Kyle Strohmaier

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.

Nonlinear electron magnetohydrodynamic physics. VII. Magnetic loop antenna in a field-free plasma

Nonlinear whistler phenomena near a magnetic loop antenna in a field-free plasma have been investigated experimentally. The loop field oscillates at a frequency far below the electron plasma frequency, hence all linear electromagnetic modes are cut off. However, the peak antenna field is so large that the electrons become magnetized allowing whistler modes to exist in the near zone of the antenna. The shielding magnetic field propagates at a speed which increases with magnetic field strength and decays slower than the rf period, resulting in a remnant field when the antenna field vanishes. A field-reversed configuration (FRC) is produced when the antenna field reverses direction. The FRC expands into the magnetized plasma and produces self-consistent magnetic helicity consistent with that of whistler modes. Thus, the new field penetrates in the whistler mode in a background field left over from the previous half-cycle. The electrons become unmagnetized at large distances, and the field convection goes ove...

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...

Whistler spheromaks, instabilities and triggered emission experiments

Whistler instabilities are observed in an EMHD spheromak in a large laboratory plasma. The spheromak slowly propagates along the ambient magnetic field, decays and converts its magnetic energy into electron kinetic energy. Anisotropies in the electron distribution function create magnetic oscillations below the electron cyclotron frequency. These are identified as whistler modes by measuring the frequency spectrum, wave field topology, polarization, helicity and propagation velocity. The instability is triggered by the transient formation of the spheromak but loses coherence in time. In order to investigate spatial and temporal growth a test whistler wave is propagated into the source region. The test wave does not grow but triggers a much larger instability amplitude. The triggered emission has a slightly different frequency from that of the test wave. The field topology of the triggered emission differs from that of the test wave. Space–time measurements in the source region show both convective wave amplification and an absolute instability in the current ring. These laboratory observations complement earlier studies of triggered whistler emissions in space plasmas.

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.

Nonlinear electron magnetohydrodynamic physics. VI. Magnetic loop antenna across the ambient field

Laboratory experiments on nonlinear whistler phenomena near a magnetic loop antenna with axis across the magnetic field are presented. The field topologies, radiation efficiency, electron energization and instabilities in the low-frequency whistler regime have been investigated. The time-dependent magnetic fields are fully three-dimensional. Radial and spiral magnetic null points are produced near the antenna but not in the propagating wave fields whose amplitudes can exceed that of the ambient field. Electron energization and light emissions are observed near the antenna. The electron drift excites whistler emissions at frequencies higher than the applied frequency. Source region and propagation characteristics of these waves have been investigated. The results are important for understanding nonlinear effects of loop antennas in plasmas and nonlinear effects in strong whistler turbulence.