First in operando transient grating spectroscopy measurements on tungsten during high flux plasma operation in PISCES-RF

This work details the construction and first in operando transient grating spectroscopy measurements conducted on PISCES-RF during plasma operation. A preliminary study on a tungsten sample with varied plasma species (Ar and D${}_2$), ion flux (4 × 10²¹ to 6 × 10²² ion/m${}^2$/s), ion energy (5 to 80 eV), and surface temperature (20 to 200 °C) is presented. Prior to plasma exposure, the thermal diffusivity and surface acoustic wave (SAW) speed, (6.9 ± 0.2)$\times$10⁻⁵ m${}^2$/s and (2.66 ± 0.02) km/s, were measured in situ with a thermal grating period of 12.5 $\mu \text{m}$. During Ar plasma exposure, these two quantities vary according to the sample surface temperature. At low flux and varied ion energy, D${}_2$ plasma exposure also results in thermal diffusivity and SAW speed varying with surface temperature alone. At high flux, D${}_2$ plasma exposure results in no convincing change to the thermal diffusivity, but the SAW speed is reduced. After returning to 20 °C, post plasma exposures, the thermal diffusivity and SAW speed almost recover to the initial value prior to exposure.