Observation of coherent mode induced by a molybdenum dust on EAST

The influence of a molybdenum dust buildup on plasma edge turbulence has been studied in the EAST tokamak. The motion of the dust from the upper divertor region is detected by a fast visible CCD camera, the XUV spectrometer arrays, and the EUV spectrometer. The MoXV emission intensity sharply increases compared with the spectral lines of various ionization states of other elements, which implies that the dust particles are the molybdenum impurities. The radial distribution of Mo¹⁴⁺ ion simulated by a simplified 1D transport model indicates that the molybdenum dust mainly deposits in the pedestal bottom region. Moreover, it is observed that the coherent mode (CM) appears at ρ = 0.94 after the molybdenum impurities enter the main plasma region. The influx of molybdenum impurities results in increasing pedestal electron density and decreasing pedestal electron temperature in contrast to that before the event of impurities dropping. It is also found that the electron density gradient in the pedestal increases when the ablation of the molybdenum impurities is observed in the pedestal region. The qualitative experimental results indicate that the onset of CM is likely related to the increase of the density gradient and edge collisionality in the pedestal. In comparison to the density gradient, the enhancement of CM amplitude largely depends on the increase of the edge collisionality.