Ricardo A. Fonseca

Particle-in-cell (PIC) codes are a cornerstone of plasma-based accelerator research. We present a generalized parallelization algorithm for PIC simulations that achieves high efficiency across modern high-performance computing (HPC) architectures, including CPUs (x86 and Arm) and graphics processing units (GPUs) (NVIDIA, AMD). The algorithm employs a micro-spatial domain decomposition combined with a high-performance particle manager for efficient particle migration between domains. Each domain is mapped to a CPU thread or GPU thread block, ensuring balanced parallel workloads even in realistic scenarios. Implementations use architecture-specific programming models—OpenMP for CPUs and CUDA/HIP for GPUs—while maintaining a common algorithmic structure, making porting straightforward. Benchmark results for a representative 2D problem demonstrate excellent performance on both single devices and large-scale HPC systems. The algorithm is also show to be extendable for other PIC variants and architectures.

We introduce a plasma wakefield acceleration scheme capable of boosting initially subrelativistic particles to relativistic velocities within millimeter-scale distances. A subluminal light pulse drives a wake whose velocity is continuously matched to the beam speed through a tailored plasma density, thereby extending the dephasing length. We develop a theoretical model that is generalizable across particle mass, initial velocity, and the particular accelerating bucket being used, and we verify its accuracy with particle-in-cell simulations using laser drivers with energies in the joule range.

Evanescent waves, such as the ones present at the interface between a laser and an overdense plasma, provide extremely localized fields that are able to induce rapid accelerations in charged particles. We show that such accelerations can generate high-energy, low-divergence, directional photon beams even for moderately relativistic electrons ( γ ≃ 10 ). This process can also lead to superradiant emission of bright, ultrashort pulses of light, as long as the electron bunch is suitably modulated. Although never directly observed, this mechanism should leave clear signatures in laser-plasma acceleration experiments with minimal setup adaptations.

Aims. Global particle-in-cell (PIC) simulations of pulsar magnetospheres are performed with volume-, surface-, and pair-production-based plasma injection schemes to systematically investigate the transition between electrosphere and force-free pulsar magnetospheric regimes. Methods. We present a new extension of the PIC code OSIRIS that can be used to model pulsar magnetospheres with a two-dimensional axisymmetric spherical grid. The subalgorithms of the code and thorough benchmarks are presented in detail, including a new first-order current deposition scheme that conserves charge to machine precision. Results. We show that all plasma injection schemes produce a range of magnetospheric regimes. Active solutions can be obtained with surface and volume injection schemes when using artificially large plasma-injection rates, and with pair-production-based plasma injection for sufficiently large separation between kinematic and pair-production energy scales.

Self-modulation is a beam–plasma instability that is useful to drive large-amplitude wakefields with bunches much longer than the plasma skin depth. We present experimental results showing that, when increasing the ratio between the initial transverse size of the bunch and the plasma skin depth, the instability occurs later along the bunch, or not at all, over a fixed plasma length because the amplitude of the initial wakefields decreases. We show cases for which self-modulation does not develop, and we introduce a simple model discussing the conditions for which it would not occur after any plasma length. Changing bunch size and plasma electron density also changes the growth rate of the instability. We discuss the impact of these results on the design of a particle accelerator based on the self-modulation instability seeded by a relativistic ionization front, such as the future upgrade of the Advanced WAKefield Experiment.

The head-on collision between electrostatic shocks is studied via multi-dimensional particle-in-cell simulations. A strong magnetic field develops after the interaction, which causes the shock velocities to drop significantly. This transverse magnetic field is generated by the Weibel instability, which is driven by pressure anisotropies due to longitudinal electron heating while the shocks approach each other. The possibility to explore the physics underpinning the shock collision in the laboratory with current laser facilities is discussed.