Experimental study of two-fluid effects on magnetic reconnection in a laboratory plasma with variable collisionality

Abstract

This article describes the recent findings on two-fluid effects on magnetic reconnection in plasmas with variable collisionality in the magnetic reconnection experiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. The MRX device has been upgraded to accommodate a variety of reconnection operation modes and high energy density experiments by increasing its capacitor bank energy and extending the discharge duration. As our experimental operation regime has moved from the collisional to the collision-free, two-fluid effects have become more evident. It is observed that the two-dimensional profile of the neutral sheet is changed significantly from the rectangular shape of the familiar Sweet-Parker type to a double wedge shape as the collisionality is reduced and the reconnection rate increases. The recent evolution of our experimental research from the magnetohydrodynamics (MHD) to the two-fluid analysis is presented to illuminate the physics of Hall MHD in a collision-free reconnection layer. In particular, a clear experimental verification of an out-of-plane quadrupole field, a characteristic signature of the Hall MHD, has been made in the MRX neutral sheet, where the sheet width is comparable to the ion skin depth. It is important to note that the Hall effect, which occurs due to two-dimensional laminar flows of electrons in the reconnection plane, is observed together with the presence of low and high frequency magnetic turbulence, which often has three-dimensional structures. These observations in MRX have striking similarities to the recent magnetospheric measurements of reconnection region, in which the quadrupole component has been detected together with magnetic fluctuations.

Reconnection in Marginally Collisionless Accretion Disk Coronae

“…Since SP involves , which is normally governed by collisions (but see the discussion of inverse-Compton drag below), the condition SP < d i can be restated in terms of the electron mean free path, as k e; mfp > L( e m e /m i ) 1/2 , where e is the ratio of the electron pressure within the reconnection layer to the magnetic pressure outside it (Yamada et al 2006). This condition highlights the role of plasma collisionality in determining the regime of the reconnection process (Uzdensky 2007).…”

“…pi is the ion plasma frequency (e.g., Cassak et al 2006;Yamada et al 2006;Uzdensky 2007). Hereafter we will assume that most of the ions are protons, so that m i ¼ m p and Z i ¼ 1.…”

On the physical interpretation of Malyshkin’s (2008) model of resistive Hall magnetohydrodynamic reconnection

“…[3,4]). Note that the resistivity has dropped out, because similar resistive terms enter in both the z and y components of the electron equation of motion, balancing the electric force − ne E z and the Lorentz force B x j ez /c, respectively.…”

Identification of the Electron-Diffusion Region during Magnetic Reconnection in a Laboratory Plasma

“…A key signature of the Hall effect, a quadrupole out-of-plane magnetic field, has been observed in both space [13][14][15] and laboratory plasmas [16][17][18]. In addition, the electron diffusion region has also been observed in space [19][20][21][22], where it was identified by examining the violation of the frozen-in condition for the electrons.…”