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Title: Development of electrostatic turbulence from drift-interchange instabilities in a toroidal plasma

Abstract

Electrostatic instabilities develop on TORPEX (TORoidal Plasma EXperiment) [A. Fasoli et al., Phys. of Plasmas, 13, 55902 (2006)] in the bad curvature region and propagate consistently with the drift wave dispersion relation. The wave number and frequency spectra are coherent at the location where the instabilities are generated, then broaden along the ExB convection. The phase coupling between spectral components at different frequencies, measured at different locations over the plasma cross section, indicates that the transition from a coherent to a turbulent spectrum is mainly due to three-wave interaction processes. Nonlinear interactions are measured between the linearly unstable mode and fluctuations with larger frequency, with transfer of energy away from the linearly unstable mode. The results are consistent with a nonlinearity induced by the convection of density fluctuations by the ExB fluctuating velocity.

Authors:
; ;  [1]
  1. CRPP-EPFL, Association EURATOM-Confederation Suisse, Lausanne (Switzerland)
Publication Date:
OSTI Identifier:
20974981
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2731323; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; CONVECTION; DISPERSION RELATIONS; DRIFT INSTABILITY; ELECTROMAGNETIC FIELDS; FLUCTUATIONS; FLUTE INSTABILITY; NONLINEAR PROBLEMS; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA DRIFT; TOKAMAK DEVICES; TURBULENCE; WAVE PROPAGATION

