This PDF file contains the front matter associated with the SPIE Proceedings Volume 11039, including the title page, copyright information, table of contents, and author and conference committee lists.

An in-depth analysis of two recent studies on nonlinear QED effects relevant to large-scale numerical simulations of laser-plasma interactions is presented. We demonstrate how accounting for a plasma as a background different from vacuum affects fundamental effects of nonlinear QED, quantify hitherto neglected coherences and propose an improved approach to better account for such effects in simulations. In particular, we show how the background plasma can be included in the calculation of nonlinear QED amplitudes on the example of the emission of a single high-energy photon by a laser-driven electron with the laser experiencing a non-trivial dispersion relation due to its propagation through a background plasma. Second, we discuss the failure of the so-called local-constant- field approximation, which is employed in all state-of-the-art numerical codes implementing QED effects both in nonlinear Compton scattering and in laser-assisted pair production. Finally, we show how in laser-assisted pair production by a high-energy photon emitted from a laser-driven electron the usually employed incoherent assumption, i.e., that the two QED processes of photon emission and pair production occur at separate points in space-time, can become invalid.

As an alternative to Compton backscattering and bremsstrahlung, the process of colliding highenergy electron beams with strong laser fields can more efficiently provide both cleaner and brighter source of photons in the multi-GeV range for fundamental studies in nuclear and quark-gluon physics. In order to favor the emission of high-energy quanta and minimize their decay into electron-positron pairs the fields must not only be sufficiently strong, but also well localized. We here examine these aspects and develop the concept of a laser-particle collider tailored for high-energy photon generation. We show that the use of multiple colliding laser pulses with 0.4 PW of total power is capable of converting more than 18 % of the initial multi-GeV electron beam energy into photons, each of which carries more than half of the electron energy

The local-constant-field approximation (LCFA) is an essential theoretical tool for investigating strong-field QED phenomena in background electromagnetic fields with complex spacetime structure. In [1] we have analyzed the shortcomings of the LCFA in nonlinear Compton scattering for the case of a background plane-wave field. Here, we generalize that analysis to background fields, which can feature a virtually arbitrary spacetime structure [2]. In addition, we provide an explicit and simple implementation of an improved expression of the nonlinear Compton scattering differential probability that solves the main shortcomings of the standard LCFA, and is suitable for background electromagnetic fields with arbitrary spacetime structure such as those of particle-in-cell (PIC) codes. Finally, we carry out a systematic procedure to calculate the probability of nonlinear Compton scattering per unit of emitted photon light-cone energy and of nonlinear Breit-Wheeler pair production per unit of produced positron light-cone energy beyond the LCFA in a plane-wave background field, which allows us to identify the limits of validity of this approximation quantitatively. [1] A. Di Piazza, M. Tamburini, S. Meuren, and C. H. Keitel, Phys. Rev. A vol. 98, 012134 (2018) [2] A. Di Piazza, M. Tamburini, S. Meuren, and C. H. Keitel, arXiv:1811.05834

Typical extremely intense laser-matter interactions include ionization, plasma production and generation of secondary particles. Modern laser systems are able to generate short and intense laser pulses ionizing matter in the poorly explored barrier-suppression regime. The classical and quantum models of barrier-suppression ionization are proposed and a simple formula of the ionization rate both for the tunnel and the barrier-suppression regimes is derived. After ionization free electrons move in extremely intense laser field in radiation-dominated regime when the radiation losses strongly affect electron dynamics. We show that the electron trajectories in this regime become close to some asymptotic trajectories where the radiation losses are minimal. The particle velocity of the asymptotic trajectory is completely determined by the local and instant EM field. At high laser intensity the laser-matter interaction can be accompanied by the avalanche-like production of electron-positron plasma via QED cascading. We demonstrate that QED cascade can develop even in a plane electromagnetic wave. The cascade front propagates as vacuum breakdown waves which is similar an avalanche breakdown at a gas discharge developing via ionization waves. The cascading makes the radiation pressure acceleration inefficient at extremely high intensities. QED cascades may also develop because of Weibel instability in two counterstreaming hot relativistic plasma flows. If the plasma flows are dense, fast, and hot enough, the overall energy of the synchrotron photons can be much higher than the energy of the generated electromagnetic fields. Furthermore, a sizable number of positrons can be produced due to the pair photoproduction in the generated magnetic field. We propose a rough criterion to judge copious pair production and considerable synchrotron losses. By means of this criterion, we conclude that the incoherent synchrotron emission and the pair production during the Weibel instability can have implications for the collapsar model of gamma-ray bursts.

