The mechanism of cosmic magnetic fields explored in the laboratory

The mechanism of cosmic magnetic fields explored in the laboratory

Experimental setup and representative snapshots of self-generated Weibel magnetic fields. (A) Sketch of the experimental plan. (B) Representative frames from the movie of the electron beam deflection by the plasma fields. The first frame shows e beam profile without laser. The following frames show the evolution of the self-generated fields in the plasma. The yellow dotted ellipse on the 0 ps frame outlines the estimated 1014 l/cm2 (ionization threshold) intensity contour of CO2 laser. Dotted white lines on the 3.3 ps and 116.7 ps frames are added to highlight the orientation of the selected density bands. On the 36.7 ps frame, the white arrows mark the structures caused by traversing the trajectory of the probe electrons that move the effective object plane closer to the plasma. All images were rotated counterclockwise by 12° to correct for PMQ-induced tilt and to put the longer dimension of the elliptical plasma parallel to the laser propagation direction. Credit: Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.221171311.

Plasma is matter so hot that electrons are separated from atoms. Electrons float freely and atoms become ions. This creates an ionized gas – plasma – that makes up almost all of the visible universe. Recent research shows that magnetic fields can arise spontaneously in a plasma. This can happen if the plasma has a temperature anisotropy – temperature that is different along different spatial directions.

This mechanism is known as Weibel instability. It was predicted by plasma theorist Eric Weibel more than six decades ago, but only now has it been unambiguously observed in the laboratory. New research, now published in Proceedings of the National Academy of Sciences, find that this process can convert a significant part of the energy stored in temperature anisotropy into magnetic field energy. They also find that the Weibel instability could be a source of magnetic fields that permeate the cosmos.

The matter in our observable universe is plasma and is magnetized. Magnetic fields at the micro-gauss level (about one millionth of Earth’s magnetic fields) penetrate galaxies. These magnetic fields are thought to be amplified from weak seed fields by the spiral motion of galaxies, known as the galactic dynamo. How the seeds’ magnetic fields are created is a long-standing question in astrophysics.

The mechanism of cosmic magnetic fields explored in the laboratory

Evolution of measured electron probe bunching. Credit: Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.221171311

This new work provides a possible solution to this vexing problem of the origin of seed magnetic fields at the microgauss level. The research used a novel platform that has great potential for studying the ultrafast dynamics of magnetic fields in laboratory plasmas, which are relevant to astrophysics and high energy density.

First theorized six decades ago, Weibel instability driven by temperature anisotropy is believed to be an important mechanism for the self-magnetization of many laboratory and astrophysical plasmas. However, scientists faced two challenges to unambiguously demonstrate the Weibel instability. First, until recently, researchers have not been able to generate a plasma with a known temperature anisotropy, as Weibel originally envisioned. Second, the researchers lacked an adequate technique to measure the complex and rapidly evolving topology of the magnetic fields subsequently generated in the plasma.

This work, enabled by the unique capability of the Accelerator Test Facility, a Department of Energy (DOE) user facility at Brookhaven National Laboratory, used a new experimental platform that allowed the researchers to create a hydrogen plasma with a known distribution of highly anisotropic electron speeds. on a time scale of tens of trillionths of a second by using an ultrashort but intense carbon dioxide laser pulse.

The mechanism of cosmic magnetic fields explored in the laboratory

Evolution of recovered magnetic field components. Credit: Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.221171311

Further thermalization of the plasma occurs through the self-organization of plasma currents that produce magnetic fields driven by the Weibel instability. These fields are large enough to deflect the relativistic electrons to reveal an image of the magnetic fields at some distance from the plasma. The researchers obtained a movie of the evolution of these magnetic fields with exquisite spatiotemporal resolution by using a picosecond relativistic electron beam to probe these fields.

More information:
Mapping self-generated magnetic fields due to Weibel thermal instability, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.221171311. www.pnas.org/doi/10.1073/pnas.2211713119

Provided by the US Department of Energy

Citation: Mechanism of Cosmic Magnetic Fields Explored in the Lab (2023, January 17) Retrieved January 17, 2023, from https://phys.org/news/2023-01-mechanism-cosmic-magnetic-fields-explored.html

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