Citation Formats

Poli, F. M., Podesta, M., and Fasoli, A. Development of electrostatic turbulence from drift-interchange instabilities in a toroidal plasma. United States: N. p., 2007. Web. doi:10.1063/1.2731323.
Poli, F. M., Podesta, M., & Fasoli, A. Development of electrostatic turbulence from drift-interchange instabilities in a toroidal plasma. United States. doi:10.1063/1.2731323.
Poli, F. M., Podesta, M., and Fasoli, A. Tue . "Development of electrostatic turbulence from drift-interchange instabilities in a toroidal plasma". United States. doi:10.1063/1.2731323.
@article{osti_20974981,
title = {Development of electrostatic turbulence from drift-interchange instabilities in a toroidal plasma},
author = {Poli, F. M. and Podesta, M. and Fasoli, A.},
abstractNote = {Electrostatic instabilities develop on TORPEX (TORoidal Plasma EXperiment) [A. Fasoli et al., Phys. of Plasmas, 13, 55902 (2006)] in the bad curvature region and propagate consistently with the drift wave dispersion relation. The wave number and frequency spectra are coherent at the location where the instabilities are generated, then broaden along the ExB convection. The phase coupling between spectral components at different frequencies, measured at different locations over the plasma cross section, indicates that the transition from a coherent to a turbulent spectrum is mainly due to three-wave interaction processes. Nonlinear interactions are measured between the linearly unstable mode and fluctuations with larger frequency, with transfer of energy away from the linearly unstable mode. The results are consistent with a nonlinearity induced by the convection of density fluctuations by the ExB fluctuating velocity.},
doi = {10.1063/1.2731323},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Low frequency electrostatic instabilities are investigated on TORPEX [Fasoli, Labit, McGrath, Mueller, Podesta, and Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device for basic plasma physics experiments with a toroidal magnetic field 100 mT and a small vertical magnetic field ({<=}4 mT). A two-dimensional (2D) profile of the frequency and amplitude of density and potential fluctuations is reconstructed using electrostatic probes with high space and time resolution. The measured phase velocity, corrected for the Doppler shift induced by the ExB drift, is consistent with the electron diamagnetic drift velocity. The local dispersion relation, measured along and acrossmore » the magnetic field, is in agreement with the predictions of a linear kinetic slab model for drift waves. Unstable modes are generated in regions of unfavorable curvature, where the pressure gradient is colinear with the magnetic field gradient. It is demonstrated that the curvature of the magnetic field lines is essential for driving the observed instabilities, which are therefore identified as drift-interchange modes.« less
  • By solving numerically the two-field nonlinear model equations for the potential and density fluctuations in the presence of a density gradient, magnetic curvature with shear, and poloidal velocity shear, turbulent spectra and diffusion rates are studied. When the adiabatic parameter ({Omega}{sub {ital e}}/{nu}{sub {ital e}})({rho}{sub {ital s}}{sup 2}/{ital R}{sup 2})/({kappa}{rho}{sub {ital s}}) is small, a trend of a dual cascade, normal cascade of the density fluctuations, and an inverse cascade of the potential fluctuations, is seen in the wave number spectra, producing a large particle flux proportional to {nu}{sub {ital e}}{sup (1/3)}. Here {ital R} is the major radius, {rho}{submore » {ital s}} is the ion Larmor radius at the electron temperature, {Omega}{sub {ital e}} is the electron cyclotron frequency, {nu}{sub {ital e}} is the electron ion collision frequency, and {kappa} is an inverse scale length of the background density gradient. Parallel wave numbers are represented by 1/{ital R}. For large ({Omega}{sub {ital e}}/{nu}{sub {ital e}})({rho}{sub {ital s}}{sup 2}/{ital R}{sup 2})/({kappa}{rho}{sub {ital s}}), the electrons become adiabatic, with a significantly reduced particle flux proportional to {nu}{sub {ital e}}. In the presence of an externally imposed radial electric field with a negative (positive) polarity, {ital E}{sub {ital r}}{lt}0 ({ital E}{sub {ital r}}{gt}0), the poloidal velocity shear in the {bold E}{times}{bold B} drift motion suppresses (enhances) the fluctuation level in the growth phase; however, these effects practically disappear in the saturated state. The radial electric field produced by the inverse cascade due to the convective nonlinearity is also studied; however, its effect on the particle transport is mild.« less
  • The effects of spatially varying magnetic field strength on the scaling properties of plasma turbulence, modelled by an extended form of Hasegawa-Wakatani model, are investigated. We study changes in the intermittency of the velocity, density, and vorticity fields, as functions of the magnetic field inhomogeneity C=-{partial_derivative} ln B/{partial_derivative}x. While the velocity fluctuations are always self-similar and their scaling is unaffected by the value of C, the intermittency levels in density and vorticity change with parameter C, reflecting morphological changes in the coherent structures due to the interchange mechanism. Given the centrality of vorticity in conditioning plasma transport, this result ismore » of interest in scaling the results of transport measurements and simulations in tokamak edge plasmas, where drift-interchange turbulence in the presence of a magnetic field gradient is likely to occur.« less
  • The transition from a regime dominated by drift instabilities to a regime dominated by pure interchange instabilities is investigated and characterized in the simple magnetized toroidal device TORPEX [TORoidal Plasma EXperiment, A. Fasoli et al., Phys. of Plasmas 13, 055906 (2006)]. The magnetic field lines are helical, with a dominant toroidal component and a smaller vertical component. Instabilities with a drift character are observed in the favorable curvature region, on the high field side with respect to the maximum of the background density profile. For a limited range of values of the vertical field they coexist with interchange instabilities inmore » the unfavorable curvature region, on the plasma low field side. With increasing vertical magnetic field magnitude, a gradual transition between the two regimes is observed on the low field side, controlled by the value of the field line connection length. The observed transition follows the predictions of a two-fluid linear model.« less
  • In this Letter, the influence of the ''Coriolis drift'' on small scale instabilities in toroidal plasmas is shown to generate a toroidal momentum pinch velocity. Such a pinch results because the Coriolis drift generates a coupling between the density and temperature perturbations on the one hand and the perturbed parallel flow velocity on the other. A simple fluid model is used to highlight the physics mechanism and gyro-kinetic calculations are performed to accurately assess the magnitude of the pinch. The derived pinch velocity leads to a radial gradient of the toroidal velocity profile even in the absence of a torquemore » on the plasma and is predicted to generate a peaking of the toroidal velocity profile similar to the peaking of the density profile. Finally, the pinch also affects the interpretation of current experiment000.« less