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Author: L. J. Zheng
Requested Type: Poster
Submitted: 2016-02-15 17:01:24

Co-authors: W. Horton, H. Miura, T. H. Shi, H. Q. Wang, and M. Furukawa

Contact Info:
University of Texas at Austin
RLM 11.222 Speedway
Austin, Texas   78712

Abstract Text:
At tokamak edge the magnetic field topology changes abruptly from the closed magnetic flux surfaces to the open field line structure in the scrape-off layer (SOL). The transport properties differ completely across the last closed flux surface (LCFS). Inside LCFS particles are confined on the magnetic surfaces and the primary transport is perpendicular to magnetic surfaces; while in SOL the parallel transport dominates. This difference in the transport properties causes the charge and current densities to become discontinuous at tokamak edge. This work investigates the effects of these two discontinuities on tokamak edge stability and turbulent transport. First, as it is known that, due to the larger ion orbit losses, non-neutralized electrons can accumulate at tokamak plasma edge. We point out that, beside the E x B drift, the excess electrons can can contribute an electric compressive stress in the direction parallel to magnetic field by their mutual repulsive force. We show that this newly recognized compressive stress can significantly change tokamak edge stability. Second, we also point out that there is a current jump across LCFS, since the current in SOL is saturated by the divertor plates. The current jump can lead the external kink modes, like peeling-balloong modes, to convert to the tearing modes, due to the current interchange effects. Our results indicate that the charge and current discontinuities at tokamak edge need to be taken into account in evaluation of tokamak edge stability and transport.

Physics of Plasma Edge and Divertor Region