Periodic Reporting for period 1 - XPACE (Extreme Particle Acceleration in Shocks: from the laboratory to astrophysics)
Berichtszeitraum: 2023-06-01 bis 2025-11-30
Astrophysical shocks are among the most powerful particle accelerators in the Universe. Generated by violent interactions of supersonic, and often relativistic, plasma flows with the ambient medium, shock waves involve a complex and highly nonlinear interplay between the dynamics of flows, magnetic fields, and accelerated particles through mechanisms not yet fully understood. “What is the origin of cosmic rays?”, “What controls particle injection and the acceleration efficiency in collisionless shocks?”, “How is the physics of relativistic shocks modified by electron-positron pair production?”, “Can these mechanisms be studied in the laboratory?” These are long-standing scientific questions, closely tied to extreme plasma physics processes, and where the interplay between micro-instabilities and the global dynamics is critical. Advances in high-power lasers and particle beams are just now opening unique opportunities to probe the microphysics of shocks and particle acceleration in controlled laboratory experiments for the first time. Together with the fast-paced developments in fully-kinetic plasma simulations, computational power, and astronomical observations, the time is ripe to deploy a research program focused on particle acceleration in shocks that can transform our ability to address these questions. We aim to use first-principles massively parallel simulations and laboratory experiments to study the microphysics of non-relativistic and relativistic shocks, and to use data-driven techniques to develop multi- scale models that bridge the gap between the microphysics and the global dynamics. This project will build comprehensive models of the plasma processes that shape magnetic field amplification, particle acceleration, and radiation emission in shocks, with the goal of solving central questions in extreme plasma phenomena, opening new avenues between theory, computation, laboratory experiments, and astrophysical observations.
XPACE is addressing long-standing scientific questions by developing and exploring state-of-the-art computational tools and experimental platforms. The project is making very significant progress in different fronts.
How is energy partitioned between electrons and ions in a collisionless shocks? When stars explode, their remnants drive violent shock waves that heat electrons in the interstellar medium to three orders of magnitude above the temperature expected from adiabatic compression on the shock. The mechanisms behind this heating have remained a long-standing puzzle, but the work developed by this project presents a new theory for the energy partition between electrons and ions in collisionless plasma flows and strong shock waves.
Initially, most of the energy in weakly magnetized astrophysical flows is carried by the ions, which are much heavier than electrons. Because these shocks are collisionless, collective electromagnetic processes must be responsible for exchanging energy between the ions and the electrons at the shock. Our work reveals that the difference in inertia between electrons and ions leads to differential scattering between the two species in the turbulent magnetic field produced ahead of the shock, driving an electric field. It is this electric field that, in turn, leads to efficient electron heating in a Joule-type process, with electrons acquiring a temperature that is a significant fraction of that of the ions, as inferred from astronomical observations. The new theoretical model was validated against first-principles simulations showing very good agreement. The generality of this model opens up promising avenues for studying electron transport and heating in different settings dominated by magnetic turbulence, which range from stellar explosions to fusion plasmas.
How are electrons accelerated in collisionless shocks? Most of the radiation observed from high-energy astrophysical shocks is associated with accelerated electrons that radiate when gyrating in the local magnetic field. How these electrons manage to be accelerated has long remained a challenge. Using a combination of theory, kinetic simulations and laboratory experiments performed at the most energetic laser system in the world (the National Ignition Facility) we have developed a new model for electron acceleration in shocks. Acceleration occurs as electrons scatter in small-scale magnetic fields produced at the shock transition. The model is consistent with particle-in-cell simulations and with the results of recent laboratory experiments where nonthermal electron acceleration was observed. This injection model represents a very significant advancement that could finally explain electron injection in high-Mach-number astrophysical shocks, such as those associated with young supernova remnants and accretion shocks in galaxy clusters.
Can we probe the physics of relativistic collisionless shocks in the laboratory? In many of the high-energy astrophysical environments associated with efficient particle acceleration, plasmas are relativistic. The direct study of these relativistic processes in controlled laboratory experiments has long been an important scientific challenge. We have conducted novel pump-probe experiments that directly imaged the relativistic current-filamentation instability in dense plasmas for the first time. By combining a high-power optical laser to drive the relativistic electrons with a high-brightness, narrow-bandwidth X-ray free-electron laser in an X-ray phase contrast microscopy configuration, we have enabled an unprecedented characterization of the filamentation instability in solid-density plasmas. Our work elucidates how the competition between electromagnetic, electrostatic and collisional effects shape the development of this instability. Furthermore, it indicates that magnetic fields are amplified to levels significantly beyond those predicted by previous models. This work opens up a new experimental route for detailed studies of the microphysics of relativistic streaming instabilities of relevance to both laboratory and astrophysical plasmas.
