Session GO5: AB: Plasma Wake-field Acceleration/Computational Techniques for Laser-Plasma Interactions

Modeling Challenges and Opportunities with High-Repetition Rate Ultra-Intense Laser Systems

Over the next decade, experimental capabilities for high repetition rate ($>$ 1 Hz) ultra-intense laser systems will grow significantly. As a proxy for the kind of science enabled by these capabilities, I briefly review experiments that have been performed on the Extreme Light ultra-intense laser system at Wright-Patterson Air Force base since 2015. I will explain how the ability to operate the laser at a kHz repetition rate played an important role in research from our group on electron and ion acceleration exceeding MeV energies. Looking towards the future, the overabundance of data from high repetition rate experiments can be a challenge to simulate and learn from compared to experiments with only a few shots. A promising new approach that our colleagues in ICF are beginning to use involves “training” a statistical model using simulations and experimental data. I will describe our efforts to use a similar approach to optimize ultra-intense laser-plasma interactions. These and other statistical methods, such as machine learning, will be very useful in our future work in a few specific ways that I will outline.   

Positron transport and acceleration in beam-driven plasma wakefield accelerators using finite radius plasmas

The transport and acceleration of positron beams is a crucial challenge on the path towards plasma-based particle colliders. In this work, a finite radius plasma is proposed to generate wakefields that can focus and accelerate positron beams in a plasma wakefield accelerator. The finite radius plasma reduces the restoring force acting on the plasma electrons forming the plasma wakefield, resulting in an elongation of the on-axis return point of the electrons and, hence, creating a long, high-density electron filament. This results in a region with accelerating and focusing fields for positrons, allowing for the acceleration and quality-preserving transport of high-charge positron beams.   

Emittance Preservation Through Matching the Witness Beam in Plasma Wakefield Acceleration

In plasma wakefield acceleration, the witness beam's emittance needs to be preserved when it propagates through a plasma stage. The plasma includes density ramps at both the entrance and the exit. Using the WKB solution of a single particle's motion, analytical expressions for the evolution of the Courant-Snyder parameters and the beam emittance in an arbitrary adiabatic plasma profile is provided neglecting the acceleration of the beam inside the plasma. It shows that the beam emittance can be preserved under the matching condition even when the beam has an initial energy spread. An expression for the emittance of an unmatched beam with energy spread is also provided. The emittance evolution from 3D QuickPIC simulation results agree well with the theoretical results. In the some of the proposed experiments on FACET II, the matching condition may not be perfectly satisfied. With a given set of beam parameters that are consistent with FACET II capabilities, the witness beam emittance growth can be minimized by choosing an optimal focal plane position.   

The FLASHForward X-1 Experiment: High brightness beams from a plasma cathode

The FLASHForward facility at DESY provides a unique facility for the study of Plasma Wakefield Acceleration (PWFA). At FLASHForward, a several kA electron beam with energies up to 1.25 GeV interacts with a plasma in a dedicated windowless, differentially pumped beamline. The X-1 Experiment aims to demonstrate the injection and acceleration of ultra-high quality electron bunches from within the plasma. This will be achieved by tailoring the plasma profile via laser ionisation with pulses from a 25~TW, fs-class, synchronised laser system prior to interaction with the electron beam. We present the recent progress made towards this internal injection, including results from the latest experimental campaigns.   

Plasma Wakefield Acceleration Experiments at FACET-II

Commissioning for the FACET-II facility at SLAC National Accelerator Laboratory is scheduled to begin in Fall 2019, and electron beam-driven plasma wakefield acceleration will feature prominently in its experimental program. Early experiments will aim to achieve pump depletion of the drive beam while accelerating the witness beam by 10 GeV and preserving emittance at the 10 mm-mrad level in a Lithium oven plasma source. Later experiments will aim to achieve the same goals while preserving emittance at the 1 mm-mrad level in a laser-ionized gas plasma source. Experimental considerations and plans will be discussed, as well as simulated predictions of results for the first run of FACET-II.   

Optimization of beam qualities on Plasma Wakefield Acceleration

The plasma wakefield accelerator (PWFA) concept has emerged as a promising candidate for making compact accelerators that can produce high-quality beams. In order for PWFA to produce high-quality beams, the energy spread and emittance must be optimized. This has led to much theoretical analysis and computer simulations on how to choose plasma and beam parameters to inject and accelerate high quality beams. However, such theories and computer simulations have yet aimed at fine tuning the parameters in order to optimize the output beam. In addition, drive beams do not have symmetric Twiss parameters or spot sizes and the shapes are not perfect Gaussians. This makes finding the optimal parameters quite challenging. We have combined the PWFA simulation code QuickPIC with POPAS, a parallel optimization toolbox developed at ANL, to efficiently find the optimal parameters of the beam and plasma that produce the highest-quality beams, and to potentially explore new acceleration regimes. We present preliminary results that show the newly developed optimization tool can find the optimal Twiss parameters for the drive beam in order to preserve the witness beam emittance in a PWFA stage. We also show that the optimization tool can help to minimize the witness beam energy spread.   

