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Plasma waves generated in the wake of intense, relativistic laser1,2 or particle beams3,4 can accelerate electron bunches to gigaelectronvolt energies in centimetre-scale distances. This allows the realization of compact accelerators with emerging applications ranging from modern light sources such as the free-electron laser to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre wakefields can accelerate witness electron bunches that are either externally injected5,6 or captured from the background plasma7,8. Here we demonstrate optically triggered injection9–11 and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ‘plasma photocathode’ decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical11 density down-ramp injection12–16 and is an important step towards the generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness17. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh-brightness beams. Electron bunches are generated and accelerated to relativistic velocities by tunnel ionization of neutral gas species in a plasma. This represents a step towards ultra-bright, high-emittance beams in plasma wakefield accelerators. [This summary has been amended from ‘laser-plasma’ to ‘plasma wakefield’ accelerators.]

Upon inclusion of collisions, the speed-limited particle-in-cell (SLPIC) simulation method successfully computed the Paschen curve for argon. The simulations modelled an electron cascade across an argon-filled capacitor, including electron-neutral ionization, electron-neutral elastic collisions, electron-neutral excitation, and ion-induced secondary-electron emission. In electrical breakdown, the timescale difference between ion and electron motion makes traditional particle-in-cell (PIC) methods computationally slow. To decrease this timescale difference and speed up computation, we used SLPIC, a time-domain algorithm that limits the speed of the fastest electrons in the simulation. The SLPIC algorithm facilitates a straightforward, fully-kinetic treatment of dynamics, secondary emission, and collisions. SLPIC was as accurate as PIC, but ran up to 200 times faster. SLPIC accurately computed the Paschen curve for argon over three orders of magnitude in pressure., 17 pages, 2 figures

This paper discusses temporally continuous and discrete forms of the speed-limited particle-in-cell (SLPIC) method first treated by Werner et al. [Phys. Plasmas 25, 123512 (2018)]. The dispersion relation for a 1D1V electrostatic plasma whose fast particles are speed-limited is derived and analyzed. By examining the normal modes of this dispersion relation, we show that the imposed speed-limiting substantially reduces the frequency of fast electron plasma oscillations while preserving the correct physics of lower-frequency plasma dynamics (e.g. ion acoustic wave dispersion and damping). We then demonstrate how the timestep constraints of conventional electrostatic particle-in-cell methods are relaxed by the speed-limiting approach, thus enabling larger timesteps and faster simulations. These results indicate that the SLPIC method is a fast, accurate, and powerful technique for modeling plasmas wherein electron kinetic behavior is nontrivial (such that a fluid/Boltzmann representation for electrons is inadequate) but evolution is on ion timescales., Comment: 29 pages, 6 figures

Sources of high-energy photons have important applications in almost all areas of research. However, the photon flux and intensity of existing sources is strongly limited for photon energies above a few hundred keV. Here we show that a high-current ultrarelativistic electron beam interacting with multiple submicrometer-thick conducting foils can undergo strong self-focusing accompanied by efficient emission of gamma-ray synchrotron photons. Physically, self-focusing and high-energy photon emission originate from the beam interaction with the near-field transition radiation accompanying the beam-foil collision. This near field radiation is of amplitude comparable with the beam self-field, and can be strong enough that a single emitted photon can carry away a significant fraction of the emitting electron energy. After beam collision with multiple foils, femtosecond collimated electron and photon beams with number density exceeding that of a solid are obtained. The relative simplicity, unique properties, and high efficiency of this gamma-ray source open up new opportunities for both applied and fundamental research including laserless investigations of strong-field QED processes with a single electron beam., Comment: 17 pages, 11 figures, matches published article

