Topic/Type: 1.2 Fusion Plasmas (magnetic & inertial confinement), Oral
University of California, Irvine
Large scale simulations of collisionless trapped electron mode (CTEM) turbulence using gyrokinetic toroidal code1 (GTC)  finds the electron heat transport exhibiting a gradual transition from Bohm to gyroBohm scaling when the device size is increased. The deviation from the gyroBohm scaling could be induced by large turbulence eddies, turbulence spreading and non-diffusive transport process. Radial correlation function shows that CTEM turbulence eddies are predominantly microscopic but with a significant tail in the mesoscale. The macroscopic, linear streamers are mostly destroyed by the zonal flow shearing in this simulation, which is confirmed by our comprehensive analysis of kinetic and fluid time scales showing that zonal flow shearing is the dominant decorrelation mechanism. The mesoscale streamers result from a dynamical process of radial streamers breaking by zonal flows and merging of microscopic eddies. It is further found that the radial profile of the electron heat conductivity only follows the profile of fluctuation intensity on a global scale, whereas the ion transport tracks more sensitively local fluctuation intensity. This suggests the existence of a nondiffusive component  in the electron heat flux, which arises from the ballistic radial ExB drift of trapped electrons due to a combination of the presence of mesoscale eddies and the weak detuning of the toroidal precessional resonance that drives the CTEM instability. In contrast, the ion radial excursion is not affected by mesoscale eddies due to the parallel decorrelation, which is not operational for trapped electrons because of the bounce averaging process associated with the fast parallel motion of electrons. This is in contrast to the good agreement between quasilinear transport theory and simulation results of transport of ion heat , toroidal momentum , and energetic particle  in ITG turbulence and. Work supported by SciDAC GPS, GSEP, CPES centers. In collaborations with Y. Xiao, I. Holod, W. L. Zhang.
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