Session GP20: Poster Session: Particle Acceleration, Beams and Relativistic Plasmas I (9:30am - 12:30pm)

New developments in the OSIRIS 4.0 framework

The OSIRIS [1] Electromagnetic particle-in-cell (EM-PIC) code is widely used in the numerical modeling of many kinetic plasma laboratory and astrophysical scenarios. In this work, we report on the new developments recently introduced into the framework. In particular, we will update on the progress of our dynamic load balancing algorithm and our customized solver for high fidelity laser particle interaction and removal of unphysical fields as well as our progress on the Quasi-3D algorithm, including its integration with many other modules. We will focus on our ability to study ionization injection of spin polarized electron beams and the use of an improved the absorber region to prevent reflux in laser solid interactions. We will also address the new features on our Compton scattering module and our advanced diagnostics for radiation shorter than the grid wavelength. Finally, we will discuss our new analytic particle pushers using 9D phase space for high fields (Lorentz force, Lorentz plus RR, Spin plus RR). [1] R. A. Fonseca et al., Lecture Notes in Computer Science \textbf{2331}, 342-351 (2002)   

Adapting PIC for Memory-Constrained Supercomputers: High-Order Particle Shape for Single-Precision VPIC

Supercomputers with new architectures require different computational and numerical approaches to obtain optimal performance for the largest plasma simulations. Specifically, particle-in-cell codes require significant modification to run and perform well on present and upcoming GPU-based machines. We report on progress adapting the code \textsc{vpic} for use on GPU supercomputers as part of ongoing performance-portable efforts utilizing the Kokkos framework. We focus here on the challenges of running large simulations on relatively memory-starved GPUs. High-order particle shape functions can often use far fewer particles without sacrificing simulation fidelity, increasing the size of simulations that can fit in memory. The single-precision nature of \textsc{vpic} additionally increases the number of particles that can be used. Combining these two features greatly reduces memory footprint, allowing simulations to be run on fewer nodes, reducing communication, or for larger simulations to be run on memory-constrained supercomputers.   

Self-force of a Lorentz-contracted charged-sphere under uniform circular motion

The radiation reaction in classical electrodynamics has not been completely solved. One of the main difficulties has been the unacceptable causality violation known as runaway or pre-acceleration solution. Even after the Landau-Lifshitz model resolved them, still there are some issues such as energy conservation of photons and a particle under the uniform acceleration. To solve this, we recently suggested a `Lorentz-contracted charged-sphere', where the radiation reaction and the corresponding energy loss in a uniform acceleration can be successfully explained by effective mass change. This result motivated us to investigate other motions of the same model of Lorentz-contracted sphere. In this paper, we present our new calculation of the self-force exerting on the Lorentz-contracted sphere under the uniform circular motion. The calculation result implies that the radiation reaction for circular motion is related to the retarded time. In addition, we discuss which size of the extended particle would be appropriate.   

A Particle-Mesh Method for the Modeling of Synchrotron Radiation from Electron Beams

Coherent and incoherent effects induced by synchrotron radiation, such as the collective beam microbunching instability and phase space diffusion caused by shot-noise in a dispersive beam optics element, are fundamental problems underpinning many of the accelerator design issues. To accurately and efficiently model the beam dynamics in the presence of synchrotron radiation, we investigate several existing methods for the calculation of the radiation near fields, including the finite difference method, the Jefimenko method and a near-field method recently validated where a Lagrangian adaptive mesh and a global mesh are employed[1]. This particle mesh method allows radiation propagation on the mesh with a greatly reduced error. We compare the accuracy and efficiency of these methods in both 1D and multi-dimensions, for the steady-state and dynamical beam trajectories, for the radiation field and space charge field, as well as for the coherent field and the incoherent field. We also discuss the multiple levels of parallelisms inherent in this particle mesh framework, which can be implemented on modern computing platforms using MPI, multi-threading, and GPUs. [1] F. Li, et al., in Proceedings of 10th Int. Part. Acc. Conf., Melbourne, Australia, 2019, pp. 397--399.   