First-principles particle-in-cell (PIC) simulation is a powerful tool for understanding plasma behavior, but this power often comes at great computational expense. Artificially reducing the ion/electron mass ratio is a time-honored practice to reduce simulation costs. Usually, this is a severe approximation. However, for steady-state collisionless, electrostatic (Vlasov-Poisson) systems, the solution with reduced mass ratio can be scaled to the solution for the real mass ratio, with no approximation. This 'scaled mass' method, which works with already-existing PIC codes, can reduce the computation time for a large class of electrostatic PIC simulations by the square root of the mass ratio. The particle distributions of the resulting steady state must be trivially rescaled to yield the true distributions, but the self-consistent electrostatic field is independent of the mass ratio. This method is equivalent to 'numerical timestepping,' an approach that evolves electron and ion populations with different timesteps. Numerical timestepping can be viewed as a special case of the speed-limited PIC (SLPIC) method, which is not restricted to steady-state phenomena. Although the scaled-mass approach is simplest, numerical timestepping and SLPIC more easily generalize to include other effects, such as collisions. The equivalence of these new approaches is demonstrated by applying them to simulate a cylindrical Langmuir probe in electron-argon plasma, speeding up simulation by two orders of magnitude. Methods such as SLPIC can therefore play an invaluable role in interpreting probe measurements by including geometric effects, collisions, secondary emission, and non-Maxwellian distributions., Comment: 22 pages, 3 figures

Many high power electronic devices operate in a regime where the current they draw is limited by the self-fields of the particles. This space charge limited current poses particular challenges for numerical modeling where common techniques like over-emission or Gauss' Law are computationally inefficient or produce nonphysical effects. In this paper, we show an algorithm using the value of the electric field in front of the surface instead of attempting to zero the field at the surface, making the algorithm particularly well suited to both electromagnetic and parallel implementations of the particle-in-cell algorithm. We show how the algorithm is self-consistent within the framework of finite difference (for both electrostatics and electromagnetics). We show several 1D and 2D benchmarks against both theory and previous computational results. Finally, we show the application in 3D to high power microwave generation in a 13 GHz magnetically insulated line oscillator.

In 1963, L. M. Chanin and G. D. Rork [Phys. Rev. 133, A1005 (1964)] [1] measured the first Townsend ionization coefficient for various gases and a range of pressures experimentally in a vacuum tube. A Townsend discharge is an ionization avalanche that occurs between two electrodes when secondary electron emission caused by ion impact on the cathode is negligible. The first Townsend coefficient, α, is essentially a measure of how many ionization events a single electron will cause when subject to a uniform electric field. In this study, we reproduce the experimental setup of Chanin and Rork's device in VSim [C. Nieter and J. R. Cary, J. Comp. Phys. 196, 448 (2004)] [2], a highly parallelized particle-in-cell/finite-difference time-domain code. Various particle interactions are included, and the first Townsend coefficient is calculated and compared to the reported value.

A current challenge that is facing the plasma wakefield accelerator (PWFA) community is transverse beam emittance preservation. This can be achieved by balancing the natural divergence of the beam against the strong focusing force provided by the PWFA plasma source in a scheme referred to as beam matching. One method to accomplish beam matching is through the gradual focusing of a beam with a plasma density ramp leading into the bulk plasma. Here, the beam dynamics in a Gaussian plasma density ramp are considered, and an empirical formula is identified that gives the ramp length and beam vacuum waist location needed to achieve near-perfect matching. The method uses only the beam vacuum waist beta function as an input. Numerical studies show that the Gaussian ramp focusing formula is robust for beta function demagnification factors spanning more than an order of magnitude with experimentally favourable tolerances for future PWFA research facilities. This article is part of the Theo Murphy meeting issue ‘Directions in particle beam-driven plasma wakefield acceleration’.

