Radial Electric Fields and Global Electrostatic Microinstabilities in Tokamaks and Stellarators

There is mounting theoretical and experimental evidence that radial electric fields play a pivotal role in the dynamics of turbulence responsible for anomalous transport in tokamaks. In this paper, the effects of these radial electric fields on various electrostatic modes are investigated with a full radius gyrokinetic formulation that unifies the axisymmetric tokamak and helical stellarator perspectives. In particular, it is shown that the shearing rate criterion for stabilization [T .S. Hahm and K.H. Burell, Phys. Plasmas 2, 1648 (1995)] established for tokamaks can be generalized to helical configurations. In addition, another stabilizing mechanism independent of the shearing rate has been found for the cases of toroidal and helical Ion Temperature Gradient (ITG) modes. On the other hand, it is shown that the inclusion of trapped particle (ion and electron) dynamics can have strong consequences: in some cases the presence of radial electric fields is destabilizing rather than stabilizing. (author)