Exploring the mechanism of cosmic magnetic fields in the laboratory

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. was expected before 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 Sciencesit 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.

It’s in our area visible universe is the state of plasma and it 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.

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.

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 for relativistic deflection electrons To detect the 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.