Speed-limited particle-in-cell (SLPIC) modeling is a new simulation technique [G. R. Werner et al., Phys. Plasmas 25,123512 (2018)], potentially much faster than conventional PIC, for modeling plasmas characterized by low-velocity kinetic processes. Numerical constraints (e.g. timestep limitations associated with particle cell-crossing times or stability limits) often place challenging restrictions on PIC models of these plasmas; even though the kinetic physics of interest predominantly involves slow particles, the fastest particles dictate the maximum allowable timestep. For high-Z plasmas, large ion/electron mass ratios separate the species timescales to the point that kinetic simulation may be prohibitive, and computational costs can be high even in hydrogenic plasmas. SLPIC provides a possible solution. SLPIC (like PIC) retains a fully kinetic description of the plasma, but imposes an artificial speed limit on fast particles whose kinetics do not play a meaningful role in the system dynamics. Larger simulation timesteps, which enable faster simulations of such discharges, are thus permitted. The speed-limiting is done in a mathematically rigorous sense to maintain accuracy over longer timescales; we may, for instance, speed-limit the bulk of the electron distribution to evolve only on characteristic ion timescales (and use larger simulation timesteps, which need only resolve these scales, to simulate the discharge). In this paper we'll demonstrate the use of SLPIC methods using the VSim code [C. Nieter and J. R. Cary, J. Comp. Phys. 196, 448 (2004)], moving from simple models of collisionless sheath formation (for which SLPIC has achieved >260x overall speedup relative to PIC with comparable accuracy) to discuss SLPIC applications in more general low-temperature plasma discharges (e.g. collisional plasmas). We'll also discuss prospects for using SLPIC in the rapid modeling of plasma discharge evolution through transient or fluid-like phases, and its capability to transition mid-simulation to a smaller-timestep conventional PIC model as kinetic processes in the discharge become important.

An electron beam experiences chromatic emittance growth in a plasma-based accelerator if it is not matched to the focusing force in the plasma wake. A ramped plasma density profile at the entrance and exit of the plasma source can control the focusing of the beam into and out of the plasma accelerator, limiting emittance growth. Here, we present a comprehensive, analytic theory to describe the transverse beam dynamics and emittance growth in a nearly arbitrary plasma ramp profile. For a given incoming beam, this theory can be used to determine the length of the ramp required to correctly focus the electron beam, the optimal location of the beam's vacuum focus, and the chromatic emittance growth in the ramp. In addition, the theory can be used to determine the effect that errors in the beam focusing and plasma profile have on the emittance of the beam. We illustrate two example ramps to demonstrate the theory: one that provides very fast focusing for beam matching, and one that is robust to errors in the plasma density profile.

Computational accelerator physics has changed and broadened over the last decade or so. Part of the change is due to the advent of multiple ways of parallel computing. Another part comes from algorithmic developments. The multiple ways of parallel computing include distributed memory parallelism and on-chip parallelism, with the latter coming from architectures (CPU and GPU) having multiple processing elements (cores or streaming multiprocessors) and wide vector (SIMD) instruction units. The basics of these new architectures and their application to computational accelerator physics are briefly reviewed. Algorithmic advances in the select areas of spin tracking, cavity calculations, plasma acceleration, and electron cooling are also reviewed. In some cases the algorithms provide increased fidelity improving the overall accuracy, while in other cases, such as controlled dispersion, the algorithms provide increased fidelity by better modeling the essential physical interaction. Finally, the use of computational frameworks, which provide the basic computational infrastructure, while allowing the capability developer to concentrate on the math and physics, is reviewed in the context of the Vorpal application, which has found use across accelerator physics and many other fields.

International audience; We present a laser-ionized, beam-driven, passive thin plasma lens that operates in the nonlinear blowout regime. This thin plasma lens provides axisymmetric focusing for relativistic electron beams at strengths unobtainable by magnetic devices. It is tunable, compact, and it imparts little to no spherical aberrations. The combination of these features make it more attractive than other types of plasma lenses for highly divergent beams. A case study is built on beam matching into a plasma wakefield accelerator at SLAC National Accelerator Laboratory’s FACET-II facility. Detailed simulations show that a thin plasma lens formed by laser ionization of a gas jet reduces the electron beam’s waist beta function to half of the minimum value achievable by the FACET-II final focus magnets alone.

