Experimental investigation on spectral broadening of lower hybrid waves with different frequencies in the EAST long-pulse plasmas

The frequency spectral broadening of lower hybrid (LH) waves at 2.45 and 4.6 GHz, which denotes the change in original properties of the LH wave, was investigated by using a radio frequency (RF) probe in Experimental Advanced Superconducting Tokamak long-pulse plasmas. The dependency of the RF spectrum on plasma density, LH power and magnetic configuration is reported and analyzed. A link between the degradation of current drive (CD) efficiency and the spectral broadening is found, which shows that the spectral broadening has a negative and significant effect on CD efficiency for both of the LH waves. In addition, the LH power absorption characteristic is also found to be correlated with the LH pump spectral broadening. Parametric instability (PI) modeling was performed to identify the mechanisms responsible for the observed pump broadening and the causal connection between spectral broadening and the loss of CD efficiency. The modeling results show that ion-sound quasi-mode-driven PI can redistribute the launched parallel refractive index (N //) spectrum to some extent, thus leading to a pump power depletion. However, the ion-sound quasi-mode-driven PI effect cannot fully account for the experimental observations and the loss of CD efficiency.