Wall damage due to oblique high velocity dust impacts

TL;DR

This paper addresses wall damage in fusion devices caused by high-velocity debris ejected during runaway electron termination. It experimentally investigates oblique tungsten-on-tungsten impacts using a two-stage light-gas gun to accelerate nearly monodisperse $D_{ ext{d}} \\approx 63 \,\\mu$m dust to $v_{ ext{imp}} \\sim 2000$–$3000 \,\\mathrm{m/s}$ at incidence angles $\\theta = 0^\\circ$–$80^\\circ$, and characterizes crater depth $H$, length $L$, and width $W$ via SEM and profilometry. A key finding is that crater depth scales approximately as $H( heta) \\approx H_{ ext{n}}(D_{ ext{d}},v_{ ext{imp}}) \\cos \\theta$, while crater length and width exhibit non-monotonic dependence on $\\theta$, with maxima near $\\theta \\sim 60^\\circ$ and $\\theta \\sim 30$–$45^\\circ$ respectively; crater volume shows a shallow maximum around $\\theta \\sim 30$–$\\45^\\circ$, deviating from some MD predictions. These results provide angle-dependent empirical damage laws essential for predicting indirect wall damage from debris in tokamaks and have implications for diagnosing ejected dust properties and improving predictive modeling of runaway-electron driven explosions. The work also outlines future steps to study elevated dust temperatures and re-entry heating effects to better emulate real debris conditions.

Abstract

Runaway electron termination on plasma facing components can trigger material explosions that are accompanied by the expulsion of fast solid debris. Due to the large kinetic energies of the ejected dust particles, their subsequent mechanical impacts on the vessel lead to extensive cratering. Earlier experimental studies of high velocity micrometric tungsten dust collisions with tungsten plates focused exclusively on normal impacts. Here, oblique high velocity tungsten-on-tungsten mechanical impacts are reproduced in a controlled manner by a two-stage light gas gun shooting system. The strong dependence of the crater characteristics and crater morphology on the incident angle is documented. A reliable empirical damage law is extracted for the dependence of the crater depth on the incident angle.

Figures (9)

• Figure 1: SEM image of a damaged bulk W target after the high velocity impact of spherical $63\,\mu$m W dust (here with $1988\,$m/s and $60^{\circ}$). Nearly half of the realized impact craters are overlapping. Only the isolated craters are considered in the statistical analysis of the crater morphology.
• Figure 2: SEM image of damaged bulk W targets after the high velocity impact of spherical $63\,\mu$m W dust (here with $2451\,$m/s and $45^{\circ}$). The definition of the 2D crater figures of merit (length and width) within the elliptical approximation. The major axis is parallel to the projection of the dust impact velocity on the target surface.
• Figure 3: SEM images of damaged bulk W targets after the high velocity impact of spherical $63\,\mu$m W dust. Here for $2052\,$m/s and $30^{\circ}$ (left), $2036\,$m/s and $45^{\circ}$ (center), $1988\,$m/s and $60^{\circ}$ (right). For a near-constant impact speed and dust size, the crater morphology transitions from near-circular to near-elliptical as the impact angle increases.
• Figure 4: Optical image of a damaged bulk W target after the high velocity impact of spherical $63\,\mu$m W dust (here with $2412\,$m/s and $80^{\circ}$). At grazing angles, clear non-elliptical features emerge. The elliptical approximation is still applicable, but becomes less accurate. The shallow head feature is barely visible at the left. The projection of the dust impact velocity on the target surface is also illustrated.
• Figure 5: Optical image of a damaged bulk W target after the high velocity impact of spherical $63\,\mu$m W dust (here with $2008\,$m/s and $80^{\circ}$). Two neighboring elliptical craters that feature a shallow head. The projection of the dust impact velocity on the target surface is also illustrated.