Nonlinear Gyrokinetic Analysis of Linear Ohmic Confinement to Saturated Ohmic Confinement Transition

Full text: One of the long lived conundrums in ohmically heated plasmas is that the energy confinement time τE shows a transition from a linear regime proportional to the density (LOC) to a saturation regime (SOC) weakly dependent on the density. In the viewpoint of the first principle nonlinear global gyrokinetic simulations, we here present an investigation of LOC to SOC transition for the first time. In this study, by varying a single parameter plasma density, the confinement time estimated by τ∝l/χ_eff shows a transition from a linearly increasing regime to a saturation regime as the plasma density increases. The effective transport diffusivity is defined as χ_eff ≡ (n_eχ_e∇T+e+n_i∇T_i)/n_e∇T_x+n_i∇_i where n_e,i, T_ei and χ_e,i are density, temperature and heat diffusivity for electron (e) and ion (i). The above nonlinear result follows the trend from the mixing length quasi-linear estimation for the heat transport. A transition of trapped electron dominant heat transport from TEM to ion dominant heat transport from ITG is observed when the LOC to SOC transition occurs. In the simulations, the Coulomb collision operator for ion-ion collision and the pitch-angle scattering operator for electron-ion collision are included. The physical effects of the collisions in the LOC to SOC transition can be understood by analyzing the phase space dynamics. Physics of intrinsic rotation reversal and E x B staircase, both of which were found to have close relations with LOC-SOC transition, will be discussed. (author)