Toroidal microinstability studies of confinement trends in TFTR

In examining the possible relevance of toroidal drift-type microinstabilities for tokamaks, attention is here focused on applications of realistic kinetic stability calculations to confinement trends observed in TFTR. The principal theoretical tool used is a comprehensive toroidal code which carriers out self consistent, ballooning-formalism eigenmode calculations including all resonances, finite beta effects in the equilibrium as well as the perturbations, and finite trapped-ion banana orbital dynamics, along with FLR effects to arbitrary order. This code has been interfaced with realistic numerical MHD equilibria and physical parameters specific to the TFTR experiments of interest. Results from the calculations of the linear properties and quasilinear fluxes of toroidal drift modes driven by trapped-particle and ion temperature gradient effects indicate the following: (i) A strong temperature dependence (diffusivity ∝ Tα, with α between 3/2 and 5/2) is found which appears to be consistent with the temperature scaling observed in the confinement region of perturbative transport experiments carried out on L-mode discharges. Dependences on radial variations of equilibrium quantities (density, temperature, safety factor, etc.) and their gradients are also explored. (ii) An important trend observed in TFTR supershot discharges is that, unlike the L-mode cases, confinement actually appears to improve with increasing ion temperature. The theoretical results here show that, at sufficiently high Ti (approx-gt 10 keV), confinement for supershot cases indeed ceases to degrade with increasing ion temperature. Variations with Te are also examined, and the implications of the results discussed. 1 ref