Evolution of surface melt damage, its influence on plasma performance and prospects of recovery

Abstract

Experiments have been carried out in the TEXTOR, ASDEX Upgrade (AUG) and Alcator C-Mod (C-Mod) tokamaks to study melt-layer motion, macroscopic W-erosion from the melt as well as the changes of material properties such as grain-size and voids. In addition the effect of multiple exposures is studied to judge the potential amelioration of inflicted melt damage. The parallel heat flux at the radial position of the PFCs in the plasma ranges from around q ∼ 45MW/m 2 at TEXTOR up to q ∼ 500MW/m 2 at CMod which covers scenarios close to ITER parameters, allowing samples to be exposed and molten even at shallow divertor angles. Melt-layer motion perpendicular to the magnetic field is observed consistent with a Lorentz-force originating from thermoelectric emission of the hot sample. While melting in the limiter geometry at TEXTOR is rather quiescent causing no severe impact on plasma operation, exposure in the divertors of AUG and C-Mod shows significant impact on operation, leading to subsequent disruptions. The power-handling capabilities are severely degraded by forming exposed hill structures and changing the material structure by re-solidifying and re-crystallizing the original material. Melting of W seems highly unfavorable and needs to be avoided especially in light of uncontrolled transients and misaligned PFCs.

Plasma operation with an all metal first-wall: Comparison of an ITER-like wall with a carbon wall in JET

“…Since then, ITER has selected a full tungsten divertor for its DT phase and there is currently a strong interest in proving its suitability as a first divertor for ITER [23]. High Z materials have the potential to severely restrict the operational space of a tokamak [4] and melting [24] is an additional risk not present with carbon. Beryllium on the other hand has a high and still uncertain erosion rate with the potential for short component lifetimes and high tritium retention [3].…”

Thermal analysis of an exposed tungsten edge in the JET divertor

“…Other experiments have addressed this questions before and studied molten tungsten under limiter and divertor plasma conditions in the TEXTOR [2,3] and ASDEX Upgrade [4][5][6] tokamaks for example. They achieved melting conditions under steady plasma flow -what we call bulk melting.…”

Overview of the JET results

“…A dedicated experiment has been carried out in JET to address the uncertainties associated with predicting the impact of transient tungsten melting in ITER due to ELMs [26][27][28][29]. The horizontal tile of the JET divertor is made of solid tungsten arranged in four stacks of lamellae in order to minimize the electromagnetic loads during disruptions (figure 2).…”