First results of LHW-generated fast electron-induced melting of TZM limiter in EAST

Abstract In tokamak devices, the lower hybrid current drive (LHCD) is remarkably efficient for steady-state plasma operation. However, hotspots are often found on the guard limiters in multiple machines with increasing injected power from the LHCD. This study reports the first experimental observation of TZM (titanium–zirconium–molybdenum alloy) limiter melting induced by lower hybrid wave (LHW)-generated fast keV electrons in the Experimental Advanced Superconducting Tokamak (EAST) tokamak. Infrared camera and hard x-ray diagnostics reveal that when 4.6 GHz LHW2 power was injected, fast electrons deposited along magnetic field lines onto the TZM limiter surface magnetically connected to the antenna, forming localized hotspots that ultimately led to melting. Heat load simulations with ANSYS demonstrate that parallel heat flux deposited on the TZM surface was increased by about 13 times compared with the background plasma heat flux. Post mortem analysis shows melt motion of up to 19.6 mm along the J × B direction, which also coincides with the direction of gravity without droplet ejection. The lesson learned from the melting of TZM tiles is that the melt pool would remain relatively stable with melt motion driven by forces tangential to the melt pool. Unlike previous tungsten melting on a divertor with droplet ejection, which eventually caused plasma disruption, a stable melt pool is not likely to eject droplets. However, such a scenario could change if a geometric obstacle is in the way of melt motion. Thus, melting of a large tile in a future fusion device may not have a noticeable impact on plasma operation provided that the melt pool does not move to the edge of the plasma-facing component (PFC). The molten region exhibited distinct grain layering, including columnar grains (depth ∼2.2 mm) and equiaxed grains (recrystallized zone), indicating deep thermal gradients due to prolonged fast electron exposure. The presence of frozen capillary waves and high apparent contact angles (∼90°) on the surface confirms solidification-dominated suppression of melt spreading, previously observed in ASDEX Upgrade experiments. These findings provide critical insights for optimizing PFC design and mitigating thermal damage under LHW heating in future fusion devices.