified as the main source of material erosion in machines with carbon PFCs.…”