QED Laser Plasmas

The talk starts with an introduction into strong-field QED employing the fundamental scenario of a collision of a high-energy electron beam with an ultra-strong laser pulse. For this purpose, radiative reaction, spin dynamics and pair production are discussed. Emphasis on recent progress involving more complex quantum systems will be given on the generation of high-energy electron, positron and gamma-ray beams, plasma diagnostics, and tests of fundamental physics. Also, high-quality proton beam acceleration and vacuum birefringence in a plasma wakefield are discussed along with an outlook on further attractive feasible experiments in the light of upcoming facilities.

The Sauter-Schwinger process of electron-positron pair creation in the presence of an electric field is one of the oldest theoretical predictions of quantum electrodynamics. Here,we shall present a novel approach, based on the Feynman- and anti-Feynman solutions of the Dirac equation [1], to treat the process in arbitrary time-dependent pulsed electric fields. In contrast to other treatments such as the quantum field theoretical approach [2] or the quantum Vlasov equation approach [3], it allows to study, for instance, polarization and spin effects in pair creation. These effects will be illustrated numerically with the main focus on vortex structures; thus, showing great versatility of the proposed method. [1] I. Białynicki-Birula and Z. Białynicka-Birula, Quantum Electrodynamics (Pergamon, Oxford, 1975). [2] A. A. Grib, S. G. Mamaev, and V. M. Mostepanenko, Vacuum Quantum Effects in Strong External Fields (Atomizdat, Moscow, 1988). [3] S. Schmidt, D. Blaschke, G. Röpke, S. A. Smolyansky, A. V. Prozorkevich, and V. D. Toneev, Int. J. Mod. Phys. E 07, 709 (1998). J. Z. Kamiński, F. Cajiao Velez, A. Bechler, and K. Krajewska Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warszawa, Poland

Nonlinear Breit-Wheeler is the process through which a high-energy photon in a strong electromagnetic field decays into an electron-positron pair. It was first demonstrated in the mid-1990's at SLAC where multi-GeV electron beams were collided with high-intensity (a0 ~ 0.36) laser pulses, leading to the first laboratory evidence for inelastic light-by-light scattering involving only real photons [1]. The advent of multi-petawatt laser facilities offers the scientific community with new opportunities for investigating this process in a potentially cleaner environment (no electrons involved). In particular, experiments are envisioned [2,3,4] where high-energy gamma photons are directly collided with a multi-petawatt light pulse. Realizing such an experiment would on the one hand lead to the first matter-antimatter creation from pure light in the lab, it will also require important theoretical, numerical and experimental developments. In this talk, I will present the work that has been done toward realizing such an experiment on the Apollon laser facility. A simple model [3] for nonlinear Breit-Wheeler pair production in the head-on collision of a high-energy gamma photon flash with an ultra-intense light pulse will be discussed. Supported by state-of-the-art Particle-In-Cell (PIC) simulations with SMILEI [5], the model allows us to identify the key parameters and optimal conditions for the experiment. Two possible gamma photon sources will be discussed, one relying on Bremsstrahlung emission by a LWFA electron beam, the other on nonlinear Inverse Compton scattering in the direct irradiation of a near-critical foam plasma. The latest developments being carried out in SMILEI to allow for modeling both processes will be discussed. Last, the current state of the Apollon facility will be presented. [1] Burke et al., Phys. Rev. Lett. 79, 1626 (1997) [2] Blackburn et al., Plasma Phys. Control. Fusion 60, 054009 (2018) [3] Mercuri-Baron et al., New J. Phys. 23, 085006 (2021) [4] Salgado et al., New J. Phys. 23, 105002 (2021) [5] Derouillat et al., Comp. Phys. Comm. 222, 351 (2018)

The simulation of relativistic plasma with a substantial domination of strong field quantum electrodynamic effects like electron-positron pair creation is a challenging task for well established and legacy plasma codes. In this talk I will concentrate on two problems namely a selection of optimal numerical methods capable to handle extreme conditions and development of new codes intended to run on modern high performance clusters. The decision to develop of a new code is not the easy one also because it may require to invest into a descent hardware infrastructure to support such undertaking. Although some baremetal HPC clusters may offer an interactive mode, a key for successful development, this however is not a general case especially for very computationally demanding QED plasma codes. To remedy this problem a virtual MPI cluster may deployed on the cloud to mimic the hardware architecture and network topology of production HPC system. The talk I will describe the steps how to setup a virtual multi-node HPC playground on the Openstack platform, an open source operational system for the cloud. From numerical side the focus will be given to HPC friendly and energy efficient methods which aim to achieve a higher order accuracy of solution for a cheaper computational price. Also I will shortly summarise a traditional approach to simulation of intense field QED effects within local constant crossed field approximation and give a brief outlook to the more sophisticated Wigner transport equations adopted to simulate the Schwinger effect of electron-positron pairs creation.

