Deuterium retention in tungsten under combined high cycle ELM-like heat loads and steady-state plasma exposure

A Huber,G Sergienko,M Wirtz,I Steudel,A Arakcheev,S Brezinsek,A Burdakov,T Dittmar,H.G Esser,M Freisinger,A Kreter,J Linke,Ch Linsmeier,Ph Mertens,S Möller,M Reinhart,A Terra,B Unterberg

doi:10.1016/j.nme.2016.04.007

A Huber, G Sergienko + Show 16 more

Open Access

https://doi.org/10.1016/j.nme.2016.04.007

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Abstract

To investigate the synergistic effects of fuel retention in tungsten, experiments were performed in the linear plasma device PSI-2 where the transient heat loads were applied by a high energy laser during the steady-state plasma operation. The impact on the deuterium (D) retention of simultaneous and sequential exposures to laser and plasma has been investigated. A significant increase of D retention, more than a factor of 12, has been observed during the simultaneous transient heat loads and plasma exposure. Successive exposures to transient heat loads and deuterium plasma also show the increased content of D-atoms by more than a factor of 3.6 in comparison to the pure plasma loading. In both cases the increase is most likely due to enhanced hydrogen clustering by the thermal shock exposures, due to an increased mobility of D atoms along the shock-induced cracks as well as to increased diffusion of D atoms into the W material caused by strong temperature gradients during the laser pulse exposure.Comparison of the NRA and TDS measurements shows that only 34% of the retained deuterium in the tungsten specimen is located inside the near-surface layer (d<4µm) after simultaneous as well as sequential exposures of W to heat load.Enhanced blister formation has been observed under combined loading conditions at power densities close to the threshold for damaging. It is shown that blisters are not mainly responsible for the pronounced increase of the D retention.

Highlights

Tungsten (W) is foreseen as the main plasma-facing material (PFM) in fusion devices, such as ITER with full-tungsten divertor [1,2], because of its excellent material properties such as a high threshold energy for sputtering [3], a high melting point [4] and a low tritium inventory [5,6]
We investigate the effect of the simultaneous exposure of tungsten to steady-state plasma and transient heat loading conditions on the surface modification of bulk tungsten as well as on the fuel retention
To compare local nuclear reaction analysis (NRA) measurements and total number of desorbed D atoms from the pieces measured by thermal desorption spectroscopy (TDS), the surface density of D atoms [CD] inside the laser spot has been evaluated from the TDS data by the following equation: CD[at/m2] =

Summary

Introduction

Tungsten (W) is foreseen as the main plasma-facing material (PFM) in fusion devices, such as ITER with full-tungsten divertor [1,2], because of its excellent material properties such as a high threshold energy for sputtering [3], a high melting point [4] and a low tritium inventory [5,6]. The combination of transient heating and hydrogen plasma exposure has been experimentally shown to lead to severe surface damage and modifications, such as crack formation, enhanced erosion/ejection, roughening, formation of melt layers and blisters [7,8,9] These results show that the damage behaviour strongly depends on the loading conditions and the sequence of the particle and heat flux exposure. The additional impact of the enhanced particle flux during the ELMs is outside of the scope of this work In this contribution, we investigate the effect of the simultaneous exposure of tungsten to steady-state plasma and transient heat loading conditions on the surface modification of bulk tungsten as well as on the fuel retention

Experimental set-up

Findings

Result and discussion

Conclusions