Structural evolution of tungsten surface exposed to sequential low-energy helium ion irradiation and transient heat loading

摘要

Highlights ? Surface melting and mass loss by ELM-like heat loading were measured on W fuzz. ? Both laser and electron beam loading were used to replicate type-I ELMs. ? Surface melting on fuzz samples is driven by the local conglomeration of W fibers. ? Increased penetration depth of electron beam reduces degree of surface melting. ? In situ mass loss measurements reveal exponential increase in particle ejection. Abstract Structural damage due to high flux particle irradiation can result in significant changes to the thermal strength of the plasma facing component surface (PFC) during off-normal events in a tokamak. Low-energy He + ion irradiation of tungsten (W), which is currently the leading candidate material for future PFCs, can result in the development of a fiber form nanostructure, known as “fuzz”. In the current study, mirror-finished W foils were exposed to 100eV He + ion irradiation at a fluence of 2.6?×10 24 ionsm ?2 and a temperature of 1200K. Then, samples were exposed to two different types of pulsed heat loading meant to replicate type-I edge-localized mode (ELM) heating at varying energy densities and base temperatures. Millisecond (ms) laser exposure done at 1200K revealed a reduction in fuzz density with increasing energy density due to the conglomeration and local melting of W fibers. At higher energy densities (～ 1.5MJm ?2 ), RT exposures resulted in surface cracking, while 1200K exposures resulted in surface roughening, demonstrating the role of base temperature on the crack formation in W. Electron beam heating presented similar trends in surface morphology evolution; a higher penetration depth led to reduced melt motion and plasticity. In situ mass loss measurements obtained via a quartz crystal microbalance (QCM) found an exponential increase in particle emission for RT exposures, while the prevalence of melting from 1200K exposures yielded no observable trend.