Current status and upgrade activities on the guard limiter of 4.6 GHz lower hybrid antennas for EAST tokamak

Abstract

The guard limiter (GL) with graphite tiles of 4.6GHz Lower Hybrid Antennas (LHA) has experienced the discharges in EAST tokamak. Much more hot spots were observed on the surface of GL tiles during the 2015–2016 campaigns. The serious ablations of GL induced by the hot spots limit the superconductor current, the plasma density and the heating power at higher parameter operations. In this paper, the GL issues are introduced with the original design. The previous improvements on the GL tiles are discussed with the ablation problems. In particular, the hotspots and the heat affected zones are discussed to predict the GL profile in accordance with the physical estimation. A trimmed region with a bevel angel of 11° is applied to avoid the heat load deposition at millimeter magnitude, which is caused by the fast electrons hitting due to the LHA heat driving. As a result, the GL ablations are confirmed to be reduced to the minimum. Moreover, it indicates that the plasma hitting would contribute most on the tiles fracture in the mid plane of the GL. In addition, new design activities on the tungsten GL are summarized in the engineering design. The new GL would be expected in the year 2017.