Periodic Reporting for period 1 - PLAXI (Unraveling the ultrafast plasma dynamics giving rise to the measured evolution filamentary structures using high-resolution X-ray Imaging)
Période du rapport: 2024-09-01 au 2026-08-31
The PLAXI project explored the ultrafast physics of relativistic plasma filamentation in solid‑density targets driven by ultra‑high‑intensity lasers, building on recent breakthroughs in high‑resolution X‑ray imaging at free‑electron‑laser facilities. Its overall objective was to identify and characterise the fundamental physical mechanisms that govern the onset and nonlinear evolution of filamentation instabilities in hot, dense plasmas—phenomena that play a central role in high‑energy‑density physics, laboratory astrophysics and advanced inertial‑fusion schemes. By combining state‑of‑the‑art kinetic simulations, novel diagnostic development, and experimental design, the project aimed to transform our understanding of how energy is transported and reorganised in extreme plasma environments, providing insights relevant to astrophysical scenarios such as gamma‑ray bursts as well as laser‑driven applications.
From the beginning of the project, extensive numerical work was carried out using the Portuguese HPC cluster Deucalion (project 2025.00196.CPCA.A3) with the PIC code OSIRIS, enabling a detailed identification of the dominant mechanisms involved in the development of filamentation instabilities in laser-solid interactions. These simulations revealed that plasma collisionality, strongly temperature‑dependent, plays a crucial role in shaping the instability and leads to rich spatiotemporal dynamics, with plasma filaments primarily developing at the front surface of the target. They also highlighted the unexpectedly strong importance of electrostatic effects (ion motion), contrasting with the commonly adopted picture of hot-electron‑driven filamentation. These findings led to two major research directions: (i) a collaboration with IFN‑GV (UPM, Madrid) to model collisionality—absent in standard PIC codes—and to design new diagnostics capable of tracking its spatiotemporal evolution; and (ii) the development, both numerically and analytically, of a novel quasi‑static spatiotemporal framework for studying filamentation instabilities, culminating in a new analytical model capturing the full unstable electromagnetic spectrum. On the experimental side, a proposal informed by this modelling effort and the new collision diagnostics was submitted to the HED instrument at XFEL. Although not awarded beamtime in this first round due to limited availability, the exceptionally positive feedback strongly encouraged resubmission.
The project delivered results that go significantly beyond the state of the art, particularly through the forthcoming quasi‑static spatiotemporal model, the first analytical framework capable of capturing filament growth across the full electromagnetic spectrum—including both electrostatic and electromagnetic modes. This model will have a major impact by enabling large‑scale simulations of filamentation instabilities in astrophysical contexts with drastically reduced computational cost. In parallel, the emerging numerical framework under development with UPM—combining atomic physics, collisional processes and kinetic plasma modelling—will be essential for understanding how laser energy is deposited and evolves in hot, dense plasmas. This knowledge is expected to have substantial implications for next‑generation plasma‑based accelerators, advanced inertial‑fusion schemes, and X‑ray plasma diagnostics.