We simulate the possibility of scaling channel formation to low densities plasmas of low atomic number gas over a large range of pulse duration including (1) pulses up to 300 ps in duration, using inverse bremsstrahlung (IB) heating and (2) ultrashort pulses up to 100s of femtoseconds for generating tenuous plasmas of centimeter to meter lengths by optical field ionization (OFI) [1]. Results show IB heating up to tens of eV, and channels formed from an initial density of 1×1018 cm−3 with axial densities as low as 1×1017 cm−3, and radius of 50 μm. It has been shown that centimeter-scale waveguides can be generated via OFI heating at densities of approximately 1×1017 cm−3[2]; we show calculations and theory of this channel formation using an axicon. Lastly, we outline the experimental setup to be used in future experiments at the University of Texas Tabletop Terawatt (UT3) facility.

The ion channel laser (ICL) was originally proposed as a compact, plasma-based alternative to the free electron laser (FEL) [1]. It is, in many ways, analogous to the FEL, though it offers some distinct advantages all on its own. Most notably, the ICL can accommodate a larger electron energy spread, making it better suited for high-brightness plasma-injected beams. In addition, the same radiator (plasma source) can be used to produce elliptically polarized light without alteration, a feature that is absent in an FEL. Historically, electron beam quality and plasma source development were insufficient for the demonstration of the ICL. In addition, the ICL appeared unfavorable due to the inherently short Rayleigh length of the radiation it produced. Recent literature, however, has shown that high gain can be achieved, despite the short Rayleigh length [2]. In addition, current and near-future facilities are able to provide appropriate beams and plasma sources for the ICL. Experimental opportunities to demonstrate an ICL at the Facility for Advanced Accelerator Experimental Tests II (FACET-II) are presented, utilizing both a 10 GeV beam originating from the SLAC National Accelerator Laboratory linac, and a 1 GeV high-brightness, plasma-injected beam.

Plasma wakefield accelerators (PWF A) have demonstrated GeV/m scale accelerating gradients; however, current PWFAs tend to increase the particle beam's emittance (volume of the beam in transverse phase-space) as the beam propagates through the plasma. The beam's emittance can be preserved by tailoring the longitudinal density profile of the plasma to properly focus the beam into the plasma. We present optical techniques to create controlled plasma densities via laser ionization. The longitudinal plasma density profile is determined by the evolution of the intensity of a high power laser pulse as it propagates through a gas. Control of the pulse intensity is achieved using a superposition of Bessel beams created by a tandem lens setup. The width of the plasma is limited by the refraction of the laser pulse off of the ionizing gas, but we have developed several optical techniques to create wider plasmas. Our simulations show that suitable plasma filaments with lengths up to a meter and widths up to a millimeter can be generated in an argon gas using a~ 1 0 TW peak-power Ti:sa laser pulse.

A new collisional framework with both CPU and GPU functionality has been implemented into the Vorpal1 code providing a significant performance improvement over the previous implementation. Vorpal is a versatile plasma simulation tool capable of PIC and fluid modeling. The collisions use the Monte Carlo method where reaction pairs are chosen from kinetic or fluid species at random (but spatially localized), and the pairs interact with a certain probability determined by the reaction cross-section. The collision framework is designed to allow agnostic particle species to interact (ie. species data can exist on the CPU or GPU and the reaction framework performs collisions equivalently). Using a standard thermal diffusion problem2, the framework has been successfully validated, reproducing analytic results for heat flux in the low, transition, and high collisionality regimes.

Beam-driven, underdense thin plasma lenses are a compact and tunable means of providing strong focusing to relativistic electron beams. It is necessary to focus electron beams to small sizes in a beam-driven plasma wakefield accelerator to preserve the beam emittance, or volume in phase space. The thin plasma lens can be used to achieve these small beam sizes that are otherwise out of reach of conventional electromagnetic beam delivery systems. Ultrashort (~ 30 fs), low energy (~ 10 mJ) laser pulses can be used to ionize a small region of gas to generate the thin plasma lens. The interplay between the ionized gas and the focusing of the laser pulse are modeled in detail to calculate experimentally achievable 3-D plasma lens profiles, which are then used in simulations of beam focusing. Plasma lens focal lengths and minimum beam sizes achievable are estimated, considering the effects of radiation reaction in both the classical and quantum regimes.

