Abstract
Strong turbulent waves (δn/n ∼0.5, f ∼5-40 kHz) are observed in the upgraded Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] on density gradients produced by an annular obstacle. Energetic lithium ions (Efast/Ti≥300, ρfast/ρs~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 wave fields. The characteristics of the fluctuations are modified by changing the plasma species from helium to neon and by modifying the bias on the obstacle. Different spatial structure sizes (Ls) and correlation lengths (Lcorr) of the wave potential fields alter the fast ion transport. The effects of electrostatic fluctuations are reduced due to gyro-averaging, which explains the difference in the fast-ion transport. A transition from super-diffusive to sub-diffusive transport is observed when the fast ion interacts with the waves for most of a wave period, which agrees with theoretical predictions.
Highlights
The transport of fast ions in electrostatic microturbulence is important in natural and laboratory plasmas
Experimental results reported in tokamaks confirms that, in the high energy regime, ions are well confined in electrostatic microturbulence
Turbulent waves associated with large density gradients and drift flow shear is observed
Summary
The transport of fast ions in electrostatic microturbulence is important in natural and laboratory plasmas. While a number of simulations reported fast ion transport in slab or toroidal geometry, the experimental study on this topic is very challenging because of the difficulty in diagnosing the fast ion population and turbulent wave fields accurately. We report the direct measurements of fastion transport in the presence of electrostatic waves with various characteristics in the LAPD. The fast ion beam cross-field transport measured for coherent waves (with long azimuthal correlation length and coherent mode structures) is at the classical level, indicating that wave-induced transport is small due to the large gyro-orbit.
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