Physics of transport barriers

Abstract

The concept of ExB flow velocity shear suppression is utterly fundamental in modern fusion research. It is asserted that there are models enabling to understand the physics involved in LH transitions. To improve the understanding of the mechanisms leading to the formation of Transport Barriers, especially the relation between Internal and Edge barriers it is necessary to invoke the issue of electric fields. Edge transport barriers are the feature of the H-mode, the baseline regime of ITER, whereas Internal Transport Barriers are used to develop regimes that might be employed for steady state operation of ITER , definitely beneficial for design and operation of fusion power plants in the future. Their synergy will be addressed. Plasma flows are closely connected to electric fields. Therefore, their role is crucial for understanding of tokamaks aimed at the achievement of fusion energy. This appears in the well known neoclassical theory as the most accomplished and selfconsistent basis for understanding of fusion plasmas. It pertains to the novel concept of "zonal flows" emerging from the recent development of gyro-kinetic transport codes. The equilibrium poloidal and toroidal flows are also crucial for the concept of the electric field shear suppression of plasma turbulence in tokamaks. Yet, this timely and topical issue has remained largely unaddressed experimentally because of great difficulties in measuring flows in plasmas.

Anomalous Ion Diffusion and Radial-Electric-Field Generation in a Turbulent Edge Plasma Potential Weakly Correlated in Time and Space

“…The cross-field diffusion of ions is, therefore, not accompanied by the analogous electron diffusion and a deficit of positive charge will appear. According to [7], this charge-separation effect may contribute to the generation of a radial electric field. The first attempt to estimate the field strength of the latter was made in [10] where, taking into account a realistic concentration of C + , a field strength on the order of 1 kV/m was found.…”

“…In our foregoing brief comments on the simple model [1] (e.g., [7]), we used a strongly simplified model of the turbulent potential structures, namely a spatially periodic and timeindependent potential. Using the Hamiltonian approach (which takes into account the cyclotron motion) we have found [2] for C + impurity ions and usual potential amplitudes a substantial increase in the diffusion of these ions, exhibiting both the Gaussian and Lévy-walk forms and resulting in radial-electricfield generation.…”

Derivation of radial electric fields using kinetic theory in tokamak

“…These fields and their effects have been considered theoretically and experimentally in a variety of devices such as tokamaks, stellarators, reversed field pinches, mirrors, etc. In theoretical evaluations these fields can follow the standard neoclassical theory in the edge of plasma (Tendler et al 2004), or they can follow the equation that is obtained from two fluids theory (Burrell 1996). The plasmas are described as fluids or statistical systems.…”