Impact of repetitive ELM transients on ITER divertor tungsten monoblock top surfaces

TL;DR

This paper investigates how repetitive ELM transients can cause surface damage to tungsten monoblocks in ITER's divertor, affecting long-term power handling, using advanced melt dynamics modeling.

Contribution

It introduces a coupled melt motion and surface deformation model to assess damage evolution under ELM transients in ITER divertor conditions.

Findings

Repetitive ELMs can cause significant surface damage to tungsten monoblocks.

Surface deformation increases vulnerability to subsequent heat loads.

Damage accumulation depends on the history of transient events.

Abstract

Owing to the high stored energy of ITER plasmas, the heat pulses due to uncontrolled Type I edge localized modes (ELMs) can be sufficient to melt the top surface of several poloidal rows of tungsten monoblocks in the divertor strike point regions. Coupled with the melt motion associated with tungsten in the strong tokamak magnetic fields, the resulting surface damage after even a comparatively small number of such repetitive transients may have a significant impact on long-term stationary power handling capability. The permissible numbers set important boundaries on operation and on the performance required from the plasma control system. Modelling is carried out with the recently updated MEMENTO melt dynamics code, which is tailored to tackle melt motion problems characterized by a vast spatio-temporal scale separation. The crucial role of coupling between surface deformation and shallow angle heat loading in aggravating melt damage is highlighted. As a consequence, the allowable operational space in terms of ELM-induced transient heat loads is history-dependent and once deformation has occurred, weaker heat loads, incapable of melting a pristine surface, can further extend the damage.