The pepper-pot method is a widely used technique, originally proposed for measuring the emittance of space-charge-dominated electron beams from radio-frequency photoinjectors. With recent advances in producing high-brightness electron beams via laser-wakefield acceleration (LWFA), the method has also been applied to evaluate emittance in this new regime. Here, the limitations of this method for measuring the emittance of LWFA electron beams are investigated, particularly in parameter regimes where the true beam emittance is overestimated. Conducting an experiment at the JETi200 laser system, we measured an upper bound for the geometric beam emittance of (26.2 + 7.3) mu mmrad using the pepper-pot method. This result is consistent with Geant4 Monte Carlo simulations of the pepper-pot diagnostic, with an input beam-emittance parameter that matches both PIC simulations of the laser-plasma accelerator and an independent measurement using the transient optical grating method.

Effects of radiation reaction are considered a key factor for lasers-plasma-interactions at ultra-high intensities, and gaining precise understanding of these phenomena will be crucical for discoveries of novel strong-field QED effects. In this paper we investigate an experimental geometry that allows a direct single-shot measurement of radiation reaction energy losses in high-intensity laser-beam collisions. This is made possible by simultaneous measurements of the pre-collision and post-collision electron beam spectra in a single shot by employing a dedicated 90-degree scattering geometry between the electron beam and the colliding high-power laser. We discuss the principal requirements and design constraints, e.g. for the electron beam divergence and source size. We derive an analytic expression for the correlation between the final particle energy and the scattering angle. Numerical simulations are performed employing both classical and stochastic quantum radiation reaction models to predict the experimental outcome and demonstrate the feasibility of the scenario.

The interaction of charged particles with an intense laser pulse gives rise to a number of characteristic spectral features of emitted radiation, including the generation of harmonics, spectral broadening due to the phase-dependent ponderomotive red shift, and the emergence of intricate sub-harmonic structures. These effects are accumulated over the course of the interaction with the electromagnetic field and are therefore inherently nonlocal in nature. For a deeper understanding of strong-field quantum electrodynamics (SFQED) processes and their practical applications, it is desirable to employ tools that enable simultaneous analysis in the time and energy domains. In time-frequency analysis, such tools are provided by joint distributions (JDs). In this work, we demonstrate how a JD can be devised within the SFQED framework. Specifically, we focus on constructing a non-negative JD, which allows for a clear probabilistic interpretation. We study the properties of the proposed distribution and test its utility by applying it to the nonlinear Compton scattering in complex laser pulse configurations with carrier-envelope phase and variable polarization.

Coherent synchrotron radiation (CSR) is vital for developing powerful ultrashort light sources. We introduce a CSR generation mechanism using surface plasmon polaritons (SPPs) resonantly excited on a solid, near-critical-density microtube. A high-intensity, circularly polarised laser pulse, propagating along the microtube axis, efficiently couples the cylindrical SPP modes. This process creates azimuthally structured, rotating electromagnetic fields. These rotating fields subsequently confine, modulate, and directly accelerate surface electrons, causing them to emit CSR in the Valilov-Cherenkov angle. We further demonstrate that by improving the azimuthal symmetry, the helical modulation enables CSR emission across all azimuthal directions, significantly enhancing radiation intensity even when full coherence is imperfect. The harmonics can be well isolated for a high charge beam. Our full 3D Particle-in-Cell simulations indicate this scheme can generate X-rays with coherence enhanced by up to two orders of magnitude compared to incoherent emission.

Coherent synchrotron radiation (CSR) is crucial for the development of powerful ultrashort light sources. We present a mechanism for generating CSR in the form of generalised superradiance, based on surface plasmon polaritons (SPPs), which are resonantly excited on a solid, near-critical-density inner surface of a microtube. A high-intensity, circularly polarized laser pulse, propagating along the microtube axis, efficiently couples the cylindrical SPP modes. This process creates azimuthally structured, rotating electromagnetic fields. These rotating fields subsequently confine, modulate, and directly accelerate surface electrons to emit CSR in the Vavilov-Cherenkov angle. We further demonstrate that by improving the azimuthal symmetry of these electrons, the helical modulation enables CSR emission across all azimuthal directions in the form of isolated harmonics, significantly enhancing radiation intensity even when full coherence is imperfect. Our full 3D particle-in-cell simulations indicate this scheme can generate x-rays with coherence enhanced by up to 2 orders of magnitude compared to incoherent emission. The challenges to experimentally realize this scheme are discussed, including the need for high-contrast lasers to prevent preplasma formation and the demanding tolerances for microtube fabrication and alignment, while these challenges are not beyond the scope of existing or near-future experimental capabilities.

