Turbulence spreading and transport scaling in global gyrokinetic particle simulations

An intriguing observation in magnetically confined plasma experiments and in global gyrokinetic particle simulations of toroidal ion temperature gradient turbulence is that the fluctuations are microscopic, while the resulting turbulent transport is not gyro-Bohm [Z. Lin , Phys. Rev. Lett. 88, 195004 (2002)]. A possible resolution to this puzzle is identified as turbulence spreading from the linearly active (unstable) region to the linearly inactive (stable) region. Large scale gyrokinetic simulations found that transport driven by microscopic fluctuations is diffusive and local, whereas the fluctuation intensity is determined by nonlocal effects. Fluctuations are found to spread from the linearly active region to the linearly inactive region. This turbulence spreading reduces the fluctuation intensity in the unstable region, especially for a smaller device size, and thus introduces a nonlocal dependence in the fluctuation intensity. The device size dependence of the fluctuation intensity, in turn, is responsible for the observed gradual transition from Bohm to gyro-Bohm transport scaling. (C) 2004 American Institute of Physics.