Suppression of tungsten accumulation during ELMy H-mode by lower hybrid wave heating in the EAST tokamak

Abstract

EAST tokamak has been equipped with upper tungsten divertor since 2014. The tungsten accumulation has been often observed in NBI-heated H-mode discharges suggesting deleterious tungsten confinement in the plasma core. It causes not only H-L back transition but also plasma disruption in several discharges. Suppression of the tungsten accumulation is therefore the most important issue in EAST to achieve a long pulse H-mode discharge. In order to study the tungsten behavior in the long pulse discharge, tungsten spectra have been measured at 20–140Å. The tungsten density, nw, is evaluated from the intensity of tungsten unresolved transition array (W-UTA) in a wavelength range of 45–70Å which is composed of several ionization stages of tungsten, e.g. W27+-W45+ at Te0∼2.5keV). It is found that the tungsten accumulation can be suppressed when the 4.6GHz LHW with PLHW∼0.8MW is superimposed on the NBI phase (PNBI= 1.9MW). During the superimposed phase the ELM frequency, fELM, increases from ∼30Hz to ∼60Hz and the tungsten density is halved compared to the NBI-heated discharge. The H-mode discharge can be thus steadily sustained for longer period. It is found that the nw is a large function of the ratio of LHW power to the total injection power, PLHW/(PLHW+PNBI), and the nw can be reduced, at least, in an order of magnitude smaller than that in NBI-heated discharges at PLHW/(PLHW+PNBI)≥0.8. The result strongly suggests a possible way toward the steady H-mode discharge. Keywords: EAST, Tungsten divertor, Tungsten spectrum, ELMy H-mode, Impurity accumulation