This Conference Presentation, "Ultra relativistic quantum dynamics at the high-end of extreme field laser physics,” was recorded at SPIE Optics + Optoelectronics 2019 held in Prague, Czech Republic.

The laser-plasma interactions are dominated by the QED regime since intensities of the forthcoming laser facilities are approaching 10^{23-24} W/cm^2. Here we present the high brightness γ-photon emission and e^+e^- pair creation accompanied with the high harmonic generation. Relativistic oscillating mirror reflects the incident intense laser field and generates the focused attosecond pulse with enhanced intensity. A large number of high energy photons are emitted by the collisions between the radiation trapped electrons and the high harmonic pulses. The corresponding photons are counter-propagating through the strong laser field which provide a large cross section for pair creation. Relativistic positron bunches are generated and further accelerated in the reflected laser field.

A novel regime of high frequency radiation generation via reflection at the electron density spikes in under- dense plasma is proposed. Intense driver laser pulse propagating in underdense plasma forms dense electron singularities near the front part of the bow waves, moving at relativistic velocity. By irradiating a source pulse counterpropagating to the electron density singularities, it is reflected and compressed, producing ultrashort coherent high order harmonics with frequency upshift.

Using quantum electrodynamics particle-in-cell simulations, we optimize the gamma flare (γ-flare) generation scheme from interaction of high power petawatt-class laser pulse with tailored cryogenic hydrogen target having extended preplasma corona. We show that it is possible to generate an energetic flare of photons with energies in the GeV range and total flare energy being on a kilojoule level with an efficient conversion of the laser pulse energy to γ-photons. We discuss how the target engineering and laser pulse parameters influence the γ-flare generation efficiency. This type of experimental setup for laser-based γ source would be feasible for the upcoming high power laser facilities. Applications of high intensity γ ray beams are also discussed. The paper on this research project is submitted to Physics of Plasmas and available at arXiv:1809.09594

This Conference Presentation, "Extreme light: facilities, projects, directions,” was recorded at SPIE Optics + Optoelectronics 2019 held in Prague, Czech Republic.

We will be giving an overview on the development of the “ELI-beamline facility” being currently implemented and opened as a user facility within the Extreme Light Infrastructure (ELI) project based on the European ESFRI (European Strategy Forum on Research Infrastructures) process. ELI-Beamlines is the high-energy, repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. The main objective of the ELI-Beamlines facility is the delivery of ultra-intense high-energy pulses for high field experiments and the generation and applications of high-brightness X-ray sources and accelerated particles. The high power laser systems currently prepared and used for the generation of higher repetition rate sources of x-rays and particles are L1 (Allegra) a 1 kHz diode pumped laser produced sub-20fs OPCPA system and the L3 (HAPLS) a 10 Hz, 1 PW (30fs) laser using as the active medium Ti:sapphire with new gas cooled diode pumped Nd doped Glass pump laser. The lasers will be able to provide focused intensities attaining >1018-21 Wcm-2 suitable for generation of x-rays and particles (electrons and ions). We will discuss the infrastructure concerning the availability of experimental areas, including secondary sources of particles and x-rays in the wavelength range between 20 eV-100 keV and few Mev and their practical implementation at the ELI-Beamline user facility. The sources are either based on direct interaction of the laser beams with gaseous targets (high order harmonics) or will first accelerate electrons which then will interact with laser produced wigglers (Betatron radiation) or directly injected into undulators (laser driven LUX or later X-FEL). The direct interaction (collision) of laser accelerated electrons with the intense focused laser again will lead to short pulse high energy radiation via Compton or Thomson scattering for different applications opening also the route to fundamental physics investigations in high intensity interaction due to the 4 gamma 2 Lorentz boost of the intensity seen by high energy (GeV- > 106) electrons.

