Estimation of carbon impurity from the guard limiter of the lower hybrid wave antenna on EAST tokamak

Hot spots on the EAST tokamak graphite guard limiters of the lower hybrid wave (LHW) antenna may cause a sudden increase of impurity influx and even ending with disruption. A sheath model is developed by taking into account the energetic electron component from the plasma-LHW interaction via electron Landau damping, and then a self-consistent method is used to study the interaction of the plasma across the sheath at the material surface and material thermal response. It is found that the fast electron fluxes driven by both the 2.45 and 4.6 GHz LHWs modify strongly the sheath potential, which then influence on the bombarding ion energy at the material surface and the surface temperature. By calculating the carbon physical sputtering and chemical erosion from guard limiter of the LHW antenna, the carbon production from the guard limiter in two LHW systems is found to have different behavior for the same edge plasma density and electron temperature. Our results show that carbon impurity is mainly from physical sputtering, except for the 4.6 GHz LHW in a narrow range of the high electron temperature where the chemical erosion has a very sharp increase. Due to modification of the sheath potential induced by the fast electrons, the power density is higher with the 4.6 GHz than the 2.45 GHz LHW system. As a consequence the surface temperature is higher at 4.6 GHz and susceptible to reach several hundred of degrees which is prone to chemical erosion.