Microturbulent drift mode stability before internal transport barrier formation in the Alcator C-Mod radio frequency heated H-mode

H -mode experiments on Alcator C-Mod [I. H. Hutchinson, R. L. Boivin, F. Bombarda, Phys. Plasmas 1, 1511 (1994)], which exhibit an internal transport barrier (ITB), have been examined with gyrokinetic calculations, before barrier formation. Ion temperature gradient (ITG) and electron temperature gradient (ETG) modes are unstable outside the barrier region and not strongly growing in the core; in the barrier region ITG is only weakly unstable. Linear calculations support the picture that ITG and ETG modes drive high transport outside the ITB, and that weakly unstable ITG modes in the barrier region correlate with reduced particle transport and improved thermal confinement even before the ITB is established, without the need for E×B shear stabilization. Long-wavelength mode stability in the barrier region is analyzed in the context of a phase diagram for ion and electron drift waves by varying the temperature and density scale lengths. Results from the gyrokinetic code GS2 [M. Kotschenreuther, G. Rewoldt, W. M. Tang, Comput. Phys. Commun. 88, 128 (1995)] are compared to standard threshold models and benchmark successfully against the experiment in the plasma core.