| Popis: |
Recent progress in studies of transient or perturbative plasma transport in tokamak and helical plasmas is reviewed. Perturbative transport study involves monitoring temporal evolutions of plasma perturbations induced by various perturbation sources: typically, sawtooth oscillations, modulated ECH power, modulated gas puff rate, impurity or ice pellet injection, and fast ramp-up of the plasma current. In tokamaks, the incremental particle diffusion coefficient and electron thermal diffusivity obtained through a transient transport study technique are typically larger by a factor of two to five than those obtained by steady state power balance. This indicates that the relationship between the flux and the parameter gradients is in nonlinear or offset -linear. A reversal of electron temperature perturbations delta T_e is induced in ohmically heated tokamak plasmas by edge cooling using impurity or ice pellet injection and edge heating employing fast current ramp-up. This can be explained by the sudden change in electron heat diffusivity in the plasma core region and near the edge. This strong nonlocal or non-diffusive behaviour is not yet clarified. This delta T_e-reversal is never ob- served in W7-AS stellarator. Theoretical models are being developed. A model including both ion and electron temperature gradient-driven turbulence qualitatively ex plains the plasma responses for applied perturbations observed in some tokamak plasmas, without any artificial nonlocal transport effects. In the near future, detailed comparative studies between perturbative transport experiments and theoretical models would elucidate complex behaviours in turbulent transport in a toroidal plasma. |
