Response of tungsten surfaces to helium and hydrogen plasma exposure under ITER relevant steady state and repetitive transient conditions

L Buzi,M Rasinski,S Bardin,A.E Huisman,R.A Pitts,G De Temmerman,T.W Morgan,G Van Oost

doi:10.1088/1741-4326/aa81e4

Abstract

The effect of helium (He) plasma exposure, and associated surface modifications, on the thermal shock resistance of tungsten (W) under ITER relevant steady state and transient heat and particle loads was studied. W samples were exposed to steady state and pulsed He plasmas at surface base temperatures from 670 to 1170 K. The same exposures were repeated in hydrogen (H) to allow a direct comparison of the role of the ion species on the thermal shock resistance. Exposure to He plasma pulses caused the formation of fine cracking network on W samples which occurred at a higher density and smaller depths compared to H pulsed plasma irradiation. The peak temperature reached during an ELM-like plasma pulse increased by a factor ~1.45 over the 100 s of He plasma exposure, indicating a deterioration of the thermal properties. Transient loading experiments were also performed using a high power pulsed laser during He plasma exposure, showing a significant modification of the target thermal response caused by the surface damage. The effect of He-induced morphology changes on the thermal response modification was found to be very small compared to that of transient-induced damage.

Highlights

Tungsten (W) is the selected plasma-facing material for the ITER divertor due to its favorable thermal and mechanical properties, and its low sputtering yield [1, 2]
The same exposures were repeated in hydrogen (H) to allow a direct comparison of the role of the ion species on the thermal shock resistance. Exposure to He plasma pulses caused the formation of fine cracking network on W samples which occurred at a higher density and smaller depths compared to H pulsed plasma irradiation
Surface morphology and thermal conductivity changes after exposure of W samples to He steady state and transient plasmas were investigated and compared with those caused by similar exposures in H. Exposure to He plasma pulses caused the formation of a fine cracking network on W samples which occurred at a higher density and smaller depths compared to H pulsed plasma irradiation

Summary

Introduction

Tungsten (W) is the selected plasma-facing material for the ITER divertor due to its favorable thermal and mechanical properties, and its low sputtering yield [1, 2]. One of the potential issues associated with this approach is the possibility that He-induced morphology changes may lower the transient damage threshold due, in par ticular, to sub (melting) threshold ELMs. Similar issues may be experienced during the D-T operation phases where He is the ash product of the fusion reaction and will be present in the divertor plasma. Similar issues may be experienced during the D-T operation phases where He is the ash product of the fusion reaction and will be present in the divertor plasma He nano-bubble formation in the near surface is the precursor to fuzz formation at high surface temperature [4, 5]. In this paper we have conducted a systematic study, addressing the effects of He plasma exposure, and associated surface modifications, on the thermal shock resistance of W under ITER relevant steady state and transient heat and particle loads. Controlled experiments were performed with H plasma exposures to assess differences in the thermal shock behavior of He and H exposed surfaces

Experimental setup

Pulsed plasma source

Laser heating

Findings

Conclusions