A laboratory study of asymmetric magnetic reconnection in strongly driven plasmas

Abstract

Magnetic reconnection, the annihilation and rearrangement of magnetic fields in a plasma, is a universal phenomenon that frequently occurs when plasmas carrying oppositely directed field lines collide. In most natural circumstances, the collision is asymmetric (the two plasmas having different properties), but laboratory research to date has been limited to symmetric configurations. In addition, the regime of strongly driven magnetic reconnection, where the ram pressure of the plasma dominates the magnetic pressure, as in several astrophysical environments, has also received little experimental attention. Thus, we have designed the experiments to probe reconnection in asymmetric, strongly driven, laser-generated plasmas. Here we show that, in this strongly driven system, the rate of magnetic flux annihilation is dictated by the relative flow velocities of the opposing plasmas and is insensitive to initial asymmetries. In addition, out-of-plane magnetic fields that arise from asymmetries in the three-dimensional plasma geometry have minimal impact on the reconnection rate, due to the strong flows.

First experiments probing the collision of parallel magnetic fields using laser-produced plasmas

“…11 It is likely that, in comparison to those experiments, the collision produced in these parallel magnetic fields experiments is somewhat weaker and at a more glancing incidence, as the interaction point is farther from the laser focal spot and, consequently, farther from the generation of the bubble expansion and drive mechanism. The deformation of magnetic field structures is likely less forceful in this configuration than with two bubbles generated on the same side of a foil.…”

“…As was described in Ref. [11] for experiments using anti-parallel magnetic fields, in-plane profiles of density, temperature, and flow velocity were taken from 2D draco 23 simulations. Hydrodynamic quantities fromdraco simulations have been benchmarkd against laser-foil experiments at a similar laser intensity (∼10 14 W/cm 2 ).…”

“…A nearly azimuthally-symmetric profile of path-integrated magnetic field strength is observed at the plasma bubble perimeters, as has been observed in previous experiments. 11,16,20 The interaction region of the plasma bubbles shows a slight enhancement of the path-integrated magnetic field strength, due to a weak pileup of magnetic flux. Lineouts of path-integrated magnetic field strength ( Figure 6) show that |∫ B×dl| is enhanced and slightly narrower in the collision region of the plasma bubbles, in comparison to a mere superposition of radial profiles of |∫ B×dl| from each bubble.…”

“…Previous experiments a) Electronic mail: mrosenbe@mit.edu; Current institution: Laboratory for Laser Energetics, University of Rochester driving the interaction of anti-parallel magnetic fields in high-β, laser-produced plasmas have demonstrated signatures of magnetic reconnection [7][8][9][10] and observed an extremely fast rate of magnetic flux annihilation. 8,11 The nominally super-Alfvénic reconnection has been attributed to the pileup of magnetic flux during the collision process, which amplifies the local magnetic field strength and Alfvén speed. In this strongly-driven system, where the bulk flow velocity is significantly larger than the Alfvén speed and, equivalently, ram pressure dominates magnetic pressure, the fields are forced together faster than they can naturally rearrange themselves.…”

“…This process of magnetic reconnection has been studied extensively in analytical theory, 1,2 computational studies, 3 spacecraft observations, 4 and laboratory experiments. 5,6 Recent experiments using lasergenerated plasmas have explored magnetic reconnection in strongly-driven configurations, [7][8][9][10][11] where plasma inflows are a significant source of energy and the compression and amplification of magnetic fields is expected to play a significant role in the reconnection physics. 12 This strongly-driven reconnection occurs in some astrophysical environments, such as the dayside magnetopause 13 in the interaction between the solar wind and the Earth's magnetosphere.…”