Operational conditions in a W-clad tokamak

Abstract

Key word codes: C0600, T1000, P0500, D0500, F0400Experiments with tungsten plasma facing components (PFCs) are performed in the ASDEX Upgrade divertor tokamak and the area covered by W-PFCs has been increased steadily since 1999 reaching 85% for the 2005/2006 campaign. The configurations chosen are W coatings on graphite and CFC. The different locations are subject to different power loads and erosion yields. This is taken into account by selecting different thicknesses in the W-coating manufactured either by physical vapour deposition or vacuum plasma spraying. Power loads in excess of 15 MW/m 2 can be handled in this way. The experiments on ASDEX Upgrade show that plasma operation is feasible with walls and divertor surfaces mostly covered with tungsten, but also reveal critical issues: Fast particles from plasma heating can play a crucial role in W erosion and particle transport must be kept high enough to overcome high impurity content and to prevent central impurity accumulation.

Mechanical properties of Y2O3-doped W–Ti alloys

“…4 for both alloys. It shows wide distributions in both alloys with average grain sizes of 10 15 lm in W 2Ti 0.5Y 2 The density of the alloys was 16.545 ± 0.006 g/cm 3 and 16.76 ± 0.01 g/cm 3 for the W 2Ti 0.5Y 2 O 3 and the W 4Ti 0.5Y 2 O 3 , respectively. They were slightly lower than those reported [4] for fully dense materials, leading to porosities of 1.5% and 0.8% for W 2Ti 0.5Y 2 O 3 and W 4Ti 0.5Y 2 O 3 , respectively.…”

Material mixing on plasma-facing components: Compound formation

“…Copper melting inside the castellation (T m(Cu) = 1083°C) proves that the temperature exceeded 1000°C thus making the volatilization of oxides possible. Finally, taking into account all experimental findings and thermodynamic data one may assume that the most probable pathway leading to the WO 2 deposition was initiated by the reaction of W with water vapour forming WO 3 or H 2 WO 4 [23] followed by the reduction to WO 2 which eventually condensed on the surface. Fig.…”

“…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].…”

Transport analysis of high radiation and high density plasmas in the ASDEX Upgrade tokamak

“…The work presented here has been carried out in ASDEX Upgrade (AUG), a medium size tokamak with full metallic wall. In fact since 2007 all plasma facing components are tungsten-coated [3]. Divertor cooling by nitrogen seeding has been extensively investigated, see e.g.…”