First analysis of in-situ melting on TZM tiles at high field side of the first wall in EAST

Abstract

Plasma facing materials (PFMs) are subjected to long-duration high-energy particle streams and radiation in tokamak devices. The PFMs of EAST have been upgraded several times and Titanium-Zirconium-Molybdenum (TZM) tiles were installed into EAST as its first wall since 2011. However, with the gradually increasing of plasma parameters, several unexpected TZM melting phenomena were found at the high field side by post mortem inspection after each EAST plasma experimental campaign since 2017. The resolidified melted surface is general in wave shape with unobvious motion of melting layer. Three different grain shapes, i.e., columnar grain, isometric crystal and original rolled crystal from surface to deep region are found by means of metallurgical analysis, in which the superficial layer columnar grain is very thin with a thickness of 100 ∼ 200 μm and the thickness of intermediate isometric crystal is also small only about 300 ∼ 400 μm, strongly indicating there was a large temperature gradient near surface when melting occurred. Combined with plasma operation parameters and temperature evolution, the melting of TZM tiles were concluded to be induced by the transient heat flux during plasma disruption. These results imply the transient heat flux during plasma disruption in EAST can severely destruct the metal PFMs and should not be ignored, suggesting the active mitigation of plasma disruption is necessary for future long pulse and high parameters operation.

Keywords

• EAST
• Melting
• TZM
• Transient heat flux
• Microstructure
• Grain characteristics