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Anomalous diffusion and radial electric field generation due to edge plasma turbulence

It is generally accepted that tokamak edge plasma turbulence causes anomalous diffusion. Potential structures, formed as a result of plasma turbulence, are observed in the poloidal plane of tokamaks (e.g., [1], [2]), with typical poloidal correlation lengths 20 10 − ≈ λ mm, lifetimes 20 10 − ≈ τ μs, and amplitudes U < 100 V. Theoretical studies addressing the anomalous diffusion in these fields are usually based on the test-particle drift approximation and on the electrostatic field resulting from the Hasegawa-Mima model (see, e.g. [3]) or the Hasegawa-Wakatani model [4]. In our preceding short comments (e.g. [5]), we discussed the effect of the anomalous ion diffusion in question on radial electric field generation. There we used a very simplified model of the turbulent potential structures, namely a spatially periodic and time-independent potential. Using a Hamiltonian approach (which also takes into account the cyclotron motion), we have found, for impurity ions C and usual potential amplitudes, a substantial increase in the diffusion of these ions (both of the Gaussian and Levy-walk forms) [6], resulting the generation of a radial electric field [5]. Using a drift approximation for this case, no diffusion and no electric field is observed. Hence, there is a major difference between these two approaches and there can be some uncertainty in using the drift approximation.