Transport implications of non-monotonic-q tokamak configurations

Advanced tokamak configurations attempt to make use of cross-sectional shaping and profile control to enhance the stability properties of the plasma. A comprehensive kinetic toroidal eigenvalue calculation which implements the ballooning formalism is employed to investigate this for high-n instabilities such as the toroidal drift mode destabilized by the combined effects of ion temperature gradients and trapped particles. An approach being pursued in advanced tokamak configurations is profile control for the current and/or pressure. In particular, it may be possible to produce regions of reversed shear (dq/dr < 0), as has been observed in the JET tokamak in PEP mode discharges. If sufficiently large, this reversal has the effect of reversing the ''bad curvature'' of the trapped particles and thereby suppressing the collisionless trapped-electron mode instability mechanism of magnetic-drift precession resonance. At the same time, temperature and density profiles which optimize the bootstrap current can also correspond to regions with ηi ≡ dln Ti/dln n < ηicrit, so that the ion-temperature-gradient-mode instability mechanism is also suppressed. If these regions overlap, as has happened in the plasma interior for a particular operating mode for a TPX design, then the toroidal drift mode can be completely stabilized. If this mode is responsible for the experimentally-observed anomalous transport in tokamaks, then this situation can lead to a ''transport barrier'' in the plasma interior. The stability properties of such configurations with respect to low-n ideal MHD modes will also be discussed