How is energy partitioned between electrons and ions in a collisionless shocks? When stars explode, their remnants drive violent shock waves that heat electrons in the interstellar medium to three orders of magnitude above the temperature expected from adiabatic compression on the shock. The mechanisms behind this heating have remained a long-standing puzzle, but the work developed by this project presents a new theory for the energy partition between electrons and ions in collisionless plasma flows and strong shock waves.
Initially, most of the energy in weakly magnetized astrophysical flows is carried by the ions, which are much heavier than electrons. Because these shocks are collisionless, collective electromagnetic processes must be responsible for exchanging energy between the ions and the electrons at the shock. Our work reveals that the difference in inertia between electrons and ions leads to differential scattering between the two species in the turbulent magnetic field produced ahead of the shock, driving an electric field. It is this electric field that, in turn, leads to efficient electron heating in a Joule-type process, with electrons acquiring a temperature that is a significant fraction of that of the ions, as inferred from astronomical observations. The new theoretical model was validated against first-principles simulations showing very good agreement. The generality of this model opens up promising avenues for studying electron transport and heating in different settings dominated by magnetic turbulence, which range from stellar explosions to fusion plasmas.
How are electrons accelerated in collisionless shocks? Most of the radiation observed from high-energy astrophysical shocks is associated with accelerated electrons that radiate when gyrating in the local magnetic field. How these electrons manage to be accelerated has long remained a challenge. Using a combination of theory, kinetic simulations and laboratory experiments performed at the most energetic laser system in the world (the National Ignition Facility) we have developed a new model for electron acceleration in shocks. Acceleration occurs as electrons scatter in small-scale magnetic fields produced at the shock transition. The model is consistent with particle-in-cell simulations and with the results of recent laboratory experiments where nonthermal electron acceleration was observed. This injection model represents a very significant advancement that could finally explain electron injection in high-Mach-number astrophysical shocks, such as those associated with young supernova remnants and accretion shocks in galaxy clusters.
Can we probe the physics of relativistic collisionless shocks in the laboratory? In many of the high-energy astrophysical environments associated with efficient particle acceleration, plasmas are relativistic. The direct study of these relativistic processes in controlled laboratory experiments has long been an important scientific challenge. We have conducted novel pump-probe experiments that directly imaged the relativistic current-filamentation instability in dense plasmas for the first time. By combining a high-power optical laser to drive the relativistic electrons with a high-brightness, narrow-bandwidth X-ray free-electron laser in an X-ray phase contrast microscopy configuration, we have enabled an unprecedented characterization of the filamentation instability in solid-density plasmas. Our work elucidates how the competition between electromagnetic, electrostatic and collisional effects shape the development of this instability. Furthermore, it indicates that magnetic fields are amplified to levels significantly beyond those predicted by previous models. This work opens up a new experimental route for detailed studies of the microphysics of relativistic streaming instabilities of relevance to both laboratory and astrophysical plasmas.
The results obtained in this project have a strong impact in the field of plasma physics and plasma astrophysics with implications that go beyond the physics of astrophysical shocks. The injection/acceleration of electrons in collisionless shocks as long been seen as one of the main puzzles in shock acceleration. The impact of our results was recently recognised recognized with the prestigious 2024 Lev D. Landau and Lyman Spitzer Jr. Award attributed jointly by the American Physical Society and European Physical Society for outstanding contributions to plasma physics and was presented to Anna Grassi, Frederico Fiuza, George F. Swadling, and Hye-Sook Park for this important result. Beyond astrophysical shocks, the new understanding provided by this work can help develop more efficient accelerators for terrestrial applications such as compact radiation sources and imaging systems. The new model for energy partition in shocks also has quite important implications for the fundamental understanding of how electrons and ions exchange energy in the absence of collisions, which range from stellar explosions to fusion plasmas. The importance of this work was also recognized with the 2025 EPS – PPCF Sylvie Jacquemot Early Career Prize, which was presented to Arno Vanthieghem for different contributions, including this work.