A Basic Experiment on the Production and Identification of ETG Modes

One of the strongest candidates for the anomalous electron energy transport is believed to be electron temperature gradient (ETG) mode [1, 2]. However, the high frequency (few MHz) and short wave length ($k_\perp\rho_e<1$) make the direct observation of ETG modes difficult in experiments. Using a DC bias heating scheme of the core plasma, we are able to produce the drive parameter $\eta_e = d ln \it{T_e} / d ln \it n $ from 1 to 6 ($ T_e \sim 20 eV $ in the center and $\sim 1eV $ on the edge) in Columbia Linear Machine (CLM). A high frequency mode at $ \sim 2MHz $has been observed. Its azimuthal wave number in $ m \sim 30$ has been measured. These values are consistent with the results of a simple kinetic dispersion relation on appropriate $\vec{E_0} \times \vec {B}$ Doppler shift. The problem of the measurement of the small parallel wave number with the large azimuthal wave number has been resolved by a novel diagnostic method. The scaling of ETG fluctuation level versus $\eta_e$, as well as the radial structure of the mode will be reported. \\ $[1]$ W. Dorland et al., \it {Phys. Rev. Lett.} 85, 5579 (2000). \\ $[2]$ R.E. Waltz, J. Candy and M. Fahey, \it {Phys. Plasmas} 14, 056116 (2007).