Reconnection converts the magnetic field energy to particle eruptions in astrophysical plasmas by snapping apart and explosively reconnecting the magnetic field lines — a process that occurs within what are called dissipation regions that are often enormously smaller than the regions they impact.

Stressed magnetic field

“Plasma doesn’t like reconnection,” said Hantao Ji, a PPPL physicist and Princeton University professor who is first author of a paper that details the roadmap in Nature Reviews Physics. “However, reconnection does happen when the magnetic field is sufficiently stressed,” he said.

“Dissipation scales are tiny whereas astrophysical scales are very large and can extend for millions of miles. Finding a way to bridge these scales through a multiscale mechanism is a key to solving the reconnection puzzle.”

The roadmap outlines the role of developing technologies with multiscale capabilities such as the Facility for Laboratory Reconnection Experiment (FLARE), a recently installed collaborative facility that is being upgraded and will probe facets of magnetic reconnection never before accessible to laboratory experiments. Complementing these experiments will be simulations on coming exascale supercomputers that will be 10 times faster than current computers. “The hope is for FLARE and exascale computing to go hand-in-hand,” Ji said.

The working theory the PPPL roadmap proposes is that multiple plasmoids, or magnetic islands, that arise from reconnection along lengthy plasma current sheets could bridge the vast range of scales. Such plasmoids would correspond more closely to the affected reconnection region, with multiscale laboratory experiments planned to provide the first tests of this theory and to evaluate competing hypotheses.

“Exascale will allow us to do more credible simulations based on high-fidelity FLARE experiments,” said PPPL physicist Jongsoo Yoo, a coauthor of the paper. The increased size and power of the new machine — its diameter will be twice that of the sports-utility-vehicle-sized Magnetic Reconnection Experiment (MRX), PPPL’s long-standing laboratory experiment — and will enable scientists to replicate reconnection in nature more faithfully.

“FLARE can access wider astrophysical regimes than MRX with multiple reconnection points and measure the field geometry during reconnection,” said William Daughton, a computational scientist at Los Alamos National Laboratory and a coauthor of the paper. “Understanding this physics is important for predicting how reconnection proceeds in solar flares,” he said.

Key challenge

A key challenge to the coming experiments will be innovating new high-resolution diagnostic systems free from restrictive assumptions. Once developed these systems will enable FLARE to build upon satellite sightings such as those produced by the Magnetospheric Multiscale mission, a fleet of four spacecraft launched in 2015 to study reconnection in the magnetosphere, the magnetic field that surrounds the Earth.

“Progress in understanding multiscale physics critically depends on innovation and efficient implementation of such diagnostics systems in the coming decade,” the paper said. The new findings will address open questions that include:

• How exactly does reconnection start?

• How are explosive plasma particles heated and accelerated?

• What role does reconnection play in related processes such as turbulence and space shocks?

Overall, “The paper lays out plans to provide the entire space physics and astrophysics communities with methods to solve the multiscale problem,” Yoo said. Such a solution would mark a major step toward a more complete understanding of magnetic reconnection in large systems throughout the universe.

Support for this work comes from the DOE Office of Science. Coauthors include Jonathan Jara-Almonte of PPPL and Ari Le and Adam Stanier of Los Alamos National Laboratory.

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Materials provided by DOE/Princeton Plasma Physics Laboratory. Original written by John Greenwald. Note: Content may be edited for style and length.

This new technique can also be applied to analysis of two-dimensional materials which have the potential to play a role in advanced energy systems, electronics and quantum computers.

Today in Science, the researchers in UCI’s Department of Physics & Astronomy and Department of Chemistry describe how they positioned two bound atoms of hydrogen in between the silver tip of the STM and a sample composed of a flat copper surface arrayed with small islands of copper nitride. With pulses of the laser lasting trillionths of a second, the scientists were able to excite the hydrogen molecule and detect changes in its quantum states at cryogenic temperatures and in the ultrahigh vacuum environment of the instrument, rendering atomic-scale, time-lapsed images of the sample.

“This project represents an advance in both the measurement technique and the scientific question the approach allowed us to explore,” said co-author Wilson Ho, Bren Professor of physics & astronomy and chemistry. “A quantum microscope that relies on probing the coherent superposition of states in a two-level system is much more sensitive than existing instruments that are not based on this quantum physics principle.”

Ho said the hydrogen molecule is an example of a two-level system because its orientation shifts between two positions, up and down and slightly horizontally tilted. Through a laser pulse, the scientists can coax the system to go from a ground state to an excited state in a cyclical fashion resulting in a superposition of the two states. The duration of the cyclic oscillations is vanishingly brief — lasting mere tens of picoseconds — but by measuring this “decoherence time” and the cyclic periods the scientists were able to see how the hydrogen molecule was interacting with its environment.

“The hydrogen molecule became part of the quantum microscope in the sense that wherever the microscope scanned, the hydrogen was there in between the tip and the sample,” said Ho. “It makes for an extremely sensitive probe, allowing us to see variations down to 0.1 angstrom. At this resolution, we could see how the charge distributions change on the sample.”

The space between the STM tip and the sample is almost unimaginably small, about six angstroms or 0.6 nanometers. The STM that Ho and his team assembled is equipped to detect minute electrical current flowing in this space and produce spectroscopic readings proving the presence of the hydrogen molecule and sample elements. Ho said this experiment represents the first demonstration of a chemically sensitive spectroscopy based on terahertz-induced rectification current through a single molecule.

The ability to characterize materials at this level of detail based on hydrogen’s quantum coherence can be of great use in the science and engineering of catalysts, since their functioning often depends on surface imperfections at the scale of single atoms, according to Ho.

“As long as hydrogen can be adsorbed onto a material, in principle, you can use hydrogen as a sensor to characterize the material itself through observations of their electrostatic field distribution,” said study lead author Likun Wang, UCI graduate student in physics & astronomy.

Joining Ho and Wang on this project, which was supported by the U.S. Department of Energy Office of Basic Energy Sciences, was Yunpeng Xia, UCI graduate student in physics & astronomy.

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Materials provided by University of California – Irvine. Note: Content may be edited for style and length.