Current-free double layers: A review

Abstract

During the last decade, there has been an upsurge in the research on current-free DLs (CFDLs). Research includes theory, laboratory measurements, and various applications of CFDLs ranging from plasma thrusters to acceleration of charged particles in space and astrophysical plasmas. The purpose of this review is to present a unified understanding of the basic plasma processes, which lead to the formation of CFDLs. The review starts with the discussion on early research on electric fields and double layers (DLs) and ion acceleration in planar plasma expansion. The review continues with the formation of DLs and rarefaction shocks (RFS) in expanding plasma with two electron populations with different temperatures. The basic theory mitigating the formation of a CFDL by two-electron temperature population is reviewed; we refer to such CFDLs as double layers structures formation by two-temperature electron populations (TET-CFDLs). Application of TET-CFDLS to ion acceleration in laboratory and space plasmas was discussed including the formation of stationary steady-state DLs. A quite different type of CFDLs forms in a helicon plasma device (HPD), in which plasma abruptly expands from a narrow plasma source tube into a wide diffusion tube with abruptly diverging magnetic fields. The formation mechanism of the CFDL in HPD, referred here as current free double layer structure in helicon plasma device (HPD-CFDL), and its applications are reviewed. The formation of a TET-CFDL is due to the self-consistent separation of the two electron populations parallel to the ambient magnetic field. In contrast, a HPD-CFDL forms due to self-consistent separation of electrons and ion perpendicular to the abruptly diverging magnetic field in conjunction with the conducting wall of the expansion chamber in the HPD. One-dimensional theoretical models of CFDLs based on steady-state solution of Vlasov-Poisson system of equations are briefly discussed. Applications of CFDLs ranging from helicon double-layer thrusters (HDLTs) to the accelerations of ions in space and astrophysical plasmas are summarized.

Instability limits for spontaneous double layer formation

“…Such a current threshold may also play a role in the common observation in expanding helicon plasmas that the double layers only form below some operating neutral pressure threshold, i.e., an overall plasma density threshold. We note that in his recent review article, Singh 33 suggested that if ion-acoustic instabilities arise in spontaneously forming DLs, the turbulence created by the instability should "eventually disappear, making it possible for the DL and ion beam to reappear, setting up a relaxation type of oscillations [sic]." The experimental results reported here are completely consistent with such behavior.…”

Spontaneous ion beam formation in the laboratory, space, and simulation

“…18 Such a wave-like potential structure could explain these observations. Although more recent PIC simulations have typically yielded only solitary DL structures, 19 the review by Singh 20 suggests that when perpendicular electric fields develop near density gradients due to differing electron and ion Larmor radii, the perpendicular electric fields are shorted out by conducting boundaries in a laboratory plasma. The resulting parallel electric field may then be localized at a single DL or be spread out across multiple DLs.…”

RFEA Measurements of High-Energy Electrons in a Helicon Plasma Device with Expanding Magnetic Field

“…Later, others [14][15][16][17] have pointed out the 2D nature of the CFDL, in that the electrons follow the magnetic field lines and escape toward the side walls, while the non-magnetized ions follow a straight path from the source into the expansion chamber [18]. A curved 2D CFDL potential structure [19,20] and the conics of enhanced plasma density along the outermost magnetic fields emerging from the source region [21] has been observed.…”