Current experimental efforts toward the detection of the nonperturbative nonlinear strong-field regime of Breit-Wheeler pair creation plan to combine incoherent sources of GeV $\gamma$ quanta and tightly focussed optical laser pulses. This endeavour calls for a theoretical understanding of how the pair yields depend on the applied laser field profile [1]. In the present discussion, we provide estimates for the number of pairs produced in a setup where the high-energy radiation is generated via bremsstrahlung. Particular attention is paid to the role of the transversal and longitudinal focussing of the laser field, along with the incorporation of a (super-)Gaussian pulse envelope. The results are compared with predictions from plane-wave models and determine the parameters of focused laser pulses which maximize the pair yield at fixed pulse energy. [1] A. Golub, S. Villalba-Chavez and C. Mueller, Nonlinear Breit-Wheeler pair production in collisions of bremsstrahlung $\gamma$ quanta and a tightly focused laser pulse, (to appear in Phys. Rev. D; arXiv:2203.14776)

Modern multi-PW laser technology will allow fundamental strong-field quantum electrodynamics processes to be extensively studied in the lab over the coming years. One of the key effects to be explored is the nonlinear Breit-Wheeler (BW) process: the conversion of high-energy photons into electron-positron pairs through the interaction with a high-power laser. A major challenge to observing the BW effect is first producing large numbers of high-energy gamma photons. In this work we outline a proposal to source these photons through a simple, highly efficient, all-optical setup by irradiating thin solid targets with a high-power laser. We consider the collision of these photons with a secondary laser, and systematically discuss the prospects for exploring the BW process at current and next-generation high-power laser facilities.

Motivated by corresponding experimental prospects (such as LUXE at DESY) we study strong-field Breit-Wheeler pair production by bremsstrahlung gamma-rays propagating through a high-intensity optical laser wave. The influence of the broad frequency spectrum of the bremsstrahlung on the properties of the process in the transition region between the perturbative and the non-perturbative interaction regimes is analysed by calculating the total yield of pair production and the energy and angular distributions of the produced particles for a range of laser intensities [1]. A continuous shift of the major contributing interval of the bremsstrahlung frequencies during the transition is shown and characteristic signatures of the laser-dressed particle mass are highlighted. [1] A. Eckey, A. B. Voitkiv and C. Müller, Phys. Rev. A 105, 013105 (2022)

In quantum electrodynamics, the Locally-constant-field-approximation (LCFA) is considered a cornerstone for quantifying light-light interactions to determine, e.g., the rate at which high-intensity light transforms into matter via the Sauter-Schwinger effect, i.e., a non-perturbative phenomenon. In the perturbative sector, another foundation is the Breit-Wheeler process. We propose a new, improved scheme that outcompetes the LCFA in the context of the overall pair production yield, in particular, in the regime where light interacts with light of the order of 100 keV energy -- all for only marginally higher numerical cost. To test our theory we discuss non-perturbative pair production in colliding, linearly polarized laser pulses on the basis of analytical approaches and large-scale simulations. In this regard, we show how the Dirac-Heisenberg-Wigner approach (quantum kinetic theory) allows us to numerically observe the transition from the Sauter-Schwinger regime at arbitrarily small laser frequencies to the Breit-Wheeler regime at large photon energies and highlight the qualitative agreement of our approach.

Schwinger pair creation in a purely time-dependent electric field can be reduced to an effective quantum mechanical problem using a variety of formalisms. Here we develop an approach based on the Gelfand-Dikii equation for scalar QED, and extent it to spinor QED. We discuss some solvable special cases from this point of view. It was previously shown how to use the well-known solitonic solutions of the KdV equation to construct "solitonic" electric fields that do not create scalar pairs with an arbitrary fixed momentum. We show that this construction can be adapted to the fermionic case in two inequivalent ways, both leading to the vanishing of the pair-creation rate at certain values of the Pöschl-Teller like index p of the associated Schrödinger equation. Thus for any given momentum we can construct electric fields that create scalar particles but not spinor particles, and also the other way round. Therefore, while often spin is even neglected in Schwinger pair creation, in such cases it becomes decisive

