Parametric Decay Wave Observation in HFS X-Mode Injection in QUEST

Abstract

The parametric decay wave (PDW) caused by three-wave parametric decay process was measured in a plasma-injecting X-mode electron cyclotron wave (ECW) from the high field side (HFS) of the Q-shu University Experimental Steady-State Spherical Tokamak (QUEST). The intensity of the low-frequency PDW on the HFS X-mode injection was significantly enhanced over the O-mode ECW injection from the low field side (LFS), where the mode conversion to electron Bernstein wave (EBW) was not expected. As the comparison was executed using plasmas with the same magnetic field and injection power, the wave injection method was considered as the primary cause of the difference in the PDW excitation. The frequency range of the low-frequency PDW was consistent with that of the lower hybrid wave (LHW) range, which was expected to be excited during the mode conversion to EBW. The low-frequency PDW intensity evolved in response to the plasma density, plasma current and injection power. This observation suggests that the low-frequency PDW intensity is a reliable indicator for the efficient mode conversion to EBW.