Plasma Formation Thresholds at Material Surfaces Under High-current Electron Beam Bombardment

This paper presents the energy density deposition thresholds required for plasma formation on the surfaces of tungsten, tantalum, copper, and stainless steel targets. Experiments were conducted using the irradiation of targets bombarded with high-current electron beams of up to 600 GW/m 2 power density. The electron beam was generated in a vacuum diode powered by a high-voltage pulse of up to ~250 kV with a duration of up to ∼600 ns. The radial distribution of the electron current density was determined using an array of collimated Faraday cups and X-ray diagnostics. The energy deposition thresholds for plasma formation were determined using the ion time-of-flight method, as well as time- and space-resolved spectroscopy. Additionally, scanning electron microscopy and electron backscattering diffraction were used to analyze the morphology of the targets before and after electron beam bombardment. The onset of plasma formation was observed prior to expected material melting across all ‎the investigated materials, and the thresholds of energy density deposition necessary for the plasma formation of the studied materials were determined. The results of these experiments are useful for predicting potential damage to fusion reactor walls and propulsion systems.