The Attosecond Light Pulse Source (ALPS) facility of the pan-European Extreme Light Infrastructure (ELI) project was designed as a laser-based research infrastructure in which light pulses of few optical cycles in the infrared or mid-infrared spectral range are used for basic and applied research. In particular, these pulses will be used as the driving source for generating even shorter extreme ultraviolet (XUV) pulses with durations as short as a few tens of attoseconds. All the six major laser systems available at ELI-ALPS were designed for stable and reliable operation, while featuring unique pulse parameters, such as unprecedented photon flux and extreme bandwidths. Each laser will run synchronized to the central facility clock, while femtosecond synchronization on target will be ensured by a dedicated timing system. Experimental beam time will be provided with uninterrupted operation of the primary driving lasers and associated secondary sources for at least eight hours per day. The primary focus of ELI-ALPS is the generation of the best quality attosecond XUV pulses in terms of pulse energy, repetition rate and photon energy. This goal is only achievable using the highest quality primary sources and expertly designed, innovative high-harmonic beamlines. The generation of high flux attosecond pulse trains and isolated attosecond pulses is targeted using Gas-based or Surface Plasma-based High Harmonic Generation. These secondary sources will feature dedicated target end stations (e.g. Reaction Microscope, Condensed matter end station, Velocity Map Imaging Spectrometer and Magnetic Bottle Electron Spectrometer) enabling users to perform state-of-the-art experiments. Experimental activities in the building complex started in 2018 with the installation of two 100 kHz repetition rate laser systems: the mid-infrared laser (MIR) and the first High Repetition Rate laser (HR1). They successfully served almost ten commissioning user experiments with external collaborators, for the investigation of phenomena such as electron migration in water, electron rescattering induced K-shell fluorescence, photoionization of droplets, photon statistics in harmonic generation in band gap materials etc., altogether for 51 operational weeks in 2018. In 2019 we expect to extend commissioning experiments to the SYLOS laser as well as to, at least, two attosecond and THz beamlines. The first attosecond beamline, driven by HR1 and dedicated to the investigation of ultrafast pheonemena in gas targets, is to be inaugurated mid 2019. In addition, the operation of the THz laboratory, as well as nanoplasmonic experiments are planned for 2019.

Nowadays by using the optical parametric chirped-pulse amplification (OPCPA) technique it is reasonable to expect the laser pulse energy up to 10 Joules with the repetition rate of 10-25 Hz. Development of such laser systems with the pulse compression down to 30 fs opens a way to build compact free-electron laser (FEL) based on the laser wakefield acceleration (LWFA). Combination of new laser development with constant improvement of the LWFA electron beam parameters has great potential in future development of the compact high repetition rate FEL, which is extremely demanded by the X-ray user community. The LWFA-driven FEL project called “LUIS" is currently under preparation at ELI-Beamlines in Czech Republic in collaboration with University of Hamburg. The LUIS project aims to experimentally demonstrate the stable and reliable generation of X-ray photons with a wavelength below 5 nm for user applications. An overview of the LUIS project including design features and a description of all the instrumentation used to characterize the laser, plasma, electron beam, photon generation and other subsystems will be presented in the frame of this report. The main challenges and future development of the laser-driven compact X-ray free electron laser with the radiation wavelength less than nm will also be discussed.

The radially- or azimuthally-polarized beam is an example of vector beams on a higher-order Poincare sphere. Recently, much attention has been paid to the vector beams and their basic focusing properties have been studied elsewhere [1]. Among the vector beams, because of the formation of a strong longitudinal electric field with a tight focusing geometry, the radially-polarized beam has been used for direct electron acceleration through the laser-matter interaction. Thus, it is interesting to investigate how strong longitudinal electric field can be formed by the femtosecond high-power laser pulse. In this paper, the field strength of a longitudinal field of a tightly-focused, radially-polarized femtosecond PW laser pulse has been investigated through numerical calculations based on vector diffraction theory [2]. Because a femtosecond laser pulse has a broad spectrum, the field strength for a given wavelength in the laser spectrum should be known for the accurate assessment of field strength of a tightly-focused femtosecond laser spot. In the research, the electric field strengths for all incident wavelength components are directly calculated from a laser spectrum and pulse energy, and used to assess the strength of a tightly-focused electromagnetic field in the focal region. The orbital angular momentum can be imposed to the light by introducing a spiral phase. A vector OAM beam is formed when the orbital angular momentum is imposed to a radially-polarized beam. Thus, the focusing property of the vector OAM beam is also discussed in the paper. The result will provide precise information on electric and magnetic fields which will be helpful in understanding electron motions under strong and unconventional electromagnetic field.