Generating high quality ultra-relativistic electrons beams using an evolving electron beam driver

In recent years, Plasma-Based Acceleration (PBA) has attracted a lot of interest in applications involving compact next generation linear colliders and x-ray free-electron-lasers (XFEL). To date, the most promising methods to produce high quality relativistic electron bunches for such applications involve injection triggered by plasma density down ramp or electron ionization. However, numerous challenges remain for the optimization and reproducibility of these methods since they typically require sharp plasma density gradients and multiple drivers to produce high brightness electron beams. In this talk, we propose and demonstrate a new method of controllable injection using a single electron drive bunch to control the wake phase velocity and induce electron trapping in a constant background density plasma. In this scheme, electron injection is determined by the Courant-Snyder parameters and the peak normalized charge per unit length. Injection is demonstrated in two different regimes where the driver vacuum propagation and plasma focusing effects are dominant. Using this approach, Particle-in-cell (PIC) simulations indicate that peak normalized brightnesses of $10^{21} A/m^2/rad^2$ can be achieved with projected energy spreads of $1\%$ and normalized emittances of $ 5$ nm.   

Plasma acceleration based high energy injector for future circular $e^{+}e^{-}$ collider

For the past century, tools for exploring high energy physics have been radio-frequency (RF) particle accelerators. The state-of-the-art accelerator Large Hadron Collider costs about {\$}4 billion to construct and is 27km in circumference. Detailed tests of recently discovered Higgs Bosons require a collider with higher energy and will cost much more with the same technology. Plasma based acceleration (PBA) is shown to have acceleration gradients orders of magnitude higher than RF accelerator, able to dramatically reduce the size and cost of future colliders. For next generation circular colliders such as the proposed Circular Electron Positron Collider (CEPC), which collides $e^{-}/e^{+}$ beams of 120GeV, may be greatly enhanced by PBA. We present preliminary results on a 45GeV injector for CEPC that consists of a conventional e^{-}/e^{+}$ beam source and a single PBA cell. The PBA cells for both electron and positron acceleration are custom-designed, which could also be applied to a future linear collider. The techniques for emittance preservation and energy dechirping are included to optimize beam quality. Detailed simulations show the final results satisfy the requirements proposed in CEPC design report.   

Toward the Modeling of Chains of Plasma Accelerator Stages with WarpX

One of the most challenging application of plasma accelerators is the development of a plasma-based collider for high-energy physics studies. Fast and accurate simulation tools are essential to study the physics toward configurations that enable the production and acceleration of very small beams with low energy spread and emittance preservation over long distances, as required for a collider. The Particle-In-Cell code WarpX is being developed by a team of the U.S. DOE Exascale Computing Project (with non-U.S. collaborators on part of the code) to enable the modeling of chains of tens of plasma accelerators on exascale supercomputers, for collider designs. The code combines the latest algorithmic advances (e.g., boosted frame, pseudo-spectral Maxwell solvers) with mesh refinement and runs on the latest CPU and GPU architectures. The application to the modeling of up to three successive muti-GeV stages will be discussed. The latest implementation on GPU architectures will also be reported, as well as novel algorithmic developments.   

On the self-forces of relativistic particles moving in “vacuum” in PIC codes

The particle-in-cell (PIC) method is widely used to model the self-consistent interaction between discrete particles and electromagnetic fields. It has been successfully applied to problems across plasma physics including plasma based acceleration, inertial confinement fusion, magnetically confined fusion, space physics, astrophysics, high energy density plasmas. In many cases the physics involves how relativistic particles (those with high relativistic gamma factors) are generated and interact with plasmas. However, when relativistic particles stream across the grid both in vacuum and in plasma there are many numerical issues may arise which can lead to incorrect physics. We present a detailed analysis of how numerical issues in PIC codes can lead to unphysical self-forces on particles that move at relativistic speeds across the grid. We analyze the fields on the grids excited by the relativistic particles and point out the numerical (unphysical) fields dominates especially for the electrical component along the drifting direction. Two representative solvers - the yee solver and the spectral solver are studied in details. Furthermore, a novel solver with $[k]_1=[k_1]_t$ is proposed to eliminate the numerical EM fields and it has been implemented into the PIC code OSIRIS.   