Numerical Studies of Relativistic 4-Photon Upconversion

Four-photon scattering at relativistic intensities has been proposed for efficient amplification of high frequency seed photons [1]. The amplification of the high frequency seed may be disrupted by the detuning of the resonance caused by self and cross phase modulation of all four waves. If the disrupting effects from detuning can be overcome, it may be possible to achieve greater energy transfer and higher contrast in the output pulse. Balancing the self and cross phase modulation effects, as well as structuring the seed and pump envelopes, may help to overcome the detuning. [1] Malkin, V. M., and N. J. Fisch. "Towards megajoule x-ray lasers via relativistic four-photon cascade in plasma." Physical Review E 101.2 (2020): 023211.   

Analysis of Tailored Laser Plasma Accelerator Gas Jet Targets

The ability to precisely shape gas jets for controlled injection of electrons in laser plasma accelerators (LPAs) is crucial for developing high-quality electron beams. For a popularly used tailored gas jet, that of a plume impinged by a blade, verification of features has called for more detailed simulations and gas density diagnostics than those traditionally used. We combined full plume simulations and a high-resolution diagnostic that can handle asymmetry to advance the customization process. Three dimensional full plume fluid simulations were performed to show the flow dynamics and variation of those with experimental variables, such as pressure and laser height. In parallel, planar laser-induced fluorescence (PLIF) was prototyped for characterizing LPA gas jet targets. PLIF has the advantage of isolating thin slices of the gas plume using a laser sheet, providing more direct density information at regions of interest. The study found that blade position dramatically alters characteristic flow parameters, affecting plume axis, effective Mach number, and therefore density transition length. These results are being used to understand and design flow regimes for LPA targets in the BELLA Center.   

Three-Dimensional Modeling of Capillary Discharge Plasmas for Acceleration and Control of Particle Beams

Next generation accelerators aim to achieve unparalleled beam quality, stability, and average power, for which dramatic improvements to the flexibility, control, and precision of beamline components is required. Capillary discharge plasmas are a subset of tailored plasma systems which offer strategic advantages over traditional beamline technologies for accelerating stages, focusing elements, energy compensators, and diagnostics. We present simulations of capillary discharge plasmas in 2D and 3D geometries using FLASH, a publicly-available multi-physics code with sophisticated magneto-hydrodynamic capabilities. We explore novel geometric configurations of capillary structures for use as a laser-plasma accelerator stage, and examine differences in the plasma density steady state resulting from structure, gas, and discharge parameters. We model laser energy deposition to generate sub-channels for the guiding of intense pulses. Lastly, we investigate the use of capillaries for active plasma lenses and present results from benchmark studies. These results are compared against simulation and experimental studies.   

Plasma Production by Optical Field Ionization

The 2D optical field ionization of neutral gases and the subsequent plasma expansion has been computed using the VSim particle-in-cell computational application. This model combines self-consistent electromagnetic fields with external laser fields to drive the charged particles. Those laser fields consist of either circular or linearly polarized gaussian pulses, and the ionization rates come from the ADK formula. In addition to field ionization, elastic electron-ion and electron-neutral collisions, as well as impact ionization reactions, are implemented. Comparisons with the computational and experimental work on plasma channel formation presented by Shalloo et al [1] are presented. An electromagnetic pulse polarized in the direction of laser propagation has been observed following the field ionization. Exploration of plasma channel formation and the use of this pulse to measure the normalized vector potential will be presented. 1. Shalloo et al, Physical Review, E 97, 053203.   

Theory of the Plasma Bubble Deflection and Induced Electron Self-Injection Controlled by the Carrier-Envelope-Phase (CEP) of a short laser pulse

When an intense few-cycle laser pulse propagates in tenuous plasma, electrons are pushed asymmetrically in the polarization plane and forming behind a deflected/oscillating bubble. We present a quasi-static theory of electron’s motion in a few-cycle pulse beyond the pondermotive approximation and show a quasi-static high-order correction force on the pondermotive force. This correction is carrier-envelope-phase dependent and has been implemented in the code: WAND-PIC [1]. Simulation results from WAND-PIC are compared with the full 3D particle-in-cell simulations and scaling of bubble deflection amplitude on laser parameters would be provided. We also present a Hamiltonian theory of electron self-injection in such a deflected/oscillating bubble. We derive the change of Hamiltonian of an electron caused by the periodic deflection and use the sufficient condition for the electron trapping [2] and give the threshold of injection as a function of deflection amplitude and CEP frequency. [1] WANDPIC Repository: https://github.com/tianhongg/WAND-PIC. [2] S. A. Yi, V. Khudik, S. Y. Kalmykov, and G. Shvets, Plasma Phys. Control. Fusion, vol. 53, 014012 (2010).   

