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Material assessment for ITER's collective Thomson Scattering first mirror

ITER's Collective Thomson Scattering (CTS) system is a diagnostic instrument that will measure the plasma density and velocity through Thomson scattering of microwave radiation. Some of the key components of the CTS are quasioptical mirrors used to produce astigmatic beam patterns, which have impact on the strength and spatial resolution of the diagnostic signal. The mirrors are exposed to neutron radiation, which may alter the mirror properties or deform its structure. These changes may affect the collection of the scattered radiation and consequently decrease the quality of the measurements. In this work, three different materials (molybdenum (Mo), stainless steel 316L (SS-316L) and tungsten (W)) are considered for the first mirror of the CTS. The objective is to assess the suitability of these materials for this mirror and to provide a first ranking, considering the neutron radiation loads requirements defined by ITER, based on the resultant maximum Von Misses stresses and temperatures. For it, the neutron irradiation, and subsequent heat-load on the mirrors were simulated using the Monte Carlo N-Particle (MCNP) code. Based on the MCNP heat-load results, a transient thermal-structural Finite Element Analysis (FEA) of the mirror over a 400s discharge (reasonable number for computational tests, since an ITER discharge will be between 200 s and 1000 s), with and without mirror cooling, is performed. The results obtained in this preliminary analysis show that of the tested materials Mo and W are the most suitable materials for this application, being able to reliably sustain the thermal and structural stresses imposed by the neutron loads.