Experimental production and identification of electron temperature gradient modes

The electron temperature gradient (ETG) mode, which is believed to be one of the strongest candidates for the anomalous electron energy transport in plasmas, is difficult to detect in experiments because of its high frequency (similar to MHz) and short wavelength (k(perpendicular to)rho(e)< 1). Using a dc bias heating scheme of the core plasma, we are able to produce a sufficiently strong ETG for exciting ETG modes in the Columbia linear machine [R. Scarmozzino, A. K. Sen, and G. A. Navratil, Phys. Fluids 31, 1773 (1988)]. A high frequency mode at similar to 2 MHz, with azimuthal wave numbers m similar to 14-16 and parallel wave number k parallel to(>)similar to 0.01 cm(-1), has been observed. The frequency range is consistent with the result of a kinetic dispersion relation of slab ETG modes with appropriate ExB Doppler shift. The scaling of its fluctuation level with the temperature gradient scale length and the radial structure are found to be roughly consistent with theoretical expectations. Therefore, this is one of the first direct definitive identifications of ETG modes.