M. C. Griskey

Laboratory studies of magnetic vortices. III. Collisions of electron magnetohydrodynamic vortices

Magnetic vortices in the parameter regime of electron magnetohydrodynamics are studied in a large laboratory plasma. The vortices consist of magnetic field perturbations, which propagate in the whistler mode along a uniform dc magnetic field. The magnetic self-helicity of the spheromak-like field perturbations depends on the direction of propagation. Vortices with opposite toroidal or poloidal fields are launched from two antennas and propagated through each other. The vortices collide and propagate through one another without an exchange of momentum, energy, and helicity. The absence of nonlinear interactions is explained by the force-free fields of electron magnetohydrodynamic (EMHD) vortices.

Three-dimensional electron magnetohydrodynamic reconnection. IV. Instabilities, fluctuations, and emissions

Further observations are presented on a reconnection experiment involving three-dimensional magnetic fields in the parameter regime of electron magnetohydrodynamics. The focus is on current-driven instabilities in the magnetic neutral sheet. Density fluctuations are observed in the neutral sheet and identified as current-driven ion sound turbulence. No lower hybrid turbulence or Buneman instabilities are detected. Enhanced thermal fluctuations are measured in the range of the electron plasma frequency. Microwave radiation is emitted from the plasma and explained by mode conversion of plasma waves on density gradients. The role of these instabilities in the conversion of magnetic energy and energy and transport is discussed.

Three-dimensional electron magnetohydrodynamic reconnection. I. Fields, currents, and flows

In a large laboratory plasma, reconnection of three-dimensional (3-D) magnetic fields is studied in the parameter regime of electron magnetohydrodynamics. A reversed magnetic field topology with two 3-D null points and a two-dimensional (2-D) null line is established, and its free relaxation is studied experimentally. Major new findings include the absence of tilting instabilities in an unbounded plasma, relaxation times fast compared to classical diffusion times, dominance of field line annihilation at the 2-D current sheet versus reconnection at 3-D null points, conversion of magnetic energy into electron thermal energy, and excitation of various microinstabilities. This first of four companion papers focuses on the magnetic field topology and dynamics.

A new laboratory experiment on magnetic reconnection

In a large laboratory plasma reconnection of three-dimensional (3-D) magnetic fields is studied in the parameter regime of electron magnetohydrodynamics (EMHD). A reversed-field topology with two 3-D null points and a two-dimensional (2-D) null line is established, and its free relaxation is studied experimentally. Major new findings include the absence of tilting instabilities in an unbounded plasma, relaxation times that are fast compared to classical diffusion times, dominance of field line annihilation at the 2-D current sheet versus reconnection at 3-D null points, conversion of magnetic energy into electron thermal energy, and excitation of various microinstabilities. The experiment implies that EMHD processes near absolute magnetic null points must be considered in the multiscale physics of magnetic reconnection.

Magnetic helicity reversal of a whistler vortex transmitted through a three-dimensional magnetic null point

The transmission of a magnetic vortex through a magnetic null point on a separatrix surface is studied experimentally in a large laboratory plasma. The plasma is in the electron magnetohydrodynamic parameter regime and the vortex is an antenna-produced magnetic field perturbation propagating in the whistler mode. Topologically, the background field is separated into two regions; a closed field line region and an open field line region. The two regions are separated by a surface of magnetic field lines with two cusp null points referred to as the separatrix. The vortex propagates into one of the null points. Its energy is partially transmitted through the separatrix and partially spreads away from the null along curving field lines. The self and mutual-helicity of the transmitted vortex reverses, thus the total magnetic helicity is not conserved. Helicity conservation breaks down because the field lines are not frozen to electron flows in the unmagnetized plasma region around the magnetic null point.

Three-dimensional electron magnetohydrodynamic reconnection. II. Tilt and precession of a field-reversed configuration

Further observations are presented on a reconnection experiment involving a three-dimensional magnetic field reversed configuration (FRC) in the parameter regime of electron magnetohydrodynamics (EMHD). The stability of the FRC that relaxes in a large ambient plasma free of boundary effects is investigated. No destructive instabilities are observed. However, the EMHD FRC performs a precession around the axis given by the ambient magnetic field after a tilt develops. The precession velocity corresponds to the electron drift velocity of the toroidal current. The phenomenon is explained by the convection of frozen-in field lines in a rotating electron fluid. It is a new phenomenon in EMHD plasmas.

Three-dimensional electron magnetohydrodynamic reconnection. III. Energy conversion and electron heating

Further observations are presented of a magnetic reconnection experiment with three-dimensional fields in the parameter regime of electron magnetohydrodynamics. The initial magnetic configuration is imposed via a Helmholtz coil, whose field is added to or subtracted from a uniform background magnetic field. Energy is transferred from the coil’s external power supply into thermal energy of electrons and kinetic energy of ions via the decay of the imposed magnetic field configuration. For the case when the Helmholtz coil field opposes the background field, thus creating a field-reversed configuration, the magnetic energy convects in the whistler mode and dissipates over large distances resulting in negligible heating. For the case when the Helmholtz coil field is added to the background field, magnetic field annihilation leads to strong localized electron heating and acceleration of unmagnetized ions via space-charge electric fields. The energy conversion to electron heat is observed in regions away from ma...

3D EMHD reconnection in a laboratory plasma

In a large laboratory plasma, reconnection of three-dimensional (3D) magnetic fields is studied in the parameter regime of electron magnetohydrodynamics (EMHD). The field topologies are spheromak-like with two-dimensional null lines and three-dimensional spiral null points. The relaxation of an initial vortex field by spontaneous reconnection is studied in the absence of boundary effects. Reconnection rates and energy conversion from fields to particles are measured. The frozen-in condition appears to be destroyed by viscous effects rather than inertia or collision. Finally, the non-driven merging of two EMHD spheromaks into a long-lived FRC is observed. These basic physics experiments demonstrate that reconnection is an important process in the parameter regime of unmagnetized ions, which is always encountered near absolute magnetic null points.

Laboratory studies of magnetic vortices. II. Helicity reversal during reflection of a magnetic vortex at a conducting boundary

The reflection of a magnetic vortex from a conducting boundary is studied experimentally in a large laboratory plasma. The parameter regime is that of electron magnetohydrodynamics and the vortex consists of a spheromak-like magnetic field perturbation propagating in the whistler mode along a uniform background magnetic field. In this work we focus on the helicity properties of the vortex magnetic field, electron velocity, and vorticity. The reflection conserves magnetic energy but reverses the sign of all helicities. The change in topology arises from a self-consistent reversal of one linked vector field without involving helicity injection, reconnection, or dissipation processes. The breakdown of helicity conservation and the frozen-in concept is explained by the presence of a vacuum-like sheath at the plasma–boundary interface.