Electron beam emittance preservation in a beam-driven, nonlinear plasma wakefield accelerator relies upon matching the beam to the plasma. This occurs when the linear focusing forces of the ion column acting on the electron beam are perfectly balanced against the emittance-driven divergence of the beam. The matching condition is satisfied when the square of the beam size equals the product of the geometric emittance and the inverse betatron wave number of the beam inside the plasma. In practice, however, the matched beam size is too small to achieve with conventional beam delivery systems operating in vacuum. Here, a passive, tapered plasma density ramp at the entrance (exit) of the plasma source is described that can focus the electron beam to (from) the necessary matched size at a rate exceeding the adiabatic plasma focusing limit without diluting the beam's emittance. A numerically derived empirical model predicts both the plasma ramp shape and parameter tolerances required for beam matching as a function of the incoming beam parameters. These results can be easily scaled for the purpose of designing sequential plasma stages in a high-energy linear collider.

Plasma Vapor Deposition (PVD) is used in a variety of industrial applications for coating thin films over a substrate. In the PVD process within magnetron sputtering devices, electrons ionize a background gas and the ensuing ions, accelerated by a potential gradient, strike a target cathode and sputter off neutral atoms. The sputtered material travels ballistically, interacting with the plasma through collisions or depositing on a surface within the chamber.1

An integral part of plasma wakefield acceleration (PWFA) is producing, monitoring, and controlling a plasma source. Understanding the temporal evolution of plasma parameters allows for constraints on experimental lifetime and initial conditions. Recombination factors for low temperature (kBT less than the ionization energy), singly-ionized Argon and Hydrogen plasmas (nominal PWFA plasma sources) are investigated to develop a model of plasma decay. Due to the high density of the plasma sources collisional recombination and ionization dominates radiative processes, and the problem of modelling plasma decay simplifies to finding the recombination coefficients for these plasma sources. This recombination coefficient is presented as a function of plasma temperature to determine plasma decay for a variety of initial plasma conditions. Additionally, plasma temperature is modelled from the ponderomotive force of the ionizing laser pulse. Results are utilized for designing optimal PWFA experiments.

Speed-limited particle-in-cell simulation (SLPIC) is a method of increasing the time-step in a PIC simulation by slowing down the fastest particles and adjusting the weight of these particles appropriately such that the end state of the simulation is unaffected, while significantly reducing the number of time-steps required to reach this end state1. SLPIC is useful when the simulation requires a kinetic treatment of fast particles (e.g. electrons) while the physics of interest occurs on the time-scale of slow particles (e.g. heavy ions). Implementation of SLPIC requires a speed limiting factor $\beta(v)$ to reduce the computational velocities $v_{s}$ of fast particles from their true velocities $v_{i}$ by $v_{s}=\beta(v_{i})v_{i}$ . The particle positions are updated at each time-step using $v_{s}$ and then particle values are interpolated to the grid using the weights $w_{s} =\beta(v_{i})w_{j}$ , where $w_{i}$ is the conventional macroparticle weight. A “speed limit” is imposed, for example by $\beta(v_{i})=v_{i}(v_{i}^{2}+v_{0}^{2})^{-1/_{2}}$ , so that no particle exceeds vo, and thus with $\Delta x=\beta v\Delta t , no particle will travel a distance greater than the cell length $\Delta x$ in a single time-step.

Speed-limited particle-in-cell (SLPIC) modeling is a new simulation technique1 for efficient modeling of slow, low-velocity kinetic processes. Numerical constraints (e.g. time step limitations associated with a particle's cell-crossing time) often place challenging restrictions on PIC models of these systems, since even though the physics of interest is predominantly driven by slower particles, the fastest particles dictate the time step constraint.