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Description
During the WEST experimental campaigns, various evolutions of the plasma-facing components (PFCs) have been observed. One of these degradations is erosion of material and redeposition on other components. These deposits have unknown but probably degraded thermal properties (low diffusivity and/or low thermal contact with the PFC). Some of these deposits created Unidentified Flying Object (UFO) during the recent experimental campaigns. These UFO could cause the premature termination of the discharge.
The aim of the present work is to improve our understanding of the evolution of the thermal properties of PFCs during their use in the WEST Tokamak. To achieve this, an experimental bench has been developed to measure the thermal properties of ITER-grade components (thermal diffusivity, thermal conductivity, etc.). This method can be applied directly to a component (without cutting or modifying it) to study its ageing and characterization of the deposits will be part of these analyses. As they are thin (from µm to tens of µm) a front face flash method is developed with laser heat source (10ns), or flash lamp (80µs) to identify parameters by inversion of photothermal measurements.
The present work exposes the thermal properties of theses deposits. Due to significant variations of thickness and composition, three zones are chosen on PFCs exposed to plasmas during different campaigns. Namely, a zone of high thickness (several tens of microns), an intermediate deposit zone (a few microns) and an oxidation zone (hundreds of nanometres). The thermal model is taken in consideration the surface roughness and the composition of the deposit. The identified thermal properties offer the possibility to improve existing thermal model used to access the temperature of the top surface of the PFC, but also the gradient of temperature in the deposit thickness.