The direct electron-positron pair creation via quantum-vacuum fluctuations is one of the most intriguing physics processes that remain untested under laboratory conditions. By applying a strong electric field above the Schwinger critical limit of 1.3$\times10^{18}$ V/m, the virtual pairs from the quantum fluctuations can be turned into observables. However, despite the recent advances in the peak power of high-intensity lasers, the critical limit is still far beyond achievable. To overcome such a challenge, strong fields can be reached in the rest frame of the created pairs after the interaction of a $\gamma$-ray photons with a highly intense laser beam. The experiment of the research group FOR2783 “Quantum-Vacuum” [1] has the goal to produce and diagnose the pair-creation process via the nonlinear Breit-Wheeler process for the first time with an all-optical setup at the ATLAS laser at CALA. In the experiment, a 2.5 GeV monoenergetic electron beam with a charge of 10 pC produces a highly intense bremsstrahlung $\gamma$-ray that collides with a laser beam of intensity 9.5$\times10^{21}$ W/cm² (a$_0$= 66) to trigger the pair creation process. Simulation results show that a pair creation rate of 80 pairs per hour is achievable for a laser repetition rate. To detect the electron-positron pairs created, a single-particle detection system composed of pixelated LYSO:Ce crystal tracking layers and a Cherenkov calorimeter was designed. GEANT4 Monte-Carlo simulations of the complete experiment are presented. [1] http://www.quantumvacuum.org/

Testing the response of charged particles and the quantum vacuum to extreme fields is predicted to give rise to fundamental and unexplored natural phenomena. Among these are finding a self-consistent solution to the trajectory of a single electron (so-called ‘radiation reaction’), pair production from the quantum vacuum and photon-photon scattering in violation of the superposition principle of classical theory. Experiments are currently in the planning and validation phase with experiments planned at both accelerator/laser combinations and all-optical experiments which aim to probe different parameter spaces. The challenge in all cases is to detect a weak signal against a strong background. Recent developments will be presented.

The 10PW laser facility (SULF) is now in the commissioning phase and an 100PW laser station (SEL) is being built on the hard x-ray free electron laser facility (SHINE) in Shanghai. These facilities not only provide light intensities in the range of 1021-23 W/cm2 but also the opportunity to integrate the most powerful light sources in the optical and x-ray regimes. In this presentation, I will introduce the status of both facilities, including some primary results on laser-proton acceleration in SULF and recent progress & science cases of SEL, which mainly focuses on strong-field QED phenomena and high energy density physics. In additional, our recent theoretical and simulation work will also be introduced, on the spin related dynamics in extreme laser fields, the quantum vortex effect in QED scattering processes and so on.

Multi-petawatt laser experiments generate extreme conditions where quantum electrodynamics effects are crucial. Few weeks before the first experiments using a 10 PW laser at ELI-NP, we will detail our experimental strategies for solid and gas targets experiments at this unprecedented power levels and our preparatory results obtained at 1PW level.

The Helmholtz International Beamline for Extreme Fields is a user consortium providing drivers for high-energy density and high-field science at the HED station of EuXFEL. This presentation will give an overview of the current implementation and commissioning results as well as future plans and exemplary science cases. In parallel, new exciting opportunities for high-field science are opening with the first user call at the ELI facilities. Here, we will discuss first results from flagship experiments during the commissioning phase at 1 PW level at ELI-NP on the generation of bright gamma ray flashes. We will also discuss the further plans for the upcoming 10 PW commissioning experiments.

Collective plasma effects and Schwinger pair creation Gert Brodin, Haidar Al-Naseri, Jens Zamanian, Greger Torgrimsson and Bengt Eliasson Strong field physics close to or above the Schwinger limit are typically studied with vacuum as initial condition [1,2], or by considering test particle dynamics. However, with a plasma present initially, quantum relativistic mechanisms such as Schwinger pair-creation are complemented by classical plasma nonlinearities. In this work we use the Dirac-Heisenberg-Wigner formalism [3] to study the interplay between classical and quantum mechanical mechanisms for ultra-strong electric fields. In particular, the effects of initial density and temperature on the plasma oscillation dynamics are determined. In contrast to previous works [4], the physical value of the fine structure constant has been used. 1.F. Hebenstreit, R. Alkofer, and H. Gies, Phys. Rev. Lett. Vol. 107, 180403 (2011). 2. I. A. Aleksandrov and C. Kohlfurst, Phys. Rev. D Vol. 101, 096009 (2020). 3. I. Bialynicki-Birula, P. Gornicki, and J. Rafelski, Phys. Rev. D Vol. 44, 1825 (1991). 4. J. C. R. Bloch, V. A. Mizerny, A. V. Prozorkevich, C. D. Roberts, S. M. Schmidt, S. A. Smolyansky, and D. V. Vinnik Phys. Rev. D Vol. 60, 116011 (1999).

