Investigation of European tungsten materials exposed to high heat flux H/He neutral beams

Abstract

Tungsten is being considered as a potential plasma-facing material for future fusion devices, primarily due to its low erosion rate and high heat resistance. The intrinsic problem of this material, the brittleness even at elevated temperatures, requires the development and assessment of new tungsten materials.In the frame of the European material development programme for future fusion power plants W containing 5 wt.% Ta as well as uniaxially forged ultra-high purity W and powder injection moulded W were assessed in a neutral beam high heat flux test facility at IPP Garching.The investigation of the morphology modification of tungsten, occurring during heat loading using H and He particles, which simulates the expected divertor operation conditions, is indispensable in order to develop reliable plasma-facing materials. The effects seen, erosion, gas retention and cavity formation, depend on both the loading conditions and the operating temperature.This contribution presents a comparative study of the surface morphology changes of different W materials under hydrogen and helium beam loading to surface temperatures between 1500°C (1773 K) and 2000°C (2273 K), using actively cooled mock-ups. Loading is performed with pure H and mixed 94% H/ 6% He beams (φ=4 × 10 21 m -2 s -1 ), resulting in 10 MW m -² thermal load. Pulse durations of 30 s are applied to achieve fluences up to 3×10 25 m -2 under stationary temperature conditions.

Tungsten erosion under combined hydrogen/helium high heat flux loading

“…The accuracy of this analysis is in the range of a few hundred nanometres. During the loading the higher temperature samples show substantial grain growth from initially a few microns typical size to about 60 microns after loading for one hour at 1500°C or 2000°C [14]. For such a grain size our method analyses the erosion behaviour of one individual grain only.…”

“…It is presently not clear whether these cavities are gas filled or not. The cavity size distribution for 1500°C and 2000°C has been analysed in [14]. It was found that for 1500°C the size distribution is bimodal with one maximum at 50 nm and another one at 200 nm.…”

Effects of temperature on surface modification of W exposed to He particles

“…Under these conditions He-induced surface damage similar to that of sample LT (blistering) can be expected. For loading with a mixture of H and He, the He component will dominate the evolution of the surface morphology [18]. The He fraction of the particle bombardment in a fusion divertor is very nevertheless low (just 5-10 %).…”

Thermal shock behaviour of H and H/He-exposed tungsten at high temperature

“…A Gaussian beam was used that has a particle flux of 3. [11]. The energy of the impinging particles in GLADIS is therefore much higher than the mean energy of the steady-state loading in a tokamak plasma, which is several 10 eV.…”

“…[10], and steady state power and particle loading, which result in cavity formation, surface morphology modifications, etc. [11]. As a consequence, the thermal shock behaviour of tungsten that was exposed to H and He particles beforehand, performed at ITER relevant surface temperatures, needs to be understood and will be addressed hereafter.…”