Modelling external injection of an electron beam into a laser wake field accelerator

Electron beams produced by laser-driven plasma-based accelerators have broader energy spreads and greater shot-to-shot instability than those from conventional linacs. External injection of electron bunches into laser wakefield accelerators holds the prospect of vastly improving the characteristics of the accelerated bunches. The CLARA facility at Daresbury Laboratory provides the opportunity to experimentally explore such a scheme. This poster presents the results of modelling the acceleration of an externally injected electron beam into a laser driven wakefield accelerator using the CLARA facility parameters. The campaign of simulations utilised particle-in-cell codes to scan a multitude of parameter sets with the aim of maximising the acceleration gradient experienced by the electrons without degradation of beam quality.   

Acceleration of Quasi-Monoenergetic Electrons to 15 MeV at 1 kHz with <2.7 mJ, ~5 fs Pulses

We demonstrate acceleration of quasi-monoenergetic electron bunches with a narrow angular divergence up to 15 MeV by focusing 2.7 mJ, 5 fs pulses at a kHz repetition rate onto a near-critical hydrogen gas jet. In previous experiments, we showed that the use of near-critical gas densities [1] enabled acceleration of electrons to MeV energies using mJ-scale pulses from a kHz system by lowering the critical power for self-focusing [2, 3]. These bunches, generated in the self-modulated (SM-LWFA) regime, had broad angular divergences and exponential energy distributions [2, 3]. Our recent experiments operating in the bubble regime with 7 fs, 2.5 mJ pulses generated in a hollow-core fiber (HCF) saw the acceleration of quasi-monoenergetic electrons to 5 MeV with an improved angular spread [4]. In this work, we employ 5 fs, 2.7 mJ pulses generated in an HCF from elliptically polarized pulses to accelerate quasi-monoenegetic electron bunches to 15 MeV with 7 mrad divergence and 4 mrad shot-to-shot jitter. [1] F. Salehi \textit{et al}., Rev. Sci. Instrum. \textbf{90}, 103001 (2019) [2] F. Salehi \textit{et al}., Opt. Lett. \textbf{42}, 215–218 (2017) [3] A. J. Goers \textit{et al}., Phys. Rev. Lett. \textbf{115}, 194802 (2015) [4] F. Salehi \textit{et al}., FiO+LS, JW4A.116, OSA, 2019.   

Observation of bubble formation in laser wakefield acceleration

Laser Wakefield Acceleration experiments were performed with the HERCULES laser facility at University of Michigan. Inferring the temperature with soft x-ray spectroscopy, we observed bubble formation as a function of plasma density. The electron injection threshold, as well as the onset of Betatron radiation were observed. Numerical modeling with PIC simulations are also presented.   

Optimization of high repetition-rate laser-driven particle and radiation sources using machine-learning techniques

Many applications of laser-driven particle sources benefit from operation at high repetition rate. Here, 20 milliJoule laser pulses are generated at 0.5 kilohertz repetition rate for a number of laser-plasma interaction experiments, including laser wakefield acceleration and k$\alpha$ x-ray generation. A genetic algorithm is implemented in the execution of these experiments using control of adaptive optics and a Dazzler acoustic-optic programmable dispersive filter. Utilizing the genetic algorithm in our laser-plasma interaction experiments allows for a heuristic search of optimal laser pulse parameters or target parameters for each experiment.   

