N. Wild

Directional velocity analyzer for measuring electron distribution functions in plasmas

A directional velocity analyzer has been developed for measuring electron distribution functions in plasmas. It contains a collimating aperture which selects particles from a narrow cone in velocity space and a retarding potential analyzer. The distribution function f(v, θ, φ) is obtained from a large number of analyzer traces taken at different angles θ, φ. In addition, the small analyzer can be moved in space and the measurements are time resolved so as to obtain the complete phase space information f (v, r, t). The large data flow of this seven‐variable function is processed with a high‐speed digital data‐acquisition system. The new electron velocity analyzer is applicable over a wide parameter range in electron energies and densities. Various cases of anisotropic distributions such as beams, shells, tails, and drifts have been successfully investigated.

Magnetic Field Line Reconnection Experiments

A laboratory experiment concerned with the basic physics of magnetic field line reconnection will be discussed. Stimulated by important processes in space plasmas and anomalous transport in fusion plasmas the work addresses the following topics: Dynamic magnetic fields in a high beta plasma, magnetic turbulence, plasma dynamics and energy transport. First, the formation of magnetic neutral sheets, tearing and island coalescence are shown. Nonstationary magnetic fluctuations are statistically evaluated displaying the correlation tensor BB in the ω-k domain for mode identification. Then, the plasma properties are analyzed with particular emphasis on transport processes. Although the classical fluid flow across the separatrix can be observed, the fluctuation processes strongly modify the plasma dynamics. Direct measurements of the fluid force density and ion acceleration indicate the presence of an anomalous scattering process characterized by an effective scattering tensor ν*. Turbulence also enhances the plasma resistivity η* by one to two orders of magnitude. Measurements of the three-dimensional electron distribution function fe(vx, vy, vz) using a novel energy analyzer exhibit the formation of runaway electrons in the current sheet. Associated microinstabilities are observed. Finally, a macroscopic disruptive instability of the current sheet is observed. Excess magnetic field energy is converted at a double layer into particle kinetic energy and randomized through beam-plasma instabilities. These laboratory results will be compared with related observations in space and fusion plasmas.

Electron distribution functions in a current sheet

Using a novel directional velocity analyzer the electron distribution function  f(v,r,t) is measured in a magnetic‐field‐line reconnection experiment. Runaway electrons are observed inside the current sheet, a result important for transport processes and instabilities.

Electron temperature measurements using a 12‐channel array probe

The electron temperature in a pulsed high‐β ( β≡2μnkT/B2≊1) plasma is studied as a function of time using a miniature array of 12 planar Langmuir probes. Instead of sweeping the bias voltage on the collectors, each is set at a different voltage corresponding to points along the characteristic I–V curve. A fast‐analog multiplexer, together with a computerized data‐acquisition system, allows determination of Te to within a few percent accuracy, with a time resolution of 1 μs. The probe is used to study temperature fluctuations and heat transport in a plasma current sheet.