A long-pulse small edge-localized-mode high-confinement plasma with detachment feedback control by floating potential in an experimental advanced superconducting tokamak in a metal wall environment

Abstract

One of the key challenges facing magnetic fusion research is to demonstrate the compatibility between high confinement and radiative divertors in long-pulse discharges with a metal wall environment. A small edge-localized-mode (ELM) high-confinement plasma with an energy confinement factor H₉₈ ∼ 1.1 and Greenwald density fraction f_GW ∼ 0.65 maintained for 26 s is obtained in the upgraded lower divertor of the Experimental Advanced Superconducting Tokamak (EAST), and periodical detachment is achieved through active control of neon impurity seeding in this long-pulse discharge. For the divertor region, partial detachment is achieved periodically on the outer divertor target plates with the plasma temperature near the outer strike point decreasing to below 5 eV and the peak surface temperature on the outer divertor target plates maintained below 350 °C. The peak heat flux of the lower outer divertor decreases significantly and its profile along the target becomes very flat in the detached state. Two low-frequency (<10 kHz) fluctuations that are related to the rippling mode caused by a resistive instability appear in the detached state. For the pedestal region, the electron pressure profile is flatter and the ELM amplitude is smaller in the detached state than that in the attached state. Edge-coherent mode appears in the attached state and disappears in the detached state. To achieve this experimentally, a new impurity seeding feedback control scheme is applied, where the floating potential measured by divertor Langmuir probes is used as a feedback sensor, which is more reliable in long-pulse discharges with high heat fluxes and thus more suitable for application in future devices. This study provides a new approach for the actively controlled radiative divertor as a solution to the divertor heat loads of future fusion reactors.