arXiv subjects

Q. Labro

Publications and source records attributed to Q. Labro.

3 recordsLinked to original sources

arXiv2026

Modeling ultrarelativistic streaming plasma instabilities under the quasistatic approximation

Plasma streaming instabilities excited by relativistic charged particle beams play a pivotal role in astrophysical and laboratory environments. Their numerical study, however, is challenged by the disparity in spatiotemporal scales between the background plasma and beam particles, which can differ by several orders of magnitude for tenuous, ultrarelativistic beams. Here, we exploit the quasistatic approximation (QSA) to develop a new theoretical framework capable of capturing the full unstable spectrum in the spatiotemporal regime relevant for beams that continuously encounter unperturbed plasma at their leading edge. Within this linear, fully electromagnetic model, we uncover a previously unreported spatiotemporal evolution of the current filamentation instability and elucidate its interplay with the oblique two-stream instability, predicting the dominance of filamentation in the vicinity of the beam front. The good agreement between theory, kinetic particle-in-cell (PIC) simulations, and QSA-based PIC simulations validates the robustness of the approach. By pushing QSA-based PIC simulations to extremely dilute electron-positron beams, such as those found in blazar jets, we demonstrate their unique ability to capture the rich nonlinear dynamics of streaming instabilities in parameter regimes previously inaccessible to kinetic simulations.

physics.plasm-ph↗

arXiv2026

Quasistatic modeling of ultrarelativistic beam-plasma instabilities

Relativistic particle beams propagating through dense ambient plasmas are susceptible to streaming instabilities that can govern the system dynamics in various astrophysical and laboratory settings. For an unmagnetized, collisionless plasma pervaded by a dilute, cold relativistic beam, the dominant instabilities are the quasielectrostatic, oblique two-stream (OTSI) and the essentially magnetic, current filamentation instability (CFI). While their linear and nonlinear properties have been researched for decades, most treatments assume unbounded, uniform systems and thus predict purely temporal instability growth. This assumption, however, is questionable for realistic configurations where a bounded beam continuously encounters fresh plasma. This feature causes instabilities to grow in a spatiotemporal manner. Whereas spatiotemporal perturbative treatments of streaming instabilities were derived as early as the 1960s, only recently have the spatiotemporal regimes of OTSI and CFI been addressed theoretically. Yet these models are restricted to a specific instability class, and hence cannot describe the competition between spatiotemporal OTSI and CFI. In this work, we present a unified, fully electromagnetic quasi-static model of all unstable modes arising throughout the beam. By not adopting the slowly varying envelope approximation (SVEA), we find that a previously unreported spatiotemporal CFI actually prevails near the front, precisely where the SVEA fails, and is only superseded by OTSI further back in the beam. Furthermore, we demonstrate that our model also captures the growth of the self-modulation and hosing instabilities excited by long, narrow beams. Comparisons with particle-in-cell simulations confirm the validity of the quasistatic approach for modeling streaming plasma instabilities triggered by relativistic dilute beams.

physics.plasm-ph↗

arXiv2025

Simulating ultrarelativistic beam-plasma instabilities with a quasistatic particle-in-cell code

Quasistatic particle-in-cell (PIC) codes are increasingly employed to study laser or plasma wakefield accelerators. By decoupling the slow dynamics of the driver (a laser or ultrarelativistic particle beam) from the fast plasma response, these codes can reduce the computational time by several orders of magnitude compared to conventional PIC codes. In this work, we demonstrate that quasistatic PIC codes can also be utilized to investigate relativistic beam-plasma instabilities, with a focus on the oblique two-stream instability (OTSI). For this purpose, we have developed a 2D quasistatic PIC code, QuaSSis, based on a new numerical scheme that can handle transversely periodic boundary conditions, a capability absent in previous quasistatic codes. The accuracy of QuaSSis is benchmarked first against standard PIC simulations performed with the CALDER code, and then against an analytical spatiotemporal model of the OTSI. Physically, this instability grows exponentially from initial fluctuations in the particle charge or current densities. Since the numerical noise inherent to PIC simulations can mimic these fluctuations to some extent, its control is crucial to seed the beam-plasma instability at the desired amplitude. Common methods for tuning this noise involve modifying the resolution or adding filters, but these can be computationally costly when aiming at very low noise levels. Here, we show that this noise can be finely controlled by properly initializing the positions and weights of the macroparticles.

physics.plasm-ph↗