Observed proton beam induced disruption of a tungsten powder sample at CERN

Fluidized tungsten powder has been proposed as a potential target technology for particle accelerator applications with very high power highly focused pulsed beams. This has motivated a series of experiments carried out at the HiRadMat facility at CERN to study the response of a tungsten powder sample to an impinging high energy proton beam pulse. The main observation was that of beam induced lifting of the powder sample which was recorded by high speed video. In this paper we consider three mechanisms to explain the observed powder lift including aerodynamic, thermal expansion and induced charge effects. Simulations of the aerodynamic effect revealed that this could not explain the magnitude of the observed eruptions especially during tests carried out with the powder in a vacuum. Thermal expansion of tungsten particles giving rise to the eruption seems implausible due to the propensity for the powder to absorb perturbations. We show that the observations can be explained by a Coulombic eruption of the tungsten particles. The high energy beam leaves a pattern of charge distributed in the poorly conducting powder sample, which creates an electric field that consequently results in a force acting on the individual charged particles. We calculate the charge deposited, the electric field and the resulting acceleration and show that this is a plausible mechanism for causing the observed eruptions. We believe the response of a granular conductive sample to an incident proton beam has not previously been explained.