The code Smilei has gathered users and developers from many laboratories around the world. This 3rd user & training workshop is a unique opportunity to learn about the project status and current developments, share your results to the community, and for newcomers, to follow hands-on training sessions with access to a cluster.
Participants will have the opportunity to submit an abstract for a presentation of their recent results.
Three sessions will be reserved for hands-on training for participants who wish to learn about running and optimizing PIC simulations (limited to 50 places).
Lunches will be provided on site, and one dinner will be organised. No registration fee is requested.
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Laser plasma acceleration [1] provides several advantages compared to conventional radio-frequency accelerators for electron source injectors: high accelerating gradients up to hundreds of gigavolts per meter (compactness) and short duration electron beams. However, the control of quality and stability of the produced electron bunches remain a challenge.
Here we focus on the target design studies for the PALLAS project which aims to achieve reliability of conventional RF accelerators producing 150-200 MeV electron bunches in the injector stage, with a charge of 15-30 pC, at 10Hz with less than 5% energy spread. The laser pulse is provided by the LaseriX laser driver, yielding a 35 fs laser pulse with a 40TW peak field.
A first rough design of a two-stage plasma cell is performed using OpenFOAM computational fluid dynamics simulations [2]. Assuming full ionization of the gas, simulated gas density results yield an longitudinal electronic density distribution which is then used in SMILEI [3] simulation.
Extensive numerical simulations are necessary to optimize gas cell target with multiple regions for controlled local ionization injection. For this work we used the envelope and azimuthal mode approximation in SMILEI with a low number of particles per cell allowing us to perform very-efficient computing time simulations. Two types of studies were performed: (1) extensive grid scan on plasma and target parameters, (2) Bayesian optimization implementation test with laser and plasma parameters for a fixed target geometry. Working points corresponding to optimal parameters sets with up to four free parameters have been obtained. Computations were performed at TGCC under A9 Call, some specific functions for job management , submission and control were developed in order to have an optimal workflow of Bayesian optimization process.
In order to proceed to quantitative comparisons of numerical experiment results with experimental results at PALLAS, we are currently testing approximation of realistic laser profile by flattened Gaussian profile.
[1] Tajima, Toshiki, and John M. Dawson. "Laser electron accelerator." Physical Review Letters 43.4 (1979): 267
[2] Audet, T. L., et al. "Gas cell density characterization for laser wakefield acceleration." Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909 (2018): 383-386
[3] Derouillat, Julien, et al. "Smilei: A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation." Computer Physics Communications 222 (2018): 351-373.
University-level introductory plasma physics courses typically address the theoretical modeling of plasmas with modules devoted to several different topics (e.g. charged particles’ orbits in an electromagnetic field, multi-fluid and magnetohydrodynamic models) but do not necessarily include a computational plasma module. However, computational tools can be valuable to teach both basic and advanced plasma physics concepts and, at the same time, let the students directly familiarize themselves with the theory [[1]]. This can be of special interest to the laser-plasma interaction community [[2]]. Particle-in-cell codes, besides being a well-established tool in plasma research, may offer a convenient route to hands-on teaching of plasma physics [[3]].
We have recently started a new series of classes and an innovative extracurricular teaching action on computational plasmas at Politecnico di Milano [[4]]. In this contribution, we wish to share with the Smilei community our efforts to bring hands-on activities on computational plasmas - complementary to the Plasma Physics course [[5]] - in Master of Science university curricula at Politecnico di Milano. We overview the teaching actions that we have developed and in which Smilei has been used for didactic purposes, also by training the students in its usage. Simple coding activities provide an opportunity for the students to explore non-trivial aspects of basic physics, while open-source user-friendly codes like Smilei let the students access more complex scenarios. Overall, we have obtained positive feedback on these didactive activities from the students, and we aim at extending the breadth of computational plasmas classes.
[[1]]. D. Caballero et al. "PICUP: A community of teachers integrating computation into undergraduate physics courses." The Physics Teacher 57.6 (2019): 397-399.
[[2]]. J. Pasley et al. "Innovative education and training in high power laser plasmas (PowerLaPs) for plasma physics, high power laser matter interactions and high energy density physics: experimental diagnostics and simulations." High Power Laser Science and Engineering 8 (2020).
