Measuring Tungsten Fuzz Growth with Transient Grating Spectroscopy

Tungsten is a candidate material for plasma facing components (PFCs) in magnetic confinement fusion (MCF) power plants and will be used for the first wall and divertor of ITER and Commonwealth Fusion System’s SPARC tokamak. When exposed to high temperatures and helium (He) plasmas, a nanostructured layer called fuzz forms on the originally smooth tungsten surface, which changes the local material properties and could potentially contaminate MCF plasmas. Transient grating spectroscopy (TGS) is a nondestructive, picosecond time scale characterization technique that reveals the material’s elastic properties, thermal diffusivity, and energy dissipation by measuring the decay of acoustic waves and temperature gratings on the surface of the material created from a laser-induced transient diffraction grating. Polished tungsten samples heated to 800°C will be exposed to a 13.56 MHz He plasma in the Dynamics of ION Implantation and Sputtering Of Surfaces (DIONISOS) experiment to show that the thermal diffusivity decreases with increasing fuzz production from two different TGS measurements. Scanning electron microscopy will confirm the fuzz microstructure evolution as a function of the plasma fluence. The first measurement will be taken on a benchtop TGS after He plasma irradiation, and the second will be in real-time on the Plasma In-situ Ion Irradiation TGS (PI³-TGS), which is installed on the DIONISOS beamline. Agreement between the benchtop and in-situ TGS measurements would point to the necessity for a diagnostic to detect the onset and severity of fuzz growth in future MCF devices using tungsten PFCs.