Study of Fast Ion Transport in Turbulent Waves in the Large Plasma Device (LAPD)

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Study of Fast Ion Transport in Turbulent Waves in the Large Plasma Device (LAPD) Shu Zhou, W. Heidbrink, H. Boehmer, R. McWilliams, University of California, Irvine T. Carter, S. K. P. Tripathi, S. Vincena, University of California, Los Angeles Due to gyroradius averaging and drift-orbit averaging, the transport of fast ions by microturbulence in tokamaks often is smaller than experienced by thermal ions [1]. In this experiment, strong drift wave turbulence (δn/n ~1, f ~5-20 kHz) is observed in LAPD in gradient regions produced by obstacles. Energetic lithium ions ( Efast /E thermal 300 , fast/~ s 10 ) orbit through the turbulent region. Scans with a collimated analyzer and with probes give detailed profiles of the fast ion spatial distribution and of the fluctuating fields. The fast-ion transport decreases rapidly with increasing fast-ion gyroradius, which is explained well by gyro-averaging theory [2]. The characteristics of the fluctuations are modified by changing the plasma species from helium to neon, and by modifying the bias on the obstacle. A transition from non-diffusive to diffusive transport is observed when the fast ion time-of-flight in the waves exceeds its correlation time ( corr ~0 . 1 ms ) with the waves. Different spatial correlation lengths of the wave potential fields also alter the fast ion transport. A Monte Carlo trajectory-following code simulates the interaction of the fast ions with stationary two-dimensional electrostatic wave potential fields and with three-dimensional, time-dependent fields. Comparison between observation and modeling is presented. [1] W. W. Heidbrink, M. Murakami, J. M. Park, et al., Plasma Phys. Controlled Fusion 51, 125001 (2009) [2] Shu Zhou, W. W. Heidbrink, H. Boehmer, et al., Phys. Plasmas, 17, 092103 (2010) .

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