[[3]]. R. Fonseca. Zpic - educational particle-in-cell code suite, accessed January 2021. URL https://github.com/zambzamb/zpic
[[4]]. Computational plasma physics via particle-in-cell simulations, Innovative teaching course, Politecnico di Milano
[[5]]. Plasma Physics course, Politecnico di Milano
High-energy photon emission can occur during the interaction of ultra-intense ($>10^{18}$ W/cm$^2$) lasers with plasma obtained from the ionization of a suitable target. This emission of electromagnetic radiation follows the generation of relativistic electrons during the interaction itself. Indeed, relativistic electrons can produce high-energy photons (keV-MeV energy range) thanks mainly to two processes. One is synchrotron-like emission mediated by the electromagnetic fields present in the plasma and named Non-linear Inverse Compton Scattering (NICS) when electrons scatter off an ultra-intense ($\gg10^{18}$ W/cm$^2$) laser pulse [[1]]. The other is bremsstrahlung emission, mediated by the atoms and ions inside the target [[2]]. Studying these phenomena is of great interest to have a complete picture of laser-plasma interaction and develop laser-based high-energy photon sources for applications. Accurate tools to study laser-driven high-energy photon emission are Particle-In-Cell (PIC) methods coupled with Monte Carlo (MC) modules to simulate photon emission [[3], [4]]. The open-source PIC code SMILEI [[5]] offers an MC package of this kind to simulate NICS, while a module for bremsstrahlung simulation is not yet available in this code. This contribution presents the results of simulation aimed to investigate NICS and bremsstrahlung emission in the case of laser interaction with a double-layer target (DLT) made of a low-density nanostructured carbon foam deposited on a thin solid substrate. DLTs can enhance the production of fast electrons with their nanostructured layer [[6]] and, thus, are interesting for boosting the consequent photon emission processes. An extensive set of 2D simulations to explore NICS in DLT has been performed with SMILEI. In particular, after a preliminary benchmark with other PIC codes capable of simulating NICS (EPOCH [[7]], WarpX [[8]], and PIConGPU [[9]]), SMILEI has been used to study the properties of this emission and assess the role of the target. The results show the relevance of the DLT and its capability to tune the emission, making DLTs worthy of investigation in future experimental campaigns. To support the possible development of a bremsstrahlung module in SMILEI, the modelling of this emission in laser-plasma scenarios can be further investigated. To this scope, this contribution presents the rationale of different simulation approaches for bremsstrahlung based on currently available open-source tools and applied to DLTs. These approaches consist in using the bremsstrahlung package in the PIC code EPOCH and coupling PIC codes, including SMILEI, with the MC code GEANT4 [[10]]. Although these simulation tools present some open problems that a future SMILEI implementation could solve, their results show that DLTs can enhance the high-energy bremsstrahlung emission compared to a single solid layer and enable control of the emission itself.
Strong-field QED (SFQED) effects are central in determining the dynamics of particles and plasma in extreme electromagnetic fields such as those generated with multipetawatt lasers or present in the vicinity of compact astrophysical objects. SFQEDtoolkit is a fully open source library designed to allow for a straightforward implementation of SFQED effects in existing particle-in-cell (PIC) and Monte Carlo codes. Through advanced function approximation techniques, high-energy photon emission and electron-positron pair creation probabilities and energy distributions are accurately and efficiently calculated within the locally-constant-field approximation (LCFA) as well as with advanced models beyond the LCFA [Phys. Rev. A 99, 022125 (2019)]. SFQEDtoolkit is designed to provide users with high-performance and high-accuracy, simultaneously. Moreover, its implementation in existing codes is kept as easy as possible, and neat examples showing its usage are provided. In the near future, SFQEDtoolkit will be enriched to model the full angular distribution of the generated articles, i.e., beyond the commonly employed collinear emission approximation, as well as to describe spin and polarization dependent SFQED effects.
Smilei Usage: We made use of Smilei not only to test our library, but also to compare the ensuing results (and performance) with those given by an original version of this PIC. To sum up, Smilei has been used for testing and physical benchmarking.
Intense lasers can accelerate electrons to very high energy over a short distance. Such compact accelerators have several potential applications including fast ignition, high energy physics, and radiography. Among the various schemes of laser-based electron acceleration, vacuum laser acceleration has the merits of super-high acceleration gradient and great simplicity. Yet its realization has been difficult because injecting free electrons into the fast-oscillating laser field is not trivial. Here we experimentally and numerically demonstrate free-electron injection and subsequent vacuum laser acceleration of electrons up to 20 MeV using the relativistic transparency effect. The key physics are identified through multi-dimensional particle-in-cell simulations and test-particle simulations. When a high-contrast intense laser drives a thin solid foil, electrons from the dense opaque plasma are first accelerated to near-light speed by the standing laser wave in front of the solid foil and subsequently injected into the transmitted laser field as the opaque plasma becomes relativistically transparent. It is possible to further optimize the electron injection/acceleration by manipulating the laser polarization, incident angle, and temporal pulse shaping. Our result also sheds light on the fundamental relativistic transparency process, crucial for producing secondary particle and light sources.
