Novel experimental platform enables first measurements of ion-acoustic wave bursts during magnetic reconnection

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The experimental platform using copper plates and a copper coil to create magnetic reconnection.  

A series of experiments conducted at the Omega Laser, part of the University of Rochester’s Laboratory for Laser Energetics, provide new insights into magnetic reconnection, a process that could help explain stellar flares and other astrophysical phenomena. The research confirmed that unstable ion-acoustic waves (IAWs) could be important to further understanding of the dissipation physics during magnetic reconnection.

"Now we are excited by having re-discovered [ion-acoustic waves'] importance under certain conditions which may exist in a wide range of phenomena in space and astrophysics,” said Hantao Ji of the Princeton Plasma Physics Laboratory, one of the paper’s authors.

The results were published in Nature Physics in a paper titled “Ion and Electron Acoustic Bursts during Anti-Parallel Reconnection Driven by Lasers.” A team of researchers from Princeton University led the work, with contributions from the University of Rochester, the Massachusetts Institute of Technology, and Lawrence Livermore National Laboratory (LLNL).

“This is inspiring work. It shows it’s possible to learn exquisite details from a simple experimental setup with extensive data analysis. There is a lot we don’t know about magnetic reconnection, but this work opens some interesting avenues to pursue,” said LLNL physicist John Moody, also an author. He provided insights on the mechanisms of the novel laser-powered capacitor coil currents of the experimental platform and the target design.

Magnetic reconnection is the breaking and reconnecting of non-parallel magnetic field lines in a plasma. In the process, magnetic field energy is converted to plasma kinetic and thermal energy. Magnetic reconnection is believed to power astrophysical phenomena like solar flares and the northern lights.

In the experiments, the researchers created an experimental platform to produce magnetic reconnection similar to that observed in the solar atmosphere. In the platform, two ultraviolet laser beams pass through holes in the first copper plate to reach a second plate and generate electric currents through two parallel loops connecting the plates. The geometry created between the two coils allows improved plasma confinement and efficient particle acceleration. The accelerated electrons then drive the ion-acoustic waves measured in these experiments.

The researchers theorized that a 3D kinetic dissipation mechanism in magnetic reconnection could be caused by unstable IAWs driven by the relative drift between electrons and ions, or equivalently electric current. Using this novel experimental platform, they were able to measure the sudden onset of bursts of IAWs for the first time using collective Thomson scattering diagnostics.

“This gives new insight into the dissipation physics of magnetic reconnection,” said LLNL physicist Hui Chen, also a co-author. She contributed to the project by providing the particle spectrometer and training graduate students on analyzing the particle data.

The research team plans further experiments, including with increased generation of current in the semicircular coils to address questions such as efficiency and universality of IAW to dissipate magnetic energy during reconnection in wider parameter space.