Spectroscopic Study of Molybdenum Impurity Generation in LHW Sustained Plasmas on TST-2 Spherical Tokamak

Impurity generation mechanisms including RF sheath sputtering and heating from fast electrons were explored in LHW sustained plasmas on TST-2 spherical tokamak. Molybdenum impurity was measured with a high-resolution spectrometer and the heating effect on a molybdenum target plate was estimated with a fast camera system. The LHW power modulation experiment indicates that the RF sheath sputtering dominated impurity generation from the antenna limiters (molybdenum) under the current plasma parameters. In addition, the target plate insertion experiment shows that molybdenum atoms were released from the target when heated by fast electrons accelerated by LHW. Although the heating effect was negligible for the antenna limiters, it could become significant under higher plasma parameters or during longer pulses.