Production and Optimized Spectroscopic Measurement of High Energy, Narrow Energy-Spread Electron Beams From a Modern Laser Wakefield Accelerator

A continuing goal of laser wakefield acceleration (LWFA) research is to decrease the spread in energy of the electron beams that these accelerators can produce while simultaneously increasing the maximum central-energy of these beams. High energy mono-energetic beams are a requirement for most applications of LWFA including using accelerated beams as light sources, or as probes for fast-evolving fields. These beams present a challenge to accurately measure in the laboratory due to the standard dipole electron spectrometer's decrease in energy resolution as particle energies increase. In this work, it is demonstrated through particle-in-cell simulations of various LWFA injection mechanisms that high energy (in excess of 1 GeV) electron beams with low energy-spread (a few \%) are produced using modern laser parameters and feasible experimental setups. These beams are then computationally measured using a simulated magnetic spectrometer. It is shown that a genetic algorithm approach may be applied to the configuration of the spectrometer's magnets and phosphorus screens in order to optimize energy resolutions in the energy range we are interested in. As LWFA beam energies continue to climb, further optimization of spectrometer techniques will be increasingly important.   

Controlled Electron Injection into a Laser-Wakefield Accelerator using a Long-Wavelength Single-Cycle Precursor

Single cycle laser pulse propagating inside a plasma causes controllable asymmetric plasma electron expulsion from laser according to laser carrier envelope phase (CEP) and forms an oscillating plasma bubble [1]. Bubble's transverse wakefield is modified, exhibiting periodic modulation. Injection scheme for a laser wakefield accelerator using such single cycle low frequency laser pulse and a many cycle high frequency laser pulse is proposed. While the single cycle pulse drives the bubble, background plasma electrons can be periodically trapped because of modified wakefield. The single cycle pulse quickly loses energy, after which the many cycle pulse generates a stable wakefield. Injection is terminated, and electrons are accelerated to higher energy.~By tuning the initial CEP of the single cycle laser pulse, injection dynamics can be modified independently of the many cycle pulse, enabling control of electron bunches' charge and spatiotemporal profile. [1] E.N.Nerush and I.Yu.Kostyukov Phys.Rev.Lett. 103,035001(2009)   

Experimental considerations of using coded apertures for high-energy high-resolution imaging

Laser-plasma x-ray sources have garnered interest from various communities due to their ability to generate high photon-energies from a small source size. The passive imaging of high-energy x-rays and neutrons is also a useful diagnostic in laser-driven fusion as well as laboratory astrophysics experiments which aim to study small samples of transient electron-positron plasmas. Conventional high attenuation aperture imaging techniques struggle with high-energy high-resolution imaging, due to their aspect ratios: substrate thickness needs to be substantial for high attenuation, whereas aperture elements need to be small for high resolution. Coded apertures with scatter and partial attenuation (CASPAs) are a technique used to relax this attenuation requirement, allowing thinner substrates to be used without significant detriment to the signal to noise ratio of the overall imaging system. There are manufacturing challenges associated with producing CASPAs with elements on the order of 10 μm, which require changes to the original design to make the aperture self-supporting. Here, we discuss the impact of manufacturing and experimental considerations on the implementation of CASPA-based imaging systems for high-energy high-resolution imaging.   

X-Ray Spectrum Reconstruction using an Attenuating Filter Pack

A technique to reconstruct the energy distribution of 1-10 MeV X-Rays produced by the Thomson backscattering of laser-plasma accelerator driven electrons upon a powerful laser pulse will be presented. By using an attenuator filter pack, the relative photon transmission signals can be estimated from a pixelated scintillator output and combined to deduce primary spectrum characteristics of the emitted radiation. Performed on a shot-to-shot basis and supported with electron distribution data from a magnetic spectrometer, this technique can be used to provide real-time feedback of the backscattering interaction. Analytic proof of concept will be shown, and an experimental design and limitations will be discussed.   

