Using intense lasers to understand the origin of magnetic instabilities in astrophysics


An article recounting the first direct experimental evidence of two electron instabilities: the Weibel and a resistive instability, can be found in Nature Physics [Ruyer et al., Nature Physics (2020). ]. These collisionless instabilities have been identified to explain the magnetization of the intergalactic medium and in particular they would explain the origin of two of the most intriguing phenomena in contemporary astrophysics: cosmic rays and gamma-ray bursts. It is hypothesized that collisionless shock waves generated by ejections of collisionless energetic matter during astrophysical phenomenon (supernova explosions, fusion of neutron stars, etc.) is caused by magnetic turbulence produced by Weibel's instability. And it is also this magnetic turbulence that would be responsible for the acceleration of cosmic rays as well as gamma photons by synchrotron radiation, according to the mechanism proposed by Enrico Fermi.

Our international team, led by Julien Fuchs (LULI, France) and Sophia Chen (ELI-NP), carried out an experiment at Lawrence Livermore National Laboratory (USA) using two high-intensity short pulse laser beams: a first beam drives a broadband proton beam from a first target to probe the magnetic fields created in a second target irradiated by a second beam. We observed that the fields related to instability could be measured quite easily and that they existed over much longer than expected. It then took several years of theoretical and numerical effort, led by Laurent Gremillet et Charles Ruyer (CEA, France), using the advanced simulation codes and supercomputers to model the data (see Figure). In doing so, two variants of Weibel's instability were identified depending on the region of the plasma, namely the collisionless electromagnetic modulations can be measured as the energetic electrons traversed the dense center region and later the lower density coronal regions.

With the forthcoming launch of even higher intensity short pulse lasers with multiple beams like the ones at ELI-NP we can continue to study instabilities related to energetic electrons like those produced in extreme astrophysical phenomena.

The experimental setup showing the probing proton beam driven from the first target radiographing the instabilities created in the second target. The Weibel instabilities induced by the energetic electrons generates magnetic fluctuations that deflect the protons that are then captured onto radiochromic films. A closer view of the simulated electron trajectories inside the target (inset)