A systematic study is made of the spontaneous growth of fluctuations in temperature, density, and magnetic field in a narrow ͑on the order of the electron skin depth͒ field-aligned temperature filament embedded in a large magnetized plasma. Two broad classes of fluctuation ͑''low'' and ''high'' frequency modes͒ have been identified and studied in detail. A high-frequency drift-Alfvén mode grows at frequencies about one tenth the ion gyrofrequency in the region of the filament where the temperature gradient is large. The measured radial profiles of the density and magnetic field fluctuations associated with this mode agree well with theoretical predictions. The high-frequency mode has been observed to exhibit several interesting nonlinear features, including steepening wave form, progression in azimuthal mode number, coupling to the low frequency mode with subsequent sideband generation, and eventually a transition to broad band turbulence. The nature of the low-frequency mode which has frequencies about one fiftieth of the ion gyrofrequency is less certain, but it has been identified as a spatially localized, azimuthally symmetric mode consisting primarily of temperature fluctuations. Both the high and low-frequency modes give rise to electron heat transport at rates in excess of the classical values.

The transport of heat deposited by a small low voltage electron beam in a strongly magnetized plasma is studied for a time interval in excess of two thousand ion gyro-periods. Electron temperatures are observed to evolve along and across the magnetic field at the classically predicted rates up to times approaching one thousand gyro-periods. At later times the onset of low frequency fluctuations (below one tenth the ion gyrofrequency) coincides with departures from classical behavior. It should be emphasized that the past decade has seen the development of increasingly sophisticated basic plasma experiments probing the nature of classical transport. Noteworthy among these are measurements of the effective cross-field thermal conductivity 5 , determination of velocity-space transport 6 , and test particle transport in non-neutral plasmas 7 .3The classical electron thermal conductivity in the axial direction (along the confining magnetic field, B 0 ) is given 2 by κ || = (3.16)n e T e τ e m e , and, in the transverse direction, by κ ⊥ = (1.47)κ || (Ω e τ e ) −2 , where n e is the electron plasma density, T e the electron temperature (in electron-volts), m e the electron mass and τ e = (3. (Ω e τ e ) −1 is satisfied, the rate of radial energy loss across the surface of a heated filament exceeds the rate of axial loss through the ends, and the anisotropic nature of the heat transport can be probed. Meeting this condition is the essence of the present study.

A systematic study is made of the axial and transverse heat transport that arises when a microscopic temperature filament is generated, under controlled conditions, in a magnetized plasma of large dimensions. For early times and relatively small temperature gradients the study conclusively demonstrates the two-dimensional pattern characteristic of the classical theory based on Coulomb collisions. The full nonlinear dependence of the transverse and axial electron heat conductivities is sampled through temperature changes in the range δTe/Te∼1–10. The dependence on the confining magnetic field is explored over a factor of 3 (∼ factor of 10 in transverse conductivity). It is found that under quiescent conditions, the observed behavior agrees with classical theory within the experimental uncertainties. However, over long times and/or for steep temperature gradients, fluctuations develop spontaneously and cause a significant departure from the predictions of the classical theory.

This experiment illustrates the spatiotemporal pattern of the fluctuations that spontaneously develop in a magnetized temperature filament whose transverse scale is comparable to the electron skin depth. A high-frequency mode exhibits a striking spiral structure and is identified as a drift-Alfven eigenmode. A low-frequency mode is found to be localized near the center of the filament. It is documented that the fluctuations significantly increase the transport of heat beyond the prediction of classical theory based on Coulomb collisions.

Abstract.A systematic laboratory study of controlled density and temperature filaments having transverse scale length comparable to the electron skin-depth has been performed in the large plasma device (LAPD) at UCLA. It is found that large amplitude shear Alfvén waves develop spontaneously and are localized within the filaments. As the plasma conditions change (e.g., lowering the plasma beta parameter or increasing the heating power) the highly coherent eigenmodes develop into broad band Alfvénic turbulence. A kinetic description that includes the effect of coulomb collisions has been developed to understand the linear properties of the modes. Excellent agreement with the measured eigenfunctions is found for the density filaments in the higher beta regime in which the modes remain strongly coherent. The similarity between the broad band fluctuation spectra generated in a variety of plasma configurations suggest the possibility of a universal process involving filamentary structures and spontaneously generated Alfvénic turbulence.