Experimental studies of low density and temperature ion and electron sheaths

Abstract

Full characterization of nighttime ionospheric plasma requires access to both the ion and electron thermal core populations. Efforts to measure particle distributions with rocket and satellite detectors designed to study low energy particles are hindered because the magnitude of spacecraft charging is on the order of or greater than the energy of the bulk of ionospheric particles. This paper presents initial laboratory investigations exploring the formation of plasma sheaths with ionospheric electron energies, densities, and Debye lengths. The goal is to identify the difficulties and solutions to obtaining both thermal electron and ion velocity distributions on one payload. Sheaths around a long cylinder biased with respect to the vacuum vessel wall are studied to verify measurement procedures. An experimental setup in which two conducting spheres, with area ratios in excess of 100, are biased with respect to one another and not referenced to the wall simulates a payload-detector system. Data are compared with simple planar, cylindrical, and spherical sheath models. Ion-rich sheaths conformed with expectations. The criterion for the formation of electron-rich sheaths was found not only to depend on the ratio of ion collector area to electron collector area but also the ratio of the collector area to sheath area. Nonmonotonic electron sheaths obtained by embedding a positively biased electrode within the sheath of a more negative conductor are also explored. It was found that the superposition of two different potential geometries led to the formation of the nonmonotonic potential structures. These initial plasma ion and electron sheath investigations both clarify the behavior of a thermal electron detector previously flown in the ionosphere and explore a low energy parameter regime that is understudied in the laboratory.

Particle-in-cell simulation of incoherent scatter radar spectral distortions related to beam-plasma interactions in the auroral ionosphere

“…The thermal electron distribution, going from ∼0.08 to ∼6 eV has never been measured effectively in the night time midaltitude auroral zone, where spacecraft potentials are usually a few volts negative therefore repelling those electrons. Although, recent attempts of improving the measurement of the thermal electron distribution have been made in satellites/ rockets [MacDonald et al, 2006] as well as laboratory/ simulation experiments [Frederick-Frost and Lynch, 2007], leading to a better understanding of the issues involved in this measurement process, no direct measurement of ionospheric electron distribution function has been obtained so far. Thus, it is desired to explore what would be the possible signatures on the IS spectrum if the electron beam distribution causing the NEIAL is really within this undermeasured thermal distribution range.…”

Potential of a plasma bound between two biased walls

“…When a strong positive bias is locally applied with a probe or at the wall confining the plasma, the formation of negative space-charge sheaths or electron sheaths is observed. 3,10,13 In the case of the electron sheath, a potential barrier g se < 0 accelerates electrons and prevents most of ions from arriving at the wall ( Figure 1). As a matter of fact, biasing experiments often show that the plasma is bound between an ion and an electron sheath.…”

“…Time-independent biasing is used in plasma experiments for different purposes, namely for the measure of the ion and electron temperatures with electrostatic probes, 2,3 in plasma thrusters for space propulsion, 4 to study the effect of shear flow on turbulence, [6][7][8][9] for the study of dust particles, 5 and for the control of turbulence in magnetic fusion devices. [10][11][12] A bias may induce local perturbations of the plasma potential.…”

Note: Flowing ion population from a resonance cavity source

“…This facility is being used for the design, testing, and calibration of devices for future rocket missions to study the aurora, 4 and for the study of fundamental plasma physics. 5 Despite the successful use of this facility, a rigorous understanding of its two-stage plasma generation process has a) Electronic mail: Lisa.E.Gayetsky@Dartmouth.edu. been missing.…”