A laboratory experiment to test particle acceleration in astrophysical environments. The study: “Laboratory evidence for proton energization by collisionless shock surfing” of W. Yao and J. Fuchs (École Polytechnique, Sorbonne Université) recently appeared on Nature Physics
Energetic particles, called “cosmic rays”, constantly rain down on our planet. Thanks to several years of theoretical studies and observations, we know that these particles can be accelerated by shocks propagating in certain astrophysical environments. The classical example of such environment are the supernovae remnants, which are expanding clouds created by supernova explosions, which are often interacting with surrounding material. Researchers have identified a few processes that can be responsible for accelerating particles up to the relativistic energies typical of cosmic rays, whose effectiveness depends on the properties of the expanding shock and the ambient environment.
In order to understand which physical process can trigger particle acceleration, a team of researchers led by the physicists W. Yao e J. Fuchs (École Polytechnique, Sorbonne Université) set up a laboratory experiment aimed at reproducing a shock expanding in an environment gas and a uniform magnetic field, and used magnetohydrodynamic simulations to study the evolution of the system. The gas environment is reproduced by a low density (1018 cm-3) hydrogen gas, surrounding a solid target of Teflon, and in a uniform magnetic field. A high-energy short (1 ns) laser pulse then was incident on the solid target, producing hot plasma that expanded in the surrounding gas. The resulting shocks is launched with an initial velocity of 1500 km/s, and it is characterized by collisional parameters (such as the particles mean free path and their collisional time) typical of shocks where collisions are not important (“collisionless shocks“). The direction of the environment magnetic field is transversal to the direction of the incident laser. This configuration reproduces the properties of shocks in several astrophysical environments, such as supernova remnants interacting with dense clouds and stellar winds.
The first important result of this experiment was to demonstrate that it is possible to reproduce expanding shocks in laboratory tests. Besides, the researchers have measured the final velocity of protons accelerated during the experiments by using two spectrometers. These measurements demonstrated that particles were accelerated up to energies of hundreds keV by the expanding shock. In particular, the acceleration was triggered during the first 2-3 ns of evolution of the system, during which the shock was expanding faster than 1000 km/s. The main acceleration mechanism during this early phase is the “shock surfing acceleration“, which consists in the acceleration by the electric field associated with the shock of the particles lying in front of the shock front. The experiment demonstrated that the energies at which particles are accelerated by the “shock surfing acceleration” process are high enough to allow further acceleration by another process, called “diffusive shock acceleration“, which was not reproduced by the experiment, and which is capable of accelerating particles up to the energies typical of cosmic rays. Finally, both the experiment and the simulations demonstrated the importance of the magnetic field for this process. Without magnetic field, in fact, no shock has been observed to develop.
This study demonstrates for the first time that the “shock surfing acceleration” is responsible for triggering the early acceleration of particles in shocks similar to those expanding in supernova remnants. In general, this process can play an important role in several environments. For instance, during the energetic phenomena occurring in the Sun, such as in the shock front in coronal mass ejections, in the interplanetary space in the interface between planetary magnetic fields and solar wind, at the “termination shock” of the solar wind, and, in general, in stellar winds.
The study is described in the paper: “Laboratory evidence for proton energization by collisionless shock surfing”, recently appeared on Nature Physics. The researchers S. Orlando (INAF – Astronomical Observatory of Palermo) and M. Miceli (University of Palermo and INAF – Astronomical Observatory of Palermo) were the only astrophysicists involved in the project, with the aim of studying the astrophysical implications of the experiment.
The figure (click here to visualize the entire image) shows the experiment setup. Panel a) shows the environment gas, the solid target, and both the directions of the laser and the magnetic fields. Panel b) also shows a 3D rendering of the expanding plasma in the environment gas made with FLASH simulations. Panels c) and d) show the gas density (integrated along the line of sigh) 4 ns after the laser pulse, measured in the x-y and x-z planes. The direction of the magnetic field is also shown.
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