Interaction of adhered beryllium proxy dust with transient and stationary plasmas

D Aussems,D Ripamonti,G Riva,P Tolias,S Ratynskaia,M De Angeli,T.W Morgan

doi:10.1016/j.nme.2018.11.007

Abstract

Tungsten (W) substrates with adhered beryllium (Be) proxy dust - copper, chromium, aluminium - have been exposed in the Magnum-PSI linear device. Their interaction with transient and stationary plasmas has been systematically studied under varying heat fluxes and magnetic field topologies. The dust remobilization activities, macro-morphological changes and chemical modifications induced by the plasma incidence are documented. Aluminium is identified to be the most suitable surrogate material due to the similar binary phase diagram and nearly identical evaporation rates. Extrapolation suggests that Be dust cannot survive on hot W surfaces but it can trigger mixed Be/W effects prior to its plasma removal.

Highlights

The tungsten and beryllium dust inventory limits in ITER have been established on the basis of safety considerations during potential accident scenarios [1,2,3]
The interaction of μm-size W dust adhered to bulk W surfaces with transient plasma heat loads has been systematically studied in tokamaks (DIII-D, ASDEX-Upgrade, COMPASS) and linear devices (PilotPSI) [4,5,6,7,8]
The experimental observations that have been supported by theoretical arguments or heat transfer simulations can be summarized as follows: (i) Adhered W dust and especially clusters melt under much lower heat loads than bulk W owing to the small contact area and imperfect thermal contact. (ii) W clusters can be transformed into large spherical W dust through the wetting induced coagulation mechanism, which arises from the competition between the fast resolidification rates and slower liquid spreading dynamics. (iii) W dust remobilization is inhibited by contact strengthening due to macroscopic material flow and microscopic atomic diffusion

Summary

Introduction

The tungsten and beryllium dust inventory limits in ITER have been established on the basis of safety considerations during potential accident scenarios [1,2,3]. The interaction of μm-size W dust adhered to bulk W surfaces with transient plasma heat loads has been systematically studied in tokamaks (DIII-D, ASDEX-Upgrade, COMPASS) and linear devices (PilotPSI) [4,5,6,7,8]. The interaction of μm-size Be dust adhered to bulk W surfaces with transient plasma heat fluxes has been previously studied via surrogate materials; copper, chromium in ASDEX-Upgrade [8] and aluminium in DIII-D [11]. These exposures concerned modest heat loads and involved rather limited statistics. Implications regarding the survivability of adhered Be dust on hot W surfaces are discussed

Sample composition and preparation

Sample exposure and analysis

Remobilization and macro-morphological changes

Summary of SEM observations

Discussion