The impact of radiation reaction and Breit-Wheeler pair production on acceleration of fully ionized Carbon ions driven by an intense linearly-polarised laser pulse has been investigated in the ultra-relativistic Breakout-Afterburner (BOA) transparency regime [1]. Against initial expectations radiation reaction at ultra-high laser intensities is found to enhance the energy gained by the ions. The electrons lose most of their transverse momentum and the additionally produced pair plasma of Breit-Wheeler electrons and positrons co-stream in the forward direction. This leads to changes in the phase velocity of the Buneman instability, that is known to aid ion acceleration in the BOA regime, by tapping the free energy in the relative electron and ion streams [2]. We show that this can further improve the highest Carbon ion energies [3]. References [1] L. Yin, B, J. Albright, B. M. Hegelich, K. J. Bowers, K. A. Flippo, T. J. Kwan, and J. C. Fern ́andez, Physics of Plasmas, Vol. 14 (2007) [2] B. J. Albright, L. Yin, K. J. Bowers, B. M. Hegelich, K. A. Flippo, T. J. Kwan, and J. C. Fern ́andez, Physics of Plasmas 14 (2007), [3] S. Bhadoria, M. Marklund and C. H. Keitel. "Energy enhancement of laser-driven ions by RR and BW pair production in the ultra-relativistic BOA", [to be submitted in 2022].

At intermediate initial kinetic energies (in the keV regime), a major bottle-neck of fusion reaction is the Coulomb barrier between the nuclei, which can only be overcome by quantum tunneling. This effect, however, is exponentially suppressed. Hence, one major goal of our research is to seek for a mechanism that alleviates particle tunneling, in turn, enhancing the overall tunneling probability and thus the possibility for a fusion process to happen. Solving the time-dependent Schrödinger equation (TDSE) through various numerical methods we identify high-intensity, high-energy electromagnetic fields as a most promising candidate. For example, attosecond laser pulses created by an x-ray free electron laser (XFEL).

With the ongoing progress of high-power laser developement, high-intensity QED effects are playing an increasingly important role in high-intensity laser plasma interactions. In this talk I will elaborate why electron-spin and photon-polarization will be a relevant subject in high-intensity laser-plasma and laser-beam interactions in the near future due to the fundamental QED processes in strong fields being sensitive to the particle polarization. Some of the predicted effects on particle polarization share similarities with the Sokolov-Ternov effect---the radiative polarization of electrons in storage rings---but they occur on much faster time-scales due to the large quantum efficiency parameter. I will discuss how those effects might be capitalized for generating spin-polarized beams for future plasma-based high-energy-physics colliders, which crucially depend on having spin-polarized beams available. Moreover, we have studied the relevance of spin and polarization for the formation of a avalanche-like QED cascade where copious amounts of spin-polarized matter and polarized photons can be produced. We found that the particle polarization affects the cascade growth rate. Moreover, the highest energy particles are polarized opposite the expectation from Sokolov-Ternov theory, which cannot be explained by just taking into account spin-asymmetries in the pair production rates, but results significantly from the phenomenon of ``spin-straggling''.

High-energy, spin-polarized particle beams are of relevance for several future experiments, ranging from probing the structure of the proton to polarized fusion. Over the past few years, the option of obtaining such particle beams from laser-plasma interaction has gained a lot of interest. One feasible option is Magnetic Vortex Acceleration, where a short, high-intensity laser pulse irradiates a near-critical density plasma target. If, additionally, a thin, solid foil is mounted in front of the gas, an abundance of ions can be accelerated. Our particle-in-cell simulations show that high polarization of the beam can be maintained due to the foil shielding parts of the laser fields. For high laser intensities $> 10^{22} \mathrm{W}/\mathrm{cm}^2$ at the next generation of laser facilities, radiation reaction cannot be ignored. We discuss the importance of this effect for the resulting beam polarization.