Observing multiphoton electron-positron pair production (the nonlinear Breit-Wheeler process) requires high-energy gamma rays to interact with strong electromagnetic fields. In order for these observations to be as precise as possible, the gamma rays would ideally be both mono-energetic and highly polarized. Here we perform Monte Carlo simulations of an experimental configuration that accomplishes this in two stages. First, a multi-GeV electron beam interacts with a moderately intense laser pulse to produce a bright, highly polarized beam of gamma rays by inverse Compton scattering. Second, after removing the primary electrons, these gamma rays collide with another, more intense, laser pulse in order to produce pairs. We show that it is possible to measure the gamma-ray polarization dependence of the nonlinear Breit-Wheeler process in near-term experiments, using a 100 TW class laser and currently available electron beams. Furthermore, it would also be possible to observe harmonic structure and the perturbative-to-nonperturbative transition if such a laser were colocated with a future linear collider.

Quantum reflection is a fascinating signature of the quantum vacuum that emerges from inhomogeneities in the electromagnetic fields. In pursuit of the prospective real-world implementation of quantum reflection in the back-reflection channel, we provide the first numerical estimates for the light-by-light scattering with dipole pulses, which are known to provide the tightest focusing of light possible. For an all-optical setup with a dipole pump and Gaussian probe of the same frequency, we find that the dominant signal signature is related mainly to the back-reflection channel from 4-wave mixing. Focusing on this, we study the particular case of a multiple focusing pulses configuration (belt configuration) as an approximation to the idealized dipole pulse. Using Bayesian optimization methods, we determine optimal parameters that maximize the detectability of a discernible back-reflection signal. Our study indicates that the optimization favors a three-beam collision setup, which we further investigate both numerically and analytically.

Strong-field QED (SFQED) probability rates in the locally monochromatic approximation (LMA) have become an indispensable tool for simulations of processes like gamma-ray emission or electron-positron pair production in laser-particle collisions. We revisit the LMA derivation and explicitly demonstrate that it is based on the separation of timescales, neglect of the long-range interference effects, and subsequent averaging over the cycle scale. Doing so, we obtain unambiguously LMA rates for arbitrary polarizations of the plane-wave background. Additionally, we partially restore the finite bandwidth effects that are lost in the LMA derivation. We refer to the bandwidth-restored result as the LMA+ and show that it agrees with the full SFQED predictions better than the standard LMA. We use LMA+ to address previously inaccessible observables and formulate an additional limitation on the applicability of locally monochromatic approximations in general. We provide analytical results for the angular-integrated LMA+ probability rate and the fully differential probability that account for the finite bandwidth effects.

Quantum field theory predicts a nonlinear response of the vacuum to strong electromagnetic fields of macroscopic extent. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. A particularly distinct signature of the resulting optical activity of the quantum vacuum is vacuum birefringence. This offers an excellent opportunity for a precision test of nonlinear quantum electrodynamics in an uncharted parameter regime. Recently, the operation of the high-intensity laser ReLaX provided by the Helmholtz International Beamline for Extreme Fields (HIBEF) has been inaugurated at the High Energy Density (HED) scientific instrument of the European XFEL. We make the case that this worldwide unique combination of an x-ray free-electron laser and an ultra-intense near-infrared laser together with recent advances in high-precision x-ray polarimetry, refinements of prospective discovery scenarios, and progress in their accurate theoretical modelling have set the stage for performing an actual discovery experiment of quantum vacuum nonlinearity.

