A metal-wall compatible high-confinement tokamak plasma regime for fusion energy

The international thermonuclear experimental reactor (ITER) is now considering to use tungsten as the material for plasma-facing components (PFCs) [1] and operate in the so-called high-confinement mode (H-mode) [2]. Heat pulse eruptions caused by the plasma instability named the edge localized mode (ELM) pose a critical threat to the PFCs, which can lead to severe material damage and generate metallic impurities that contaminate the core plasma, rendering stable H-mode operation difficult [3]. Therefore, ITER and future fusion reactors are desired to operate in a plasma regime without ELM but with good energy confinement, simultaneously coupled with the plasma detachment to relieve heat load on the material surface [4]. However, such an operational regime has not yet been demonstrated for long pulse without expense of energy confinement, and the mechanism for maintaining ELM suppression in a detached H-mode plasma remains unclear. Here, we report the first demonstration of the achievement of a long-pulse detached H-mode plasma regime without ELM but with energy confinement even better than the standard H-mode in a metal-wall environment, and elucidate, for the first time, the underlying physics. We find that a high-frequency turbulence is excited at the plasma edge that provides a radial transport channel, thus preventing ELM generation. Simulations suggest that in ITER the plasma edge will be more prone to turbulences, as turbulence suppression by the ExB flow shear is expected to be significantly weaker than that in present tokamaks [5]. These findings may lead to a promising operational scenario for future tokamak fusion reactors compatible with metal-wall conditions, which is essential for harvesting fusion energy.