Further Roughing of Tungsten Surface due to Transient Heat Load by Laser Irradiation during Helium Plasma Exposure

Abstract

745 J. Plasma Fusion Res. Vol.81, No.10 (2005) 745 746 Tungsten is one of the candidates for divertor materials because of its high thermal property and low sputtering yield. However, recent experimental observations have revealed that bubbles and holes are formed on the surface by helium plasma exposure even with an incident ion energy less than the threshold energy of sputtering [1]. The formation of holes and bubbles may lead to degradation of the superior properties of tungsten, especially, its thermal diffusivity. If the thermal diffusivity is degraded, transient heat loads owing to the ELMs (Edge Localized Modes) [2] and disruption may pose serious problems, such as the melting of high-Z materials. In this paper, the transient heat load to the tungsten target during exposure to helium plasmas was demonstrated using pulsed laser irradiation. Although the heat pulse duration was several ns, which is much shorter than the characteristic time of the ELMs (<~10 −4 s), the experimental observations indicate the effect of transient temperature increases on the surface modifi cation. Experiments were performed in the divertor simulator NAGDIS-II (NAGoya university DIvertor Simulator-II). A schematic view of the experimental setup is shown in Fig. 1. The helium plasma was irradiated to a W sample situated at about a 45-degree angle to the magnetic fi eld line. The specimen was powder metallurgy W 0.2 mm in thickness. Second-harmonic pulses of an Nd:YAG laser (Continuum: SLII-10), with a wavelength of 532 nm, were used for the photon source. The pulse duration was 5–7 ns, and the pulse interval was 0.1 s, suffi ciently longer than the characteristic thermal relaxation time. The laser beam was injected through a quartz viewing port at a 90-degree angle to the magnetic fi eld line. The laser-pulse energy was measured before the viewing port, and the energy per unit area at the target was deduced by taking into account the transmission rate of the viewing port and the angle of the laser beam to the tungsten sample. SEM images of the tungsten sample exposed to helium plasmas without laser irradiation are shown in Fig. 2 (a) (from top) and Fig. 3 (a) (cross section). The ion fl ux was Γi = 1.7 × 1023 m–2/s, and the incident ion energy was 27 eV. The exposure time to the helium plasma was 1800 s; the tungsten surface temperature measured with a radiation pyrometer was ~1700 K. Micron-bubbles and holes are observed on the surface, and the penetrating depth of the surface modifi cation was 1– 2 μm. Figure 2 (b) shows SEM images of the laser irradiated tungsten surface exposed to the helium plasma under the same