Electric fields and double layers in plasmas

Abstract

Various mechanisms for driving double layers in plasmas are briefly described, including applied potential drops, currents, contact potentials, and plasma expansions. Some dynamical features of the double layers are discussed. These features, as seen in simulations, laboratory experiments, and theory, indicate that double layers and the currents through them undergo slow oscillations which are determined by the ion transit time across an effective length of the system in which the double layers form. It is shown that a localized potential dip forms at the low potential end of a double layer, which interrupts the electron current through it according to the Langmuir criterion, whenever the ion flux into the double is disrupted. The generation of electric fields perpendicular to the ambient magnetic field by contact potentials is also discussed. Two different situations have been considered; in one, a low-density hot plasma is sandwiched between high-density cold plasmas, while in the other a high-density current sheet permeates a low-density background plasma. Perpendicular electric fields develop near the contact surfaces. In the case of the current sheet, the creation of parallel electric fields and the formation of double layers are also discussed when the current sheet thickness is varied. Finally, the generation of electric fields (parallel to an ambient magnetic field) and double layers in an expanding plasmas is discussed.

Particle-in-cell modeling of magnetized argon plasma flow through small mechanical apertures

“…The ion velocity in the ER shows coherent ∼ 20% variations with a periodicity of ∼ 0.3 mm, which propagate at approximately the ion sound speed. This length is similar to the length of the DL, a coincidence noted by earlier researchers [17].…”

“…It is not known whether this effect is due to the under-resolution of the aperture itself (and its fields) or of the ion acoustic mode wavelength near the aperture. Ion acoustic double layers have been reported [17,20] to form a dip in φ and n p on the low potential side, and such dips are regularly revealed in MNX simulations and experiments. Some additional discussion on the possible role of IAWs is provided in Sec.…”

“…For example, the plots reveal the DL separates two sets of distributions between the MC and ER, with quantitatively different injected and ionized populations found in each. Potentials are known to develop when plasmas of different properties come into contact [17].…”

“…As reviewed and discussed in [17], when the perpendicular +E r fields within the aperture are shorted out outside of the aperture, 2D potential structures are generated with electric fields parallel to the applied magnetic field. As also pointed out in [17], the generation of this parallel potential drop (E z ) by shorting out the perpendicular (E r ) fields away from their source region (the aperture) is equivalent to applying a potential drop. In other words, a perpendicular potential drop can become a parallel potential drop (the DL) due to boundary conditions and a magnetic field.…”

Suppression of Energetic Electron Transport in Flares by Double Layers

“…A double layer (DL) is a nonlinear electrostatic electric field localized within two adjacent layers of equal and opposite net charge (Block 1978;Raadu & Rasmussen 1988;Singh et al 1987). It is highly localized to ∼10 λ De because quasineutrality in a plasma can only be violated on λ De scales.…”