Growth, characterization and adhesion of thick deposited tungsten layers under transient loading in Magnum-PSI

The recent decision by ITER to move from a Be to a W first wall has significant implications for material transport and deposition, and can be expected to lead to increased W layer growth rate in deposition regions, which may flake leading to plasma disruption. In this work a novel sputtering set-up was implemented in the Magnum-PSI linear plasma device to create W layers under ITER relevant conditions. A W plate was inserted into the plasma edge and biased to cause sputtering of W in an Ar plasma. This sputtered W was entrained by the plasma and deposited on an array of mixed W and Mo targets at <300 oC. The deposited layer thickness was later determined using weight gain of the targets, FIB-SEM cross sections and RBS. This showed a strongly inhomogeneous deposition profile with layer thicknesses varying from 100’s nm up to >=50 μm over a lateral region of around 3 cm diameter. Analysis of the layers by EDX, RBS, XRD and indentation testing showed them to be high purity dense W layers with mixed microstructure, forming columnular grains at greater thicknesses, with properties typical for W thin film growth. IR camera images showed flaking and delamination occurring during the deposition process, initiating at the layer-substrate interface and at imperfections or edges, with the flaking region growing over time. Films deposited on rough targets were better adhered than on polished ones, and high deposition rates were more likely to lead to flaking. Subsequent loading of the grown layers under simultaneous D+ plasma and ELM-like pulsed laser loading showed flaking to occur on previously adhered but poorly attached layers, as well as melt blobs forming at the edges of such layers. The results overall indicate that conditions in ITER should be favourable for growing well adhered layers, but that flaking may still occur and requires further evaluation.</p>