We report recent results on the study of pair production by nonlinear Breit-Wheeler process in the collision of a high energy photon beam with a counter-propagating laser beam. This configuration has been used in particular in the pioneering Burke experiment [Burke et al., 1997], and all the energy of the produced particles comes from the initial energy of the beam : hence the name showers. For a fixed laser energy amount we study the optimal situation, and show that the total production rate increases more quickly with the effective surface than the decrease of the production rate with the peak intensity, if we are above treshold for the quantum parameter (>>1). We consider both Gaussian fields and Laguerre-Gauss beams with parameters l = 1- 4, carring orbital angular momentum (OAM). 3D Particle-In-Cell (PIC) simulations are performed with the opensource PIC code SMILEI [Derouillat et al., CPC (2008)]. Our study show the counterintutive result that increasing peak intensity can result in decreasing the pair production, and suggest that in this configuration the extra complexity of introducing AOM beams is not justified if the focus is on pair creation.

This Conference Presentation, "Frontiers of applications of petawatt laser physics,” was recorded at SPIE Optics + Optoelectronics 2019 held in Prague, Czech Republic.

Since research in laser plasma interaction has moved from single-shot experiments to enabling real world applications, the need to pin down what has happened in an experiment requires to compare experiment & model data in a reproducible manner. This is important to first quantify the variability still seen in experiments while working towards predictive capabilities of models even at ultrahigh intensities. We present recent results of laser acceleration experiments of electrons & ions to explain how the need for more control of the acceleration process and better knowledge of the physical effects that play a role at ultrahigh intensities require a fusion of modelling and experiment work in new ways.

Capillary discharge is a convenient tool for creation of stable quiet plasma that could be used in experiments devoted to interaction of high intensity laser beams with plasma. This talk will be devoted to review of different applications of this capillary discharge plasma in such kind of experiments as well as to methods of simulation of such plasma. Due to specific features of its steady state, such plasma can form an optical wave guide for distant enough transportation of intense laser beams that is used in laser accelerators of multi-GeV electrons. Balance between Ohmic heating and thermal conduction cooling provides not so sharp refraction index profile in the wave guides. To diminish spot size of matched laser beam in comparison of capillary radius the plasma near the capillary axis could be heated additionally by nanosecond laser beam, transported in the wave guide in self consistently manner. This aim generates a rather complicated optic-plasma-dynamic problem. Approaches to solutions of this problem will be considered. Discharges in curved capillaries could be used in multi-stage laser accelerators of electrons for merging and separating of electron and laser beams. Theory of plasma steady state in such curved capillaries as well as its guiding properties will be briefly considered also. Capillary discharge due to existence of magnetic field formed by electric current through the discharge can be used as an active plasma lens. Such magnetic lenses can be used for compression and transportation of electron beams accelerated with laser electron accelerators. Main properties of such active plasma lenses will be considered in the talk also.

The construction of 10 PW class laser facilities with unprecedented intensities has emphasized the need for a thorough understanding of the radiation reaction process. We describe simulations for a recent all-optical colliding pulse experiment, where a GeV scale electron bunch produced by a laser wakefield accelerator interacted with a counter-propagating laser pulse. In the rest frame of the electron bunch, the electric field of the laser pulse is increased by several orders of magnitude, approaching the Schwinger field and leading to substantial variation from the classical Landau-Lifshitz model. Our simulations show how the final electron and photon spectra may allow us to differentiate between stochastic and semi-classical models of radiation reaction, even when there is significant shot-to-shot variation in the experimental parameters. In particular, constraints are placed on the maximum energy spread and shot-to-shot variation permissible if a stochastic model is to be proven with confidence.

The pre-plasma effects have been extensively studied experimentally and numerically and techniques for suppressing the pre-pulse are known widely. However, the increasing availability of the (multi-)PW-class laser systems enables to perform experiments with ultra-high laser intensities. The simulations of the pre-plasma formation and the effect on the main laser pulse must be reconsidered, since the systems are always limited in the contrast available and the created pre-plasma affects the interaction considerably. Our recent investigation of the topic revealed that the non-local transport of energy going beyond the paradigm of the diffusive approximation plays an important role in the process. An over-critical plateau is formed, where the main pulse is absorbed partially before reaching the solid target. Moreover, strong filamentation of the laser field occurs in the plasma. This effect is studied further by the means of the hydrodynamic simulations of the pre-plasma followed by the kinetic simulations of the interaction of the main pulse.

We analyze theoretically the properties of two counter-propagating electromagnetic waves within the framework of the long wavelength approximation corresponding to the Heisenberg–Euler formalism in quantum electrodynamics (QED). We obtain a novel solution for electromagnetic shocks in QED vacuum and investigate the properties of the solution, the wave steepening and subsequent generation of higher order harmonics and electromagnetic shock wave formation with electron–positron pair generation at the shock wave front.