The properties of linear instabilities in the Large Plasma Device [W. Gekelman et al., Rev. Sci. Inst., 62, 2875(1991] are studied both through analytic calculations and solving numerically a system of linearized collisional plasma fluid equations using the 3D fluid code BOUT [M. Umansky et al., Contrib. Plasma Phys. 180, 887 (2009)],which has been successfully modified to treat cylindrical geometry. Instability drive from plasma pressure gradients and flows is considered, focusing on resistive drift waves, the Kelvin-Helmholtz and rotational interchange instabilities. A general linear dispersion relation for partially ionized collisional plasmas including these modes is derived and analyzed. For LAPD relevant profiles including strongly driven flows it is found that all three modes can have comparable growth rates and frequencies.Detailed comparison with solutions of the analytic dispersion relation demonstrates that BOUT accurately reproduces all characteristics of linear modes in this system.
Numerical simulation of plasma turbulence in the Large Plasma Device (LAPD)
[Gekelman et al, Rev. Sci. Inst., 62, 2875, 1991] is presented. The model,
implemented in the BOUndary Turbulence (BOUT) code [M. Umansky et al, Contrib.
Plasma Phys. 180, 887 (2009)], includes 3-D collisional fluid equations for
plasma density, electron parallel momentum, and current continuity, and also
includes the effects of ion-neutral collisions. In nonlinear simulations using
measured LAPD density profiles but assuming constant temperature profile for
simplicity, self-consistent evolution of instabilities and
nonlinearly-generated zonal flows results in a saturated turbulent state.
Comparisons of these simulations with measurements in LAPD plasmas reveal good
qualitative and reasonable quantitative agreement, in particular in frequency
spectrum, spatial correlation and amplitude probability distribution function
of density fluctuations. For comparison with LAPD measurements, the plasma
density profile in simulations is maintained either by direct azimuthal
averaging on each time step, or by adding particle source/sink function. The
inferred source/sink values are consistent with the estimated ionization source
and parallel losses in LAPD. These simulations lay the groundwork for more a
comprehensive effort to test fluid turbulence simulation against LAPD data
Turbulence calculations with a 3D collisional fluid plasma model demonstrate qualitative and semi-quantitative similarity to experimental data in the Large Plasma Device [W. Gekelman et al., Rev. Sci. Inst. 62, 2875(1991], in particular for the temporal spectra, fluctuations amplitude, spatial correlation length, and radial particle flux. Several experimentally observed features of plasma turbulence are qualitatively reproduced, and quantitative agreement is achieved at the order-of-magnitude level. The calculated turbulence fluctuations have non-Gaussian characteristics, however the radial flux of plasma density is consistent with Bohm diffusion. Electric polarization of density blobs does not appear to play a significant role in the studied case. Turbulence spectra exhibit direct and inverse cascades in both azimuthal and axial wavenumbers and indicate coupling between the drift instability and Kelvin-Helmholtz mode.
Strong drift wave turbulence is observed in the Large Plasma Device ͓H. Gekelman et al., Rev. Sci. Instrum. 62, 2875 ͑1991͔͒ on density gradients produced by a plate limiter. Energetic lithium ions orbit through the turbulent region. Scans with a collimated ion analyzer and with Langmuir probes give detailed profiles of the fast ion spatial distribution and the fluctuating fields. The fast ion transport decreases rapidly with increasing fast ion gyroradius. Unlike the diffusive transport caused by Coulomb collisions, in this case the turbulent transport is nondiffusive. Analysis and simulation suggest that such nondiffusive transport is due to the interaction of the fast ions with stationary two-dimensional electrostatic turbulence.
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