Trapped-Electron Transport via Microinstabilities

Anomalous electron thermal conduction is generally recognized to be the principal heat loss channel in most modern tokamaks. Turbulent drift-type microinstabilities have long been suspected to be the primary cause of this anomalous thermal conduction. In this work, we present a systematic formulation of trapped-electron transport based on a drift-wave turbulence model in the good confinement region (between the q = 1 and q = 2 surfaces). A pre-existing nonlinear gyrokinetic formalism has been extended to include collisional effects by using the Fokker-Planck collision operator. Following the procedures of neoclassical theory, a formal transport theory including turbulence is developed. To further explore the properties of the low-(beta) gyrokinetic Fokker-Planck equation, we consider the specific case of an axisymmetric, large aspect ratio tokamak with circular, concentric magnetic surfaces and, employ the ballooning mode representation for the fluctuating quantities. The resulting equation is solved analytically for the perturbed electron distribution function in a particular limit ((omega)(,be) > (nu)(,eff,e) > (omega)). Corresponding expressions for the fluxes of particles and energy are evaluated numerically after prescribing the mode structure along the field line and the saturated mode amplitudes. Electrostatic transport coefficients thus obtained are presented and found to be in agreement with previous heuristic arguments.