Hot spots induced by RF-accelerated electrons in the scrape-off layer on Experimental Advanced Superconducting Tokamak

Preventing impurity emission from hot spots on plasma-facing materials is a critical issue in the maintenance of high-performance plasma on the Experimental Advanced Superconducting Tokamak (EAST). In this study, experimental and theoretical analyses were performed to investigate the mechanism of hot spot formation. In the upper single null magnetic configuration of the EAST, two separatrices were connected to the upper (primary) and lower (secondary) X-points. Experiments on plasma configuration control indicated that the reduction in the gap between the lower (secondary) separatrix and lower hybrid antenna is effective in preventing hot spot formation on the lower divertor, which frequently emits impurities in long-duration discharges. This effectiveness was quantitatively confirmed by magnetic field lines tracking simulation and calorimetric measurement of divertors in the experiment. Two-frequency power modulation of the lower hybrid wave (LHW) was conducted to evaluate power deposition on the scrape-off layer (SOL) during propagation from the LHW antenna to the main plasma. This experiment clarified that LHW-accelerated electrons in the SOL via collision damping deliver their energies to hot spots along the magnetic field line. These findings help alleviate or even eliminate the formation of hot spots and maintain the performance of plasma.