We discuss two QED vacuum processes where temporal effects and spacetime symmetries are relevant. The first one concerns the creation of electron-positron pairs from vacuum and is based on the "Temporal Klein Model". This allows us to study the transition from a singular electric field spike, with infinitesimal duration, to the opposite case of a static field where the Schwinger formula would apply. The second process corresponds to coherent photon emission from vacuum and can be called "Vacuum Superradiance". These two processes, are based on the use of intense laser pulses with infinitesimal duration. Other strategies to improve the nonlinear QED vacuum response to intense laser fields are also discussed.

We investigate the radiation of several particles interacting with a strong electromagnetic plane wave and establish the conditions providing the coherent radiation. In particular we determine the spatial configuration under which 2 particles with similar initial velocities emit coherently. This effect can be important in the context of the laser interaction with a dense cold plasma.

The development of an ultrahigh-intensity laser gives an opportunity to investigate the physics associated with quantum electrodynamics (QED) phenomena under a strong electromagnetic field. The Compton scattering between an ultra-relativistic electron beam and an ultraintense laser pulse can reveal a new interaction regime of QED, known as strong-field QED. The experiment at SLAC reported the first signature of nonlinear Compton scattering involving a few laser photons, in the collision between a 46-GeV electron beam generated by a linear accelerator and a laser pulse with an intensity, represented by the classical nonlinearity parameter, $a_0 \leq1$. Since then the advances in laser plasma accelerators, including laser wakefield acceleration (LWFA), all-optical strong field QED experiment have been pursued by a number of groups including CoReLS. We have performed experiments for nonlinear Compton scattering in a strong laser field, in which a single, multi-GeV electron scatters off hundreds of laser photons simultaneously. We generated a multi-GeV electron beam via LWFA and then made it collide with another laser pulse with $a_0=14$ at 30^{\circ}, reaching quantum nonlinearity parameter $\chi= 0.46$, producing gamma-rays from multiphoton (involving several hundreds of laser photons) Compton scattering with the energy extended up to 700 MeV; this demonstrated the nonlinear Compton scattering in the strongly nonlinear regime. With higher laser intensity and electron energy, more complex strong field processes, such as nonlinear Breit-Wheeler pair production, will be explored.

Understanding and interpretation of the dynamics of ultrarelativistic plasma is a challenge, which calls for the development of methods for in situ probing the plasma dynamical characteristics. We put forward a new method, harnessing polarization properties of gamma-photons emitted from a non-pre-polarized plasma irradiated by a circularly polarized pulse. We show that the angular pattern of gamma-photon linear polarization is explicitly correlated with the dynamics of the radiating electrons, which provides information on the laser-plasma interaction regime. Furthermore, with the gamma-photon circular polarization originating from the electron radiative spin-flips, the plasma susceptibility to quantum electrodynamical processes is gauged. Our study demonstrates that the polarization signal of emitted gamma-photons can be a versatile information source, which would be beneficial for the research fields of laser-driven plasma, accelerator science, and laboratory astrophysics.

It is widely accepted that the next lepton collider beyond a Higgs factory would require center-of-mass energy of the order of up to 15 TeV. Since, given reasonable space and cost restrictions, conventional accelerator technology reaches its limits near this energy, high-gradient advanced acceleration concepts are attractive. Advanced and novel accelerators (ANAs) are leading candidates due to their ability to produce acceleration gradients on the order of 1–100 GV/m, leading to compact acceleration structures. However, intermediate facilities are required to test the technology and demonstrate key subsystems. A 20-100 GeV center-of-mass energy ANA-based lepton collider can be a possible candidate for an intermediate facility. Apart from being a test beam facility for accelerator and detector studies, this collider will provide opportunities to study muon and proton beam acceleration, make precision Quantum Chromodynamics and Beyond the Standard Model physics measurements, and investigate charged particle interactions with extreme electromagnetic fields in the quantum regime. There is a number of questions that need to be addressed in the course of designing such a facility. One of the most important is the positron beam generation, capture, and acceleration.

The next generation of pulsed lasers will have intensities in excess of $10^{23}~\mathrm{W/cm^2}$. While propagating through a pre-formed plasma channel, a laser of such intensity allows for direct laser acceleration (DLA) of leptons in the radiation reaction-dominated regime. The DLA scheme has already been demonstrated to provide high-charge electron beams (at a $\sim \mathrm{nC}$ level) with moderate laser intensities ($\sim 10^{20}~\mathrm{W/cm^2}$). In this work, we show what can be accomplished with near-future laser facilities. We have found that increasing the laser power is bound to augment the charge content even further. The field structure formed due to electron beam loading allows for accelerating positrons. What is more, the interaction in the radiation-dominated regime will provide a high flux of emitted photons, in the hard x-ray and gamma-ray range. These photons can then be used as a seed for electron-positron pair creation, as well as a radiation source for applications. \\ This work was supported by FCT grants CEECIND/01906/2018, PTDC/FIS-PLA/3800/2021, FCT UI/BD/151560/2021 and ERC-2015-AdG Grant 695088. We acknowledge PRACE for granting access to MareNostrum in BSC, Spain.

