ELM-induced arcing on tungsten fuzz in the COMPASS divertor region

Highlights

• The interaction of fuzzy tungsten surfaces with ELMy H-mode plasmas in the COMPASS tokamak was observed.

• Arcing was the main interaction mechanism; observed both in-situ and post-mortem.

• In the arc traces, localized melting of the fuzz was observed, retaining some porosity, without extending into the bulk.

Abstract

Materials exposed to plasma may undergo various forms of surface modifications. Among the important phenomena for tungsten - as the prime candidate plasma-facing material for fusion devices – is a formation of helium-induced fibreform nanostructure, so-called tungsten fuzz.

In this paper, we report direct observations of the interaction of the pre-prepared fuzzy tungsten surfaces with ELMy H-mode plasmas in the COMPASS tokamak as well as consequent ex-situ morphological analyses, with a particular focus on arcing as a potential erosion mechanism. Arcing events are documented from high-speed camera imaging. The sample surfaces are examined by scanning electron microscopy.

Arc traces were observed on all samples, while their number was dependent on the sample position and orientation. Inside the arc traces, localized melting and densification of the original fuzz was observed, resulting in thickness reduction. The modified structure still retained some porosity and did not extend into the bulk.

Introduction

Tungsten will be used as a plasma-facing material in the ITER divertor, and is the prime candidate material for the most severely loaded plasma facing components in future fusion reactors. When exposed to helium-containing plasma at elevated surface temperature, a fine nanostructure – often referred to as tungsten fuzz – forms on the surface. This has been reported from laboratory experiments [1] and also observed in the C-Mod tokamak [2]. Tungsten fuzz is a nanometric filamentary structure that forms on tungsten surfaces exposed to high fluxes of low-energy (20–60 eV) helium ions at temperatures above ∼900 K [3]. The formation mechanism of such structures is possibly connected to the formation and coalescence of helium bubbles in the near-surface region, inducing swelling of the surface [4]. Active surface diffusion of tungsten atoms is thought to play a key role in forming the protrusions and fine structures, as surface diffusion is generally faster than lattice diffusion [5]. The characteristic size of these nanostructures is correlated with surface temperature [6]. These affected surface layers feature very high porosity, and therefore low surface reflectivity and thermal conductivity [5], [7], [8], [9], with significant consequences for the plasma-material interaction. Various forms of plasma-induced damage can be largely different on these surfaces than on bulk, smooth material. Several erosion processes have been studied, such as sputtering, melting/evaporation due to pulsed heat loads, and unipolar arcing. Sputtering yields of fuzzy surfaces were found to be ∼5× smaller than for smooth surface, which was attributed mainly to their porous nature [5]. Because of the low effective thermal conductivity, pulsed heat and particle loads (such as Edge Localized Modes, ELMs) could cause enhanced erosion by splashing and droplet ejection [5]. It was demonstrated that unipolar arcing can be easily initiated on the nanostructure [1], which could represent an important erosion process. Arcing craters have been observed in several fusion-oriented devices [10], [11], [12], including COMPASS [13]. The conditions for arc occurrence on fuzzy tungsten were investigated in a fusion relevant environment using the PISCES-A, MAGNUM-PSI and Pilot-PSI facilities [10], [14]. However, those observations were made in linear devices using high power lasers or pulsed plasmas and a surface perpendicular to the main magnetic field, while in a tokamak, the conditions might be different due to the shallow incidence angle of the magnetic field. The goal of this study is to investigate the occurrence of arcing during ELMs and its effects on helium-induced nanostructures in the COMPASS tokamak.