Studies of strong laboratory double layers and comparison with computer simulation

Abstract

Strong electrostatic double layers were produced with a triple plasma configuration in the large plasma chamber (5 m long, 2·5 m diameter) at IPM in Freiburg, Federal Republic of Germany. Owing to relatively low densities (1011 1012m−3), Debye lengths of a few centimetres and layer thicknesses of the order of a metre were obtained. Layers both with and without magnetic fields were studied. Analysis of particle spectra prove that wave-particle interactions play a minor role in maintaining the strong electric field. The three-dimensional potential distribution is measured and is qualitatively discussed in terms of particle budget. For cases with a magnetic field it tends to agree with observations above the aurora. Comparisons are made with double-layer theory and computer experiments, and general agreement is found as far as the available results allow.

Review of recent laboratory double layer experiments

“…They can be produced in laboratory plasmas in a wide variety of shapes and sizes which range from very strong, with e~/T e as large 2000 (where e is the electron charge, q5 is the potential step at the double layer, and T e is the electron temperature in eV), to weak with e~/Te approximately equal to 1 . The shape of the potential step can be one dimensional (Hershkowitz etal., 1981), U shaped (Baker et aL, 1981), or three dimensional . They can be stable or transient.…”

“…There are several alternative explanations for double layers. In steady state they can be BGK solutions (Bernstein et al, 1957) which depend entirely on the distribution functions of both ions and electrons at boundaries far from the double layer potential step or they can depend on the presence of turbulence (DeGroot et al, 1977;Chan and Hershkowitz, 1982) and phase space vortices (P6cseli, 1984) near the double layer region. Another possibility is that they require ionization of plasma on the high potential side of the double layer , which can be enhanced by the energy gain that electrons receive when they accelerate across the double layer.…”

“…Solutions were then found by appropriate choice of trapped ions on the low potential side. Block (1972Block ( , 1981 showed that electrons also needed a minimum drift energy. However, both ion and electron dirft criteria can be relaxed when significant trapped particle densities are present (Kan and Lee, 1980).…”

“…The evolution of laboratory double layers has been rapid since the introduction of the triple plasma device and Q machines. They have evolved from weak (e~b/T e = 3-6) (Quon and Wong, 1976) to strong (ec~/T e > 10) (Coakley etal., 1978) to very strong and then to very weak , from one dimensional to two dimensional (Coakley etal., 1979) and to 'U' shaped (Baker et al, 1981) from stationary to moving , from single to multiple (Chart and , and most recently from BGK-like to ion acoustic . Along with this evolution there has also been an evolution in the diagnostic techniques used to study double layers and increased understanding of what is needed to characterize double layers.…”

A new hydrodynamic analysis of double layers

“…) arrivedat electricfieldsinsideof theseturbulence areas andmaybe interpreted assomesortof a doublelayer behavior. The laboratory experiments showed doublelayersin mercury discharges (Torv6n,1981;Stangeby and Allen, 1973), Q machines (Sato etal., 1976), andtripledevices where twoplasmas atdifferent elec.tric potential are connected through gridsby aplasma whichhasthena doublelayeraccording tothedifference ofthevoltages plus the difference of thetemperatures between the two outerplasmas (CoakleyandHershkowitz, 1981;Quonand Wong, 1976;Leunget al, 1980).The geometrycanbe one-dimensional ,twodimensional (Baker et al, 1981),or three-dimensional (Merlinoet al, 1984).…”