While the high frequency of modern x-ray free-electron lasers has the benefit of requiring less energy of a seed electron for triggering the development of a QED cascade, the non-linearity parameter obeys a_0 < 1, in contrast to high-intensity optical lasers. Accordingly, we analyse the phenomenon of multi-photon effects in trident pair-production in pulsed x-ray laser fields at such values of a_0. The impact of the energy spectrum and its temporal structure and the coherence of the laser field on the emergent particle distribution at the onset of further cascading is discussed. Besides the evolution of mean multiplicities in the course of an energy-powered cascade, we seek for characteristic fluctuation patterns.

Extremely intense electromagnetic fields modify the Maxwell equations due to the photon-photon scattering that makes the vacuum refraction index to depend on the field amplitude. The refraction index modification makes the ultra-relativistic electron to emit high energy photons via the Synergic Cherenkov-Compton radiation mechanism. With high power lasers the Synergic Cherenkov-Compton process can be observed by colliding the laser accelerated electrons with high intensity electromagnetic pulse. In the presence of electromagnetic waves with small but finite wavenumbers the vacuum behaves as a dispersive medium. The finite amplitude electromagnetic wave evolution in QED vacuum results in wave steepening, subsequent generation of high order harmonics and shock wave formation with electron-positron pair generation at the shock front. At extremely high photon energy end the vacuum refraction index tends to unity quenching the Cherenkov radiation. The interplay between the vacuum dispersion and the nonlinear effects in the interaction of electromagnetic waves results in the formation of solitons that can propagate without changing their shape.

The E-320 collaboration at SLAC plans to collide the FACET-II high-energy electron beam ($10-13\,\mathrm{GeV}$) with intense laser pulses ($a_0 \sim 1-10) to access the non-perturbative regime of strong-field QED. In this regime the vacuum becomes unstable to pair production and novel phenomena are expected to occur. E-320 aims to perform precision measurements of the fundamental strong-field processes pair production and photon emission from the perturbative to the non-perturbative regime. This talk will summarise the goals of the experimental program, describe the experimental setup, and provide an update on its ongoing commissioning and first beam times.

Laser radiation of extreme intensities in conjunction with high-energy electron beams permit the study of radiation reaction (RR) in the quantum regime as well as effects of strong-field quantum electrodynamics (SFQED). These are quantified by the electron acceleration within its rest frame normalized to the Schwinger critical field, denoted $\chi$, where $1 \lesssim \chi \lesssim 1600$ delimit the quantum domain of RR and SFQED phenomena where the upper limit is the conjectured breakdown of perturbative non-linear QED [1]. Probing theories from measurements in the high-$\chi$ limit can be intractable since electrons can emit multiple times during interaction while high-$\chi$ emissions from electrons may be indistinguishable to those at low-$\chi$. Moreover, rapid cascade of electron-positron pairs inhibit the peak field being reached [2]. We present simulations of an experimental scheme using an optimal laser geometry that attain high values of $\chi$ and provide strategies to isolate the signal of electron emission at large $\chi$ [3]. [1] Fedotov, “Conjecture of perturbative QED breakdown at $\alpha \chi^{2/3} \gtrsim 1$,” Journal of Physics: Conference Series, vol. 826, p. 012027, Apr. 2017. [Online]. Available: https://doi.org/10.1088/1742-6596/826/1/012027 [2] S. Bulanov, V. D. Mur, N. B. Narozhny, J. Nees, and V. S. Popov, “Multiple colliding electromagnetic pulses: A way to lower the threshold of $e^+ e^-$ pair production from vacuum,” Physical Review Letters, vol. 104, no. 22, Jun. 2010. [Online]. Available: https://doi.org/10.1103/physrevlett.104.220404 [3] C. Olofsson and A. Gonoskov, “Bi-dipole wave: optimum for attaining extreme regimes at matter-light collid- ers,” 2 2022.

