Observation of the Early Transition from Slab to mixed Slab-Toroidal ETG Turbulence

Parametric studies of the transition between the slab branch of electron temperature gradient (ETG) mode and the mixed slab-toroidal branch of the ETG mode in CLM are reported. CLM was operated in a mirror machine configuration with a cell length of 50-100 cm, and a mirror ratio of 1-2. For typical CLM parameters and a mode localized at r=2 cm this provides a range for inverse radius of curvature ${R_c}^{-1}$ between 0 and .006 ${cm}^{-1}$. Under normal conditions theory predicts transition between slab and toroidal modes would occur when the parameter ${k_{||}R_c}/{2k_{\perp}{\rho}} \sim 1$ [1]. Recent experiments have obtained an experimental scaling of mode amplitude and frequency as a function of ${R_c}^{-1}$. They indicate that even for much more modest levels of ${k_{||}R_c}/{2k_{\perp}{\rho}} \sim .1$, there are substantial increases in saturated mode, up to 5 times larger than the pure slab mode. Changes in real frequency in the mode are generally small, on the order of $<$ 5\%. \\[4pt] [1] J.Y. Kim and W. Horton, Phys.Fluids B 3, 1167 (1991).