Design of Compton Scatter Magnetic Spectrometer for Characterization of Thomson Scattered Photon Sources

Thomson scattering of intense laser pulses from laser-plasma accelerator generated electron beams can provide a source of quasi-monoenergetic few-MeV photons. Such sources will reduce dose and increase sensitivity for active interrogation techniques. Development of these sources requires proper diagnostics for the resultant MeV photon pulse. This has proven difficult due to the high scattered photon energies and tens-of-femtosecond pulse durations. A powerful diagnostic tool would allow the user to effectively attribute the energy-angle dependence of the photon spectrum to parameterized characteristics of the electron beam, such as energy and divergence distributions. Active research is underway to develop a Compton scatter magnetic spectrometer that will allow for such characterization of these LPA-sourced electron beams for 1-10 MeV scattered photons. Simulation of these Thomson scattered sources has shown that determination of electron peak energy is readily achievable, with the potential for more detailed analysis using spatial data at the magnetic spectrometer scintillator. Future efforts will focus on the development of necessary methods for calculating electron beam energy-angle distribution characteristics.   

Evolution of the laser's wavefront and mode during propagation through the amplifier and transport line of a 100-TW-class laser system

Laser Plasma Accelerators (LPAs) require high-quality drive lasers in order to generate high-quality electron beams; however, it is known that the propagation of the laser beam through the amplifier, beam expander, compressor, and transport chain will make the laser beam susceptible to unwanted spatial amplitude and phase modulations. As such, developing reliable diagnostic tools to identify the wavefront and mode evolution, and their dependence on the optical elements in the beamline, will provide critical insight. In this regard, parasitic traits that may be inherited from these optical elements can be probed and eliminated or replaced from the transport line thereby increasing the quality of the drive-laser. We will present simulations and experimental results aimed at understanding the evolution of the mode and wavefront of a high-power drive laser, and their impact on the final focus properties.   

Radiation emission at Langmuir frequency from laser wake in longitudinally stratified plasma column

Theoretical analysis shows that a laser wake (an electrostatic Langmuir wave), driven in a periodically stratified, cylindrical plasma column, generates a superluminal rotational current at a Langmuir frequency. This current emits a Cherenkov TM wave into the plasma-free space. The spatial period of stratification defines an opening angle of the emission cone. Wave breaking in the inhomogeneous plasma limits the lifetime of the wake (hence, the THz signal length) to a few tens of picoseconds. Monochromaticity and coherence distinguishes this signal from ultrashort, uncollimated, broadband THz pulses emitted from plasma filaments. The efficiency of electromagnetic energy conversion, from optical to THz, reaches the maximum when the drive pulse waist size is close to the column radius. The efficiency increases with an increase in the drive pulse wavelength, and reaches the maximum when the drive pulse power becomes near-critical for relativistic self-focusing. Theoretically, conversion efficiency of a sub-Joule, near-IR TW drive pulse is expected to reach $10^{-5}$, with the emitted energy of several $\mu$J, and a MV m$^{-1}$ electric field a meter away from the source. Approved for public release; distribution is unlimited. Public Affairs release approval AFMC-2020-0266.   

Generation of Relativistic THz Radiation by Laser Irradiation of Microplasma Waveguide with Application to Pulsed Polarimetry

Generation of relativistic pulses in the terahertz range and its application to pulsed polarimetry is proposed.~~Irradiation of a microplasma waveguide (MPW) with a relativistic (a0\textgreater 1) p-polarized sub-pico second laser results in~a~high-charge (10s nC) electron bunches with~energies~up to 100 MeV.~~As~an~electron bunch exits the MPW~its~energy is converted to THz radiation through coherent diffraction radiation.~~Particle-in-cell (PIC) simulation is used to demonstrate the generation of the electron bunches by laser irradiation of~a~plasma~at~grazing and oblique incidences~to~a sharp plasma-vacuum boundary.~~The~spacing of the electron bunches~is roughly one laser wavelength.~~We shall discuss the various competing mechanism facilitating~their~generation: direct laser transverse field driver, ponderomotive force, counter streaming (return current) electrons in the plasma.~~Moreover, we shall demonstrate the THz radiation generation by performing spectral analysis of the exiting radiation.~~2D PIC simulation shall be used to optimize the MPW shape and laser properties to maximize the outgoing THz radiation.~~Lastly, we shall discuss how the THz radiation is applicable to pulsed polarimetry to enable the measurement of internal magnetic field.