When charges interact with a laser pulse in the regime of strong fields, two main phenomena are important, hard photon emission by nonlinear Compton scattering, and electron-positron pair creation by nonlinear Breit-Wheeler. In favorable conditions of intensity and field configurations, those two processes can in principle couple to each other to reach a regime called a cascade or an avalanche, an exponential increase in the number of electron-positron pairs. Although a recent work suggests that a cascade could be initiated with one laser pulse [1], the required intensity would be far beyond what is expected in the near future experimental facilities. More promising configurations involve two counter-propagating laser pulses, as discussed in numerical and theoretical studies [2, 3, 4, 5]. However these works only consider Gaussian laser pulses. Here we present the result of 3D Particle In Cell simulations using the code SMILEI [6] in which we explore the optimal configurations to produce a cascade using Laguerre-Gauss (LG) pulses. We discuss the effect of polarisation and LG beam order in the field configuration and then on particles dynamics. Finally we identify the physical quantities which are relevant to trigger the cascade development, so as to be able to predict if a given field configuration is favorable or not, and in case improve its efficiency to maximize the number of produced pairs. References : [1] A. A. Mironov, E. G. Gelfer, and A. M. Fedotov, Onset of electron-seeded cascades in generic electromagnetic fields, Phys. Rev. A 104, 012221 [2] Grismayer T, Vranic M, Martins J L, Fonseca R A and Silva L O 2017, Seeded QED cascades in counterpropagating laser pulses, Phys. Rev. E 95 023210 [3] Elkina N V, Fedotov A M, Kostyukov I Y, Legkov MV, Narozhny N B, Nerush E N and Ruhl H 2011, QED cascades induced by circularly polarized laser fields, Phys. Rev. Spec. Top. Accel. Beams 14 054401 [4] TamburiniM, Di Piazza A and Keitel C H 2017, Laser-pulse-shape control of seeded QED cascades, Sci. Rep. 7 5694 [5] Jirka M, Klimo O, VranicM, Weber S and Korn G 2017, QED cascade with 10 PW-class lasers, Sci. Rep. 7 15302 [6] Derouillat J et al 2018, Smilei: a collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation, Comput. Phys. Commun. 222 351–73, https://smileipic.github.io/Smilei

All-optical experiments offer a promising route to unprecedented precision tests of quantum electrodynamics in strong macroscopic electromagnetic fields. So far, most theoretical studies of all-optical signatures of quantum vacuum nonlinearity are based on simplifying approximations of the beam profiles and pulse shapes of the driving laser fields. However, precise experimental tests require quantitatively accurate theoretical predictions. Our approach is based on the vacuum emission picture, and makes use of the fact that the dynamics of the driving laser fields are to an excellent approximation governed by classical Maxwell theory in vacuum. In combination with a Maxwell solver, which self-consistently propagates any given laser field configuration, this allows for accurate theoretical predictions of photonic signatures of vacuum nonlinearity in high-intensity laser experiments from first principles.

At extremely high strength of the external field, which exceeds ~1600 Sauter-Schwinger fields in the reference frame of a particle, the standard strong-field QED perturbative approach based on the Furry picture is known to be problematic. As a result, any scattering problem requires a summation of loop radiative corrections to all orders. According to the Ritus-Narozhny conjecture, the leading order contribution (for a given process) can be obtained by the summation over all the Feynman diagrams with photon lines replaced by the bubble-chains. They are given by combining the 1-loop photon polarization operators with the bare photon lines. However, there is a caveat: in effect, such dressed `photons' obtain a dynamical mass depending on the quantum parameter $\chi$. Unless $\chi$ is small, they are violently unstable. This makes formulation of the photon emission probability quite intricate. In this talk, we review this issue in detail and the possible ways to resolve it. Specifically, we discuss application of the optical theorem to SFQED with unstable states.

High-power high-intensity laser facilities have been designed to push the frontier of high field science, so finding regimes involving these lasers that are relevant to high field science and quantum electrodynamics research is paramount. The processes of matter/anti-matter production from light alone stand out because they have yet to be observed in the laboratory. While most attention has been focused on the multi-photon process, the process that involves two gamma-rays, the linear Breit-Wheeler process, has been overlooked due to a misconception that it is more difficult to observe. To objectively assess the linear BW process, we have developed a first-ever fully kinetic code for predictive simulations of the electron-positron pair production via this process in high-intensity laser-matter interactions and the subsequent positron acceleration. Using this new tool, we have discovered several regimes where one or two laser pulses propagating through a dense plasma form an effective self-organized collider of gamma-rays and an adjoining accelerator for the generated positrons. In contrast to the regimes proposed for the multi-photon process, our regime only requires a peak intensity that is already accessible at most flagship laser facilities to produce more than $10^7$ electron-positron pairs. Positrons are emitted as energetic beams with a narrow divergence angle, which is likely to facilitate their detection.

