Characterization of <i>in situ</i> damage to tungsten PFCs induced by transient heat flux during plasma disruption in EAST

Abstract Transient heat flux of up to several thousand MW m −2 in a short pulse (∼ms) in tokamaks poses great risk to plasma-facing components (PFCs), making it a major concern for ITER. Despite numerous high heat flux tests, analysis of in situ transient heat flux-induced damage to PFCs remains necessary. Such damage, including the melting and cracking of tungsten (W) PFCs, is notably observed on the divertor (dome and baffle plates) and limiter in EAST. The damage is identified as being induced by runaway electron loss during plasma disruption at the beginning of each plasma campaign. It typically occurs at the leading edges or protruding parts of PFCs, sometimes accompanied by visible macrocracks. In terms of melting phenomena, three distinct grain layers can be observed from the molten surface to deeper regions, namely columnar grain, equiaxed grain (recrystallization region) and original grain. This grain distribution indicates a steep temperature gradient from the surface to the deeper regions during melting events, a characteristic feature for W under fusion-relevant transient heat flux loading. The surface morphologies of all melted PFCs are generally similar, characterized by undulated melting waves. The motion of the melting layer is primarily along the toroidal direction, as shown in the in situ melting of PFCs. The influence of the J × B force might not be significant due to the limited lifetime of the melting pool, which results in limited acceleration times and expected bulk melt displacement. The directions of plasma pressure and Marangoni flow, both along the toroidal direction, might be the dominant forces here. Additionally, cracks at the leading edges were observed on the divertor dome and baffle plates during post-mortem inspection. In some cases, dense cracks were visible in the melting region and even in areas far from the melting zone. It should be noted that cracks were only found in partially melted PFCs, which could be related to the base temperature when PFCs were hit by the runaway electron-induced transient heat flux. Furthermore, some columnar grains were observed to exfoliate from the material, indicating severe cracking. Since EAST has similar W PFCs to ITER, the transient heat flux-induced melting and cracking damage to W PFCs by runaway electrons during plasma disruption in EAST provide important references for ITER.