Dust deposition on plasma-facing substrates extracted from the WEST Tokamak

Dust particles deposited during the first phase of operation of the WEST tokamak were collected and analyzed from substrates positioned at four distinct poloidal locations along the inner wall. Among various particle types composed of materials present in the vacuum vessel, previously unreported tungsten molten splashes exhibiting highly distinctive “stethoscope-like” morphologies were discovered. These unusual tungsten particles were found in large numbers exclusively on substrates located closest to the lower divertor. They display a well-defined log-normal size distribution, with average lengths ranging from 0.75 to 1.5 µm, and a clear angular alignment pointing predominantly away from the divertor. This directional distribution provides compelling evidence that they were ejected from the lower divertor region. Additionally, more conventional tungsten ellipsoidal particles were identified across all four poloidal positions. These elipsoids were most abundant near the divertor and exhibited progressively more elongated shapes and less distinct alignment patterns with increasing distance, suggesting a common origin but different transport histories. Their size distributions are also log-normal, with average diameters between 100 and 200 nm. The characteristics of both particle types (distribution, size scaling, and directionality) suggest a common origin in molten tungsten droplets expelled from the lower divertor, most likely as a result of arcing events. • W micron stethoscope-shaped splashes and sub-micron spheroids found on WEST’s inner wall. • Dust density, size, and elongation direction measured versus poloidal position. • Dust density decreases with divertor distance, pointing to arcing-driven droplet emission as the source. • Shape differences depend on the droplet size, collision angles, and impact velocities.