We present experimental results which show a laser polarization-dependent contribution to electron beam pointing jitter in laser wakefield accelerators (LWFAs). We develop a theoretical model for the polarization dependence in terms of the transverse dynamics of trapped electrons, resonantly driven by bubble centroid oscillations. The latter are generated by the carrier wave phase evolution at the self-steepened laser pulse front. In the model, the polarization-dependent jitter originates from shot-to-shot fluctuations of the laser carrier envelope phase (CEP). The model is verified by particle-in-cell simulations and suggests that CEP stabilization of the driving lasers might be necessary to achieve ultimate electron pointing stability in LWFAs.

The identification of prospective scenarios for observing quantum vacuum signals in high-intensity laser experiments requires both accurate theoretical predictions and the exploration of high-dimensional parameter spaces. Numerical simulations address the first requirement, while optimization provides an efficient solution for the second one. In the present work, we demonstrate the potential of Bayesian optimization in maximizing photonic quantum vacuum signals on the example of two-beam collisions. This allows us to find the optimal waist sizes for beams with elliptic cross sections, and to identify the specific physical process leading to a discernible signal in a coherent harmonic focusing configuration scenario.

In this paper, we have studied the influence of laser polarization on the dynamics of the ionization-injected electron beams, and subsequently, the properties of the emitted betatron radiation in laser wakefield accelerators (LWFAs). While ionizing by strong field laser radiation, the generated photo-electrons carry a residual transverse momentum in excess of the ionization potential via the above threshold ionization (ATI) process. This ATI momentum explicitly depends on the polarization state of the ionizing laser and eventually governs the dynamics of the electron beam trapped inside the wake potential. In order to systematically investigate the effect of the laser polarization, here, we have employed complete three-dimensional particle-in-cell simulations in the nonlinear bubble regime of the LWFAs. We focus, in particular, on the effects the laser polarization has on the ionization injection mechanism, and how these features affect the final beam properties, such as beam charge, energy, energy spread, and transverse emittance. We have also found that as the laser polarization gradually changes from linear to circular, the helicity of the electron trajectory, and hence the angular momentum carried by the beam, increases significantly. Studies have been further extended to reveal the effect of laser polarization on the radiation emitted by the accelerated electrons. The far-field radiation spectra have been calculated for the linear and circular polarization states of the laser. It has been shown that the spatial distributions and the polarization properties (Stokes parameters) of the emitted radiation in the above two cases are substantially different. Therefore, our study provides a facile and efficient alternative to regulate the properties of the accelerated electron beams and x-ray radiation in LWFAs, utilizing ionization injection mechanism.

The collision of a high-energy electron beam with a laser pulse may be used to study radiation reaction and nonlinear Compton scattering among many other processes in strong-field quantum electrodynamics. Predictions from simulation and theory for these interactions rely on a number of approximations and assumptions that have not been experimentally tested. Here, experimentally measurable signatures are identified that might be able to distinguish between radiation reaction models, i.e., classical or quantum, or between the local constant field and local monochromatic approximations used to calculate the properties of the nonlinear Compton process. These signatures are considered through Monte Carlo simulations of various experimental conditions that are relevant to today's laser facilities. Potential detection schemes for measuring the signatures are proposed. We find that single-photon counting of keV photons to resolve harmonics and scintillator-based detection of MeV photons may allow us to validate nonlinear Compton scattering models and radiation reaction models respectively. This will require electron beams with divergence angles less than 2 mrad and less than 20% energy spread.

Novel schemes for generating ultra-low emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Recent theoretical work has proposed a laser-based method capable of resolving emittances in the sub 0.1 mm mrad regime, by modulating the electron phase-space ponderomotively. Here we present a first experimental demonstration of this scheme using a laser wakefield accelerator. The observed emittance and source size is consistent with published values. We also show calculations demonstrating that tight bounds on the upper limit for emittance and source size can be derived from the 'laser-grating' method even in the presence of low signal to noise and uncertainty in laser-grating parameters.