We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm’s law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the
electromagnetic field to the plasma (J.E>0) ahead or at the DF, whereas it is the opposite (J.E<0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J.E'<0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related
to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.
Extreme-ultraviolet pulses can propagate through ionized solid-density targets, unlike optical pulses, and thus have the potential to probe the interior of such plasmas on an attosecond time-scale. We present a synthetic diagnostic method for solid-density laser-generated plasmas based on the dispersion of an extreme-ultraviolet attosecond probe pulse, in a pump--probe scheme.
In our approach, the plasma dynamics in the presence of an optical pump is simulated using Smilei, while the dispersion of the probe is calculated with an external pseudo-spectral (PS) wave solver implemented in Python, allowing for high accuracy when calculating the dispersion. To this end, the output from Smilei must be read and interpolated by the PS code; at the moment, we can read either ParticleBinning or TrackParticles diagnostics. The tools, both for reading/interpolating Smilei output and for running the PS computation, are freely available on GitHub.
We illustrate the application of this method on thin-film plastic and aluminium targets irradiated by a high-intensity pump pulse. By comparing the dispersion of the probe pulse at different delays relative to the pump, it is possible to follow the time evolution of the plasma as it disintegrates.
We will show how a multilayer target behaves under ultra high intensity laser irradiation, based on a SMILEI simulation study. We observe density oscillation, a dynamic, that has not been mentioned in plasma physics yet. It describes how neighboring layers repeatedly compress each other, causing the ion and electron density of each layer to oscillate over time. Based on that, we will show how the density oscillation affects the isochoric heating of the target.
All findings are based on SMILEI simulations, experiments are planned for the future.
At the interaction of an ultra-high intensity laser pulse (I ≥ 1018W/cm2) with a plasma, the plasma constituents will absorb a significant part of the laser energy and will be accelerated up to relativistic velocities for electrons. The most predominant mechanisms of energy transfer from the laser pulse to the plasma constituents are collisionless in this regime, being done by collective effects in plasma. The absorption of laser energy depends on the initial laser and target parameters [1, 2, 3]. The target transparency or opacity depends on the interaction process itself: a slightly over dense target can absorb or reflect the laser energy according to the laser amplitude [4].
Our main goal is to describe and model the energy transfer from laser to particles, from the transparent to less transparent regime of laser-plasma interaction in the ultra-high intensity regime, and using the results obtained to optimize the ion acceleration. We propose a theoretical model of energy transfer, assuming that most of the laser energy will be transferred to hot electrons. The model proposed is further tested and corrected through 2D particle-in-cell (PIC) simulations performed with SMILEI (Simulating Matter Irradiated by Light at Extreme Intensities) [5]. Varying the target density and thickness, we studied the optimal parameters for the maximum conversion efficiency of the laser energy to particles. We investigate a model for a near-critical density plasma between 0.5 – 20 nc (where nc ≈ 1.1·1021 cm-3 is the critical density) driven by a laser pulse of intensity in the range 1018 – 1023 W/cm2 and the pulse duration in the range 10 – 100 fs.
The laser absorption mechanisms determine the characteristics of the accelerated particles. Theoretical modelling of the predominant laser-plasma interaction mechanisms predicts the particle energy and conversion efficiency optimization [6]. The transition from the opaque to the transparent regime can lead to an enhancement of the ion acceleration process [7]. Our studies led to an optimization of the target areal density for maximizing proton acceleration for a laser intensity of 1022 W/cm2, which is in good agreement with [8].
[1] K. G. Estabrook, E. J. Valeo, and W. L.Kruer, The Physics of Fluids, 18(9):1151, 1975
[2] F. Brunel, Physical Review Letters, 59(1):52, 1987
[3] W. L. Kruer and K. Estabrook, The Physics of Fluids, 28(1):430, 1985
[4] E. Lefebvre and G. Bonnaud, Physical Review Letters, 74(11):2002, 1995
[5] J. Derouillat et al., Comput. Phys. Commun. 222, 351-373 (2018)
[6] J. Fuchs et al., Nature Physics, 2:48–54, 2006
[7] R. Mishra, F. Fiuza and S. Glenzer, New Journal of Physics, 20(043047), 2018
[8] A. V. Brantov, Phys. Rev. ST Accel. Beams 18, 021301 (2015)
