Effect of ELM pacing on morphology evolution and erosion of tungsten as a plasma-facing material in a fusion environment

Abstract

Mitigation of transient plasma events in a future fusion device is necessary to minimize thermal damage on surrounding plasma facing components (PFCs). Intense heat loading caused by an edge-localized mode (ELM) can lead to melting and splashing of the PFC surface. ELM mitigation involves triggering these events at higher frequencies, which lowers the effective heat flux imparted on the material. Unfortunately, concurrent, high-flux He+ ion irradiation on tungsten (W), which is the leading candidate material for future PFCs, has been shown to drive the development of an arborescent nanostructure, known as fuzz. Large drops in thermal conductivity and mechanical strength with fuzz formation necessitate a reconsideration of the impact of ELM mitigation on material damage and erosion. Three different types of experiments designed to characterize the synergistic effect between He+ ion irradiation and pulsed heat loading (replicated with a pulsed millisecond laser) were conducted in the UHFI-II facility at CMUXE.W samples exposed to pulsed heat loading underwent roughening in the form of a shale-like surface morphology, regardless of frequency. The addition of simultaneous He+ ion irradiation with pulsed heat loading promoted splashing of molten material and caused significant pore formation. Increasing the frequency to 10 Hz produced a smoother W surface. However, when the total average laser power was held constant, increasing the frequency decreased the laser energy density, which significantly decreased surface melting and enhanced early stage fuzz formation. Results obtained raise an important question of whether ELM mitigation parameters can be tuned to anneal out any He-induced surface structuring without causing unsustainable splashing of molten material.