Shot-to-shot electron beam pointing instability in the plasma bubble, defined here as electron beam pointing jitter (EBJ), is a long-standing problem that limits the potential of the laser wakefield accelerator (LWFA) in a range of demanding applications. In general, EBJ is caused by variations in laser and plasma parameters from shot to shot, although the exact physical mechanism by which EBJ grows in the plasma wave remains unclear. In this work we theoretically investigate the fundamental physics of EBJ inside the plasma bubble and show how the intrinsic betatron oscillation can act as an amplifier to enhance EBJ growth. The analytical formulas for electron trajectory, pointing angle, and EBJ are derived from the basic momentum equation of an electron and verified numerically. It is shown that the shot-to-shot fluctuations of the laser and plasma parameters, such as laser strength, focus, and carrier-envelope phase, as well as the ambient plasma density and profile, lead to EBJ. The evolution of EBJ is dictated by the dynamics of the plasma bubble. Two amplification processes of the betatron oscillation are found in the rapidly evolving bubbles and play important roles in EBJ growth. The first is driven by a linear resonance in the wobbling bubble due to the coupling of the betatron oscillation and the bubble centroid oscillation. The second is a parametric resonance seen in the breathing bubble, where EBJ grows exponentially due to the strong frequency modulation of the betatron oscillation. Their characteristic functions, growth rates, and resonance conditions are deduced analytically and validated numerically. Finally, we also studied how radiation reaction affects EBJ. Our research provides a clear understanding of the basics of EBJ dynamics in LWFA and will help improve the use of LWFA in demanding applications.

In this article we investigate novel signatures of radiation reaction via the angular deflection of an electron beam colliding at 90 degrees with an intense laser pulse. Due to the radiation reaction effect, the electrons can be deflected towards the beam axis for plane wave backgrounds, which is not possible in the absence of radiation reaction effects. The magnitude and size of the deflection angle can be controlled by tailoring the laser pulse shapes. The effect is first derived analytically using the Landau-Lifshitz equation, which allows to determine the important scaling behavior with laser intensity and particle energy. We then move on to full scale 3D Monte Carlo simulations to verify the effect is observable with present day laser technology. We investigate the opportunities for an indirect observation of laser depletion in such side scattering scenarios.

We report on an experimental observation of the streaking of betatron x rays in a curved laser wakefield accelerator. The streaking of the betatron x rays was realized by launching a laser pulse into a plasma with a transverse density gradient. By controlling the plasma density and the density gradient, we realized the steering of the laser driver, electron beam, and betatron x rays simultaneously. Moreover, we observed an energy-angle correlation of the streaked betatron x rays and utilized it in diagnosing the electron acceleration process in a single-shot mode. Our work could also find applications in advanced control of laser beam and particle propagation. More importantly, the angular streaked betatron x ray has an intrinsic spatiotemporal correlation, which makes it a promising tool for single-shot pump-probe applications.

Simulated electron and photon angular spectra produced by non-linear Comp-ton scattering using different models (LCFA, LMA, classical) are characterized. Signatures to differentiate between models are found and assessed for their ability to be measured experimentally.

The investigation of spin and polarization effects in ultra-high intensity laser-plasma and laser-beam interactions has become an emergent topic in high-field science recently. In this paper we derive a relativistic kinetic description of spin-polarized plasmas, where QED effects are taken into account via Boltzmann-type collision operators under the local constant field approximation. The emergence of anomalous precession is derived from one-loop self-energy contributions in a strong background field. We are interested, in particular, in the interplay between radiation reaction effects and the spin polarization of the radiating particles. For this we derive equations for spin-polarized quantum radiation reaction from moments of the spin-polarized kinetic equations. By comparing with the classical theory, we identify and discuss the spin-dependent radiation reaction terms, and radiative contributions to spin dynamics.

With the rapid development of high-power petawatt class lasers worldwide, exploring physics in the strong field QED regime will become one of the frontiers for laser–plasma interactions research. Particle-in-cell codes, including quantum emission processes, are powerful tools for predicting and analyzing future experiments where the physics of relativistic plasma is strongly affected by strong field QED processes. The spin/polarization dependence of these quantum processes has been of recent interest. In this article, we perform a parametric study of the interaction of two laser pulses with an ultrarelativistic electron beam. The first pulse is optimized to generate high-energy photons by nonlinear Compton scattering and efficiently decelerate electron beam through the quantum radiation reaction. The second pulse is optimized to generate electron–positron pairs by the nonlinear Breit–Wheeler decay of photons with the maximum polarization dependence. This may be experimentally realized as a verification of the strong field QED framework, including the spin/polarization rates.