Observation of Simultaneous Axial and Transverse Classical Heat Transport in a Magnetized Plasma

Abstract

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.

Energetic ion transport by microturbulence is insignificant in tokamaks

“…12 These observations were made in the LAPD afterglow for which the thermal background plasma transport is separately known to be classical 13 (i.e., there are no pressure gradients or turbulent fluctuations). In order to study energetic ion transport due to turbulence, an experiment 14 was conducted in the main discharge with a copper plate covering half of the cathode in order to create a large pressure gradient.…”

Exponential frequency spectrum and Lorentzian pulses in magnetized plasmas

“…This behavior corresponds quantitatively to that predicted by the classical theory of heat transport based on Coulomb collisions, 32,33 as has been documented extensively in previous work. 31,34 A second stage can be identified approximately 2 ms after beam injection when high-frequency oscillations become apparent in the electron temperature. These oscillations correspond to coherent drift-Alfvén waves driven unstable by the electron temperature gradient 35 and have been described in detail in a previous publication.…”

Spontaneous Fluctuations of a Temperature Filament in a Magnetized Plasma

“…Figure 5a displays the electron temperature profile in the direction transverse to the confining magnetic field at late times (7 ms) in the evolution of a temperature filament when the spontaneous fluctuations become broadband, at an axial distance of 410 cm from the beam injector. In Figure 5a the open circles correspond to measured values of T e deduced from a consistent analysis [7] of Langmuir probe data. The dashed curve corresponds to the prediction of a two-dimensional transport code [7] that uses classical electron heat-conduction coefficients.…”

“…At x 6.0 mm one observes a modest increase in T e with low-level fluctuations. The early increase in T e can be explained quantitatively [7] in terms of classical transport.…”

“…The experimental arrangement is identical to that used previously [7] to observe the heat transport exhibited by an expanding temperature plume. Under quiescent conditions heat was observed to be conducted at the classical rates, along as well as across the confining magnetic field.…”