Pulsation Effects of Incident Ion Energy on W Fuzz Growth

Abstract

Fuzzy nanostructure growth occurs on tungsten (W) surfaces by the exposure to helium (He) plasmas. We investigated pulsation effect in the incident energy of He ions on W fuzz growth. It is shown that He irradiation contributes to the growth of fuzzy layer even if the incident ion energy was less than the threshold energy of 20 -30 eV. When the duty cycle of the pulse was 1 -10 %, 7 -8 eV He ion irradiation have contributed to the fuzz growth in addition to high (> 60 eV) energy ion irradiation, and the growth rate was enhanced. . Because the layer significantly changes the material properties from those of bulk material, the influence on the transients in fusion devices accompanied by edge localized modes (ELMs) is one of the concerns [2]. An issue to be investigated further is whether such morphology changes will actually occur on plasma facing materials in fusion devices. It is known that the growth requires the surface temperature range of 1000 -2000 K and the incident ion energy, E i , of greater than 20 -30 eV [3]. Thus, detached plasmas, in which the temperature is lower than several eV [4], are thought to have an inhibiting effect on the growth of the fuzzy structure, because E i could be lower than the threshold energy. It is of importance to investigate the effects of transients such as ELMs, because they will convey higher energy particles even for a short period of time. Transient heating effects on fuzz growth have been investigated by Yu et al. using pulsed laser irradiation [5]; it was found that an enhanced fuzz growth was identified by transient heating events. In this study, we will investigate the effect of cyclic pulsation in E i on W fuzz growth.Experiments were conducted in the linear plasma device NAGDIS-II (Nagoya divertor simulator-II). He plasmas were produced in steady state, and a W sample (0.2 mm thick) was installed in the downstream of the cylindrical plasma; E i was controlled by biasing the sample using a bipolar power supply. The sample surface was in parallel to the magnetic field. Figure 1 (a) shows a schematic of the temporal evolution of E i . The incident ion energy was pulsated cyclically from E L (< 30 eV) to E H (65 -95 eV) at a frequency, f , of 10 or 100 Hz for 1 ms; the duty cycle, D, was 1 or 10 % at f = 10 or 100 Hz,

Morphologies of co-depositing W layer formed during He plasma irradiation

“…It was revealed from laser heating experiments that an enhancement of fuzz growth occurred with transient temperature increases [14]. An ion energy pulsation experiments demonstrated that a low energy irradiation less than 20 eV also contributed to the fuzz growth if pulses existed [15]. Another issue could be the effect of re-deposition, which would be important for various issues including deuterium accumulation [16].…”

Ignition and erosion of materials by arcing in fusion‐relevant conditions

“…The incident He ion energy near the strike point of ITER divertor would be lower than the threshold (∼20 eV) for fuzz formation . However, several recent experiments conducted to investigate transient effects on a W surface have revealed that fuzz growth could be accelerated by adding pulsation to the heat load or incident ion energy . Low‐energy He ions, whose incident energy is lower than 10 eV, are especially likely to contribute to the further growth of fuzz when combined with cyclic precipitation of high‐energy ions .…”

“…However, several recent experiments conducted to investigate transient effects on a W surface have revealed that fuzz growth could be accelerated by adding pulsation to the heat load or incident ion energy . Low‐energy He ions, whose incident energy is lower than 10 eV, are especially likely to contribute to the further growth of fuzz when combined with cyclic precipitation of high‐energy ions . As the fuzz formation would lead to significant thermal and electrical property changes, these morphology changes should be regarded as an important factor when evaluating the erosion properties of divertor plates …”