Observation of the electron thermal transport and temperature fluctuations for electron cyclotron resonance heated plasmas on J-TEXT

First dedicated experiments have been carried out on the J-TEXT tokamak for investigating the electron thermal transport and plasma fluctuations before and during electron cyclotron resonance heating (ECRH). The electron thermal diffusivities are calculated using the diffusive space-time evolution characteristic of the electron temperature perturbations produced by internal sawtooth collapse. It is found that the electron thermal diffusivity significantly increases as ECRH is turned on. The temperature fluctuations in the ECRH-assisted ohmic plasmas measured by the correlation electron cyclotron emission (CECE) diagnostic have a larger amplitude and broader bandwidth than that of pure ohmic plasmas. However, there is a slight change in the density fluctuations. The electron thermal diffusivities and the bandwidth of the electron temperature fluctuations decrease simultaneously as density increases in ohmic plasmas. However, the same trend is not found for ECRH-assisted ohmic plasmas with fixed ECRH power. The electron thermal diffusivities increase almost linearly with the ECRH power for the same discharge parameters, while the temperature fluctuations become broader at the same time. Linear stability analysis using the gyrokinetic electromagnetic numerical experiment code indicates that there is a turbulence transition from the mixed mode of ion temperature gradient mode and trapped electron mode (TEM) to the pure (broader) TEM when changing from pure ohmic to ECRH-assisted ohmic plasmas. It is consistent with the temperature fluctuations measured by CECE, which is sensitive to the long-wavelength modes. All these results suggest that the long-wavelength temperature fluctuations play an important role in electron thermal transport. In ECRH-assisted ohmic plasmas, electron thermal transport may become dominated by ETGs when TEMs are suppressed at high density.