Flying Focus (FF) technique is a novel method of spatiotemporal laser pulse shaping that enables arbitrary motion of the laser focus, including configurations in which the focus moves at the speed of light against laser phase oscillations. Focus propagation in this regime for the duration of tens of picoseconds was already experimentally demonstrated in 2018, spurring the theoretical development of this topic in recent years. Such FF pulses give us an opportunity to co-propagate ultra-relativistic electron beam with the laser focus so that the electrons stay in the region of highest EM field intensity for prolonged interaction times. In this talk, generation and analytical description of FF pulses with arbitrary focus velocity will be introduced. Then we will discuss high-intensity field applications in which the long laser-particle interaction time is beneficial. For example, FF pulses can be used to enhance radiative properties of the nonlinear Thomson scattering, amplify the effects of radiation reaction on the particle motion, and guide the electron beams without spreading over macroscopic distances.

High-intensity lasers in combination with conventional or laser-based electron accelerators provide a remarkable opportunity to observe quantum regimes of radiation reaction and other effects due to strong-field quantum electrodynamics. These regimes are characterized by large values of $\chi$ parameter, which compares electron’s acceleration in its rest frame to that the Schwinger critical field would provide. The experimental objectives may range from verification of existing theoretical predication for $\chi \sim 1$ to the detection of signs of the physics beyond the conjectured breakdown of perturbative nonlinear QED at $\chi \approx 1600$. Nevertheless, retrieving fundamental knowledge from measurements can be challenging because electrons can emit more than once during the interaction, while the signal from emissions at high $\chi$ can be indistinguishable from the signal originated from the overwhelming majority of emissions at low $\chi$. Another known difficulty is that rapid cascaded generation of dense electron-positron plasma can prevent strong electromagnetic fields from being attained. We analyze the problem of reaching high $\chi$ values and the ways to isolate the signal of emissions at high $\chi$ by the electrons that had not lost any significant energy before that. The optimal geometry for attaining high $\chi$ values and results of simulations for possible experimental layouts are to be presented.

We investigate the finite time behavior of pair production from the vacuum by time-dependent Sauter pulsed electric fields in spinor quantum electrodynamics (QED). Using the exact analytic solution of the mode function, we find the one-particle distribution function in momentum space. The longitudinal momentum spectrum of particles shows oscillatory behavior at a finite time in a small window of longitudinal momentum where the electric field diminishes to around one-hundredth of its maximum magnitude and its oscillation time is close to Compton time. This oscilla- tion is asymmetric, i.e., the amplitude of oscillation is maximum for negative longitudinal momentum in comparison to positive momentum. The oscillation in the longitudinal momentum spectrum can occur due to the quantum interference effect, and this quantum interference effect comes from the result of dynamical tunneling. The transverse momentum spectrum shows the Gaussian structure with a peak at zero transverse momentum when t = 0. After t ≈ τ/2, the smooth Gaussian structure becomes distorted, and we see inconstancy in spectrum structure either a dip at the origin with an off-axis maximum or a peak at zero transverse momentum with small peaks up to t ≈ 2τ observed. After that, the spectrum shows a maximum peak at zero transverse momentum with weakly pronounced peaks. This substructure in transverse momentum spectrum at finite time occurs due to the coupling of transverse and longitudinal momentum through the accumulated phase. Deepak (1, 2) and Manoranjan P. Singh (1, 2) (1) Theory and Simulations Lab, Raja Ramanna Centre for Advanced Technology, Indore-452013, India (2) Homi Bhaba National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India

Strong-field QED processes are central in determining the dynamics of particles and plasmas in extreme electromagnetic fields such as those generated with ultraintense lasers. SFQEDtoolkit is an open source library designed to allow for a straightforward implementation of SFQED processes in existing particle-in-cell (PIC) and Monte Carlo codes. Through advanced function approximation techniques, high-energy photon emission and electron-positron pair creation probability rates and energy distributions are calculated within the locally-constant-field approximation as well as with more advanced models [Phys. Rev. A \textbf{99}, 022125 (2019)]. SFQEDtoolkit is designed to provide users with high-performance and high-accuracy, and neat examples showing its usage are provided. In the near future, SFQEDtoolkit will be enriched to model the angular distribution of the generated particles, i.e., beyond the commonly employed collinear emission approximation, as well as to model spin and polarization dependent strong-field QED processes.