Experimental setup and representative snapshots of self-generated Weibel magnetic fields. (a) Schematic diagram of the experimental layout. (B) Representative frames from a movie of electron beam deflection by fields in the plasma. The first frame shows the letter e– Beam profile with no laser. The following frames show the evolution of self-fields in the plasma. The dotted yellow ellipse on the 0 ps frame specifies the estimated number of 1014 w / cm2 (ionization threshold) carbon dioxide intensity contour2 laser. Dotted white lines are added on the 3.3 ps and 116.7 ps frames to highlight the orientation of the selected intensity bands. On a 36.7 ps frame, the white arrows indicate the structures resulting from the path crossing of the probe electrons shifting the effective object plane closer to the plasma. All images were rotated counterclockwise by 12° to correct for the tilt caused by the PMQ and to place 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 so hot that electrons separate from atoms. Electrons float freely and atoms turn into ions. This results in an ionized gas – a plasma – that makes up nearly all of the visible universe. Recent research shows that magnetic fields can spontaneously appear in plasmas. This can happen if the plasma has a temperature anisotropy – a different temperature along different spatial directions.
This mechanism is known as Weibel instability. It was predicted by the plasma theorist Eric Whipple more than six decades ago, but only now has it been unequivocally observed in the laboratory. New research, now published in Proceedings of the National Academy of Sciences, it was found that this process can convert a large part of the energy stored in the temperature anisotropy into magnetic field energy. He also found that the Weibel instability could be the source of the magnetic fields that permeate all parts of the universe.
Matter in our visible universe is a state of plasma and is magnetized. Galaxies are permeated with magnetic fields on a microgauss scale (about a millionth of Earth’s magnetic fields). These magnetic fields are thought to be amplified from the weak seed fields by the spiral motion of galaxies, known as a galactic dynamo. How the magnetic fields of the seeds are created is a long standing question in astrophysics.
Evolution of the measured assembly of the electron probe. credit: Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.221171311
This new work offers a potential solution to this troubling problem regarding the origin of seed magnetic fields at the microgauss scale. The research used a new platform with great potential to study the ultrafast dynamics of magnetic fields in laboratory plasmas relevant to astronomical and high-energy energy density physics.
First theorized six decades ago, the 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 in unequivocally demonstrating the Weibel instability. First, until recently, researchers have not been able to produce plasmas with such a known temperature contrast as Weibel initially envisioned. Second, the researchers did not have a suitable technique for measuring the complex and rapidly evolving topology of the magnetic fields subsequently created in the plasma.
This work was enabled by the unique capability of the Accelerator Test Facility, a Department of Energy (DOE) user facility at Brookhaven National Laboratory, that used a new experimental platform that allowed the researchers to create hydrogen plasmas with known highly anisotropic electron velocity distributions. on a time scale of tens of trillionths of a second using an ultra-short but intense carbon dioxide laser pulse.
Evolution of recovered magnetic field components. credit: Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.221171311
Subsequent heat treatment of the plasma occurs by self-organizing plasma currents that produce magnetic fields driven by the Weibel instability. These fields are large enough to deflect relativistic electrons to reveal an image of magnetic fields at a certain distance from the plasma. The researchers obtained a movie of the evolution of these magnetic fields at remarkable spatio-temporal resolution by using a picosecond relativistic electron beam to probe these fields.
Mapping of self-generated magnetic domains 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
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