Speaker
Description
The WEST tokamak hosts a comprehensive set of diagnostics dedicated to monitoring heat flux, radiation, impurity transport, and local plasma parameters, providing detailed characterisation of its scrape-off layer (SOL). Among the available diagnostics, mobile Langmuir probes positioned at strategic SOL locations can be equipped with sample holders, enabling controlled exposure of material samples to extreme heat fluxes (10⁶–10⁸ W/m²) and ion fluxes (10²²–10²⁴ m⁻²s⁻¹). These capabilities allow WEST to operate as a testbed for evaluating advanced materials under plasma conditions that are otherwise challenging to reproduce. In particular, the accessible SOL heat and ion fluxes are suited for studying the thermal response and ablation dynamics of High-Temperature Ceramics (HTCs), whose excellent thermomechanical properties position them as promising heat-shield ablators for atmospheric re-entry vehicles. Two dedicated experiments have been carried out to test different HTCs thanks to the exploitation of two custom-made sample holders, fitted on two different exposure devices, targeting respectively the lower and the upper end of the accessible heat and ion flux range. The first system performs short plunges up to the separatrix on the top of the machine using a hydraulic drive, and exposed 10 BN samples to cumulated fluences up to 10²⁴ part/m². The second device, installed on the divertor, exposes a single sample to strike-point fluxes thanks to an in-situ magnetic drive, consisting of a coil that moves when a voltage is applied to it, due to the Lenz force in the tokamak magnetic field. Using the divertor device and a dedicated magnetic configuration, the ablation of a polycrystalline SiC sample has been successfully achieved, as confirmed by strong Si emission lines in VUV and visible spectroscopy, observed in both core and edge channels immediately after the exposure trigger, indicating high sublimation rates and motivating further plasma transport studies thanks to the unusual impurity release. Furthermore, divertor upper-view IR imaging shows a saturation of the sample temperature at ~1400 °C (underestimated by limited spatial resolution), behaviour consistent with a phase change. The average parallel heat flux on the sample’s leading edge, estimated with Fiber Bragg Gratings and flush-mounted divertor Langmuir probes, is about 45 MW/m². In-situ Langmuir probe collectors, together with complementary diagnostics, provide a complete record of the plasma conditions during the samples’ exposure. The experimental protocol is presented, as well as the analysis and interpretation of the data from the relevant diagnostics, and finally the post-mortem analyses on different HTCs samples.