Properties of ion temperature gradient drift instabilities in H-mode plasmas

Experimental results from tokamaks such as DIII-D [Phys. Fluids 31, 3738 (1988)] and JET [Proceedings of the 15th European Conference on Controlled Fusion and Plasma Heating (European Physical Society, Budapest, 1988), Vol. 12B, Part 1, p. 2239] have indicated that the electron density profile in H-mode ("high confinement") discharges can be nearly flat over most of the plasma and in some cases, even inverted (outwardly peaked). These conditions have very interesting implications for pictures of anomalous thermal transport, based on the presence of ion temperature gradient (η_i) drift instabilities. The present paper includes (i) a new derivation of the ion temperature gradient threshold for weak density grandient η_i modes when ion transit resonances are taken into account; and (ii) the first derivation of threshold conditions for the onset of η_i modes when η_i is negative, along with the properties of these negative η_i instabilities when the thresholds are exceeded. Possible consequences for confinement in H-mode plasmas are discussed.