A team of researchers from IST has recently published a paper entitled “Radiation reaction cooling as a source of anisotropic momentum distributions with inverted populations” in the prestigious journal Physical Review Letters. This paper has been selected as Editor’s Suggestion, assigned to papers considered of be particularly important, interesting, and well written.

The study reveals a previously undocumented effect in plasma physics that alters the behavior of plasmas subjected to strong electromagnetic fields, potentially rendering them unstable and causing the emission of intense coherent radiation. This effect is thought to occur naturally in plasmas surrounding neutron stars. The paper’s lead author, PhD student Pablo J. Bilbao, conducted the research under the supervision of Professor Luís O. Silva of the Physics Department and GoLP/IPFN.

In classical electrodynamics textbooks, we all learn that charged particles accelerated in powerful electromagnetic fields radiate. When the particles are relativistic and the fields are strong, the radiation is called synchrotron radiation, in the x-ray region or even higher energies of the electromagnetic spectrum, with many applications across science and technology. If the energy lost by particles via radiation becomes comparable to the kinetic energy of the particles, their motion is strongly modified, the particles slow down and are damped by radiation reaction or radiation damping. This phenomenon has been studied since the 1890’s, by Lorentz, and since then by many prominent physicists, including Abraham, von Laue, Born, Pauli, Dirac, and Landau. Currently, scientists anticipate radiation reaction effects to play a substantial role in plasmas surrounding the magnetosphere of pulsars and magnetars. These celestial objects possess magnetic fields that can exceed 100,000 Teslas, which is over 2,200 times stronger than the most potent continuous magnets ever produced on Earth, with a strength of 45 Teslas. To put this figure in context, suppose we amplified a small fridge magnet with a magnetic field of 0.01 Teslas to the same strength as a pulsar or magnetar. In that case, the little fridge magnet would be capable of lifting 100 metric tons of iron. The latest generation of laboratory experiments with intense lasers also enables the creation of plasmas under such intense electromagnetic fields, making the study of radiation reaction effects on collective plasma dynamics increasingly relevant.

Upon closer examination of the radiation reaction force shows qualitative differences from the Lorentz force. First, it is a damping non-conservative force, similar to friction. Secondly, the radiation reaction force cools relativistic particles in a nonlinear manner, with faster cooling of particles with higher momentum than those with lower momentum. This leads to bunching in momentum space, with the formation of rings. Previous works have focused on the individual trajectories of particles in strong electromagnetic fields when considering radiation reaction, and have missed the aforementioned bunching and the formation of rings in momentum space.

The paper’s key finding is that, in general, plasmas with relativistic temperatures under extremely strong magnetic fields will cool and will always develop ring-shaped distribution in momentum space. These ring distributions are well known for being unstable and can cause coherent radiation emission through the maser emission. In this process, the plasma behaves similarly to a laser leading to the amplification of coherent light. This is an exciting development, as previous works have conjectured that the maser process could explain the origin of coherent radiation observed from astrophysical objects, such as the enigmatic Fast Radio Bursts. Until now, however, there was no explanation for the generation of the rings that could radiate as maser radiation. This work demonstrates the resilience of ring generation and could shed light on the origin of previously unexplained astronomical observations. Additionally, the paper suggests that other physical setups could lead to the same physics being described, opening the door for further research in extreme plasma physics in the laboratory.

This figure above shows a simulation of the momentum distribution of a plasma undergoing synchrotron cooling. The initial momentum distribution is Gaussian in shape, but as the plasma undergoes cooling, it evolves into a ring-shaped momentum distribution. The simulation was performed using a particle-in-cell (PIC) code that incorporates the effects of synchrotron radiation.