Absorption of Charged Particles in Perfectly-Matched-Layers

Perfectly-Matched Layers (PML) are widely used in Particle-In-Cell simulations of plasmas, in order to absorb electromagnetic waves that propagate out of the simulation domain. However, when charged particles cross the boundary between the simulation domain and the PMLs, a number of numerical artifacts can arise (including field reflections, and accumulation of residual static field at the boundary).
 We introduce a new PML algorithm that significantly mitigates these artifacts. The benefits of this algorithm is illustrated in simulations of laser-plasma acceleration, whereby a significant fraction of the plasma particles exit the simulation domain.   

Computational modeling of multi-time scale laser-plasma interactions

The interaction of intense, short-pulse lasers with matter necessarily couple physics across a range of timescales, from the femtosecond laser period to picosecond and nanosecond time scale physics of plasma evolution after initial irradiation. Realistic computational modeling of these interactions with traditional Particle-in-Cell (PIC) methods becomes prohibitively expensive to perform over timescales longer than a few picoseconds. A number of methods have been developed in order to model longer timescale physics by using multiple simulations, for different timescales[1]. Analogous to these efforts, I present a scripted framework in Python that enables the simulation of laser-plasma interactions from the femtosecond to the nanosecond by breaking a problem into timescale "stages", over which a PIC simulation models each timescale and acts as the initial condition of the following timescale, which requires the development of tool to handle "handoffs" between each stage. An example of simulation the irradiation of a 10 micron water droplet by a 5x1018 W/sq cm intensity laser in the LSP code with this framework is presented to illustrate the utility of this method to model multi-time scale physics. [1] Derek, et al. \textit{Physics of Plasmas} 26.4 (2019): 043110   

Highly efficient quasi-static particle-in-cell algorithm using azimuthal Fourier decomposition based on QuickPIC

The particle-in-cell (PIC) algorithm based on quasi-static approximation (QSA) is widely utilized to model high-energy charged particles and short laser pulses interacting with plasma. Compared to computationally intensive full 3D explicit PIC codes, the quasi-static codes can speed up the simulations by orders of magnitude, which allows for modeling problems that require large computing resources, including the hosing instability of particle beams and ion motion for matched particle beams in plasma accelerators. In addition to QSA, the Fourier azimuthal decomposition (FAD) is another effective speedup technique already applied in some general-purpose PIC codes. However, this approach has never been combined with the quasi-static approach. We present details on a new PIC code based on the workflow and structure of QuickPIC. It combines QSA and FAD together. FAD expands the fields, charge and current density into azimuthal harmonics and truncates the expansion. The complex amplitudes of fields on an r-z grid are then solved through Maxwell's equations under QSA using finite difference solvers in conjunction with the multigrid method. Benchmarks against the full 3D and 3D quasi-static codes are presented. We call this new code QPAD for QuickPIC with Azimuthal Decomposition.   

Adjoint approach to accelerator lattice design

Traditionally, accelerator lattices are designed using computer codes that solve the equations of motion for charged particles in both prescribed and self-consistent fields. These codes are run in a mode in which particles enter a lattice region, travel through the lattice for a finite distance, and then have their phase space coordinates recorded to assess various figures of merit (FoMs) (emittance, dynamic aperture, etc.). The lattice is then optimized by varying the positions and strengths of the focusing elements. This optimization is done in a high dimensional parameter space, requiring multiple simulations of the particle trajectories to determine the dependence of the confinement on the many parameters. Sophisticated algorithms for this optimization are being introduced. However, the process is still time consuming. We propose to alter the design process using “adjoint” techniques. Incorporation of an “adjoint” calculation of the trajectories and self-fields can, in several runs, determine the gradient in parameter space of a given FoM with respect to all lattice parameters. It includes naturally self-fields and can be embedded in existing codes. The theoretical basis for the method and several applications will be presented.   

Essential parameters in comprehensive hybrid modeling of metals irradiated by femtosecond laser

Study of femtosecond laser interaction with metals using two temperature model requires accurate calculation of several major optical and thermodynamic parameters including laser photon reflectivity and penetration depth, electron thermal conductivity, heat capacity, and electron-lattice coupling factor. All these parameters are still not well determined through the theoretical studies or experiments and have great effects on the physical processes. In this work, the comprehensive hybrid, two temperature and molecular dynamics, model was developed. The effect of nonequilibrium electrons was considered, and major parameters were calculated as functions of electron and lattice temperatures and lattice density. The effect of each individual parameter and the synergistic effects will be discussed. By tuning these parameters, we benchmarked our results with recent experimental data and other simulation results. The benchmarking includes electron and lattice temperature, surface displacement, and phase transition. Verified parameters were then used for further studies such as damage threshold, laser ablation rate, and nanoparticle production.