Directed x-rays produced in the interaction of sub-picosecond laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated. Synchrotron-like (betatron) radiation occurs in the process of direct laser acceleration (DLA) of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields. In an experiment at the PHELIX laser system, high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity. The spectrum of directed x-rays in the range of 5–60 keV was evaluated using two sets of Ross filters placed at 0° and 10° to the laser pulse propagation axis. The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence. We report 1013 photons/sr with energies >5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s−1 mm−2 mrad−2 (0.1%BW)−1. The angular distribution of the emission has an FWHM of 14°–16°. Thanks to the ultra-high photon fluence, point-like radiation source, and ultra-short emission time, DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities.

We report the results of numerical particle-in-cell (PIC) simulation of the electron bunch generation by a laser pulse upon interaction with a rarefied gas target. It is shown that the mechanism of electron self-injection during the breaking of the wake wave of a laser pulse propagating along an ascending slope of the plasma target density makes it possible to produce subfemtosecond electron bunches with a charge of hundreds of picocoulombs and an electron energy of hundreds of megaelectronvolts. The considered mechanism of the formation of short bunches is the basis for the development of the injector.

Intense beams of photons and neutrons in the MeV energy range are effective tools in many areas of research, such as the diagnostics of matter in extreme states, nuclear physics, and materials science, as well as in medical and biophysical applications. A concept is presented for creating efficient sources of γ radiation and neutrons based on the generation of relativistic electrons in the mode of direct laser acceleration during interaction between a laser pulse with an intensity of 1019 W/cm2 and extended plasma with a density close to critical.

High performance of laser-driven sources of radiation is in focus of research aimed at the study of high energy density matter, pair production and neutron generation using kJ PW-laser systems. In this work, we present a highly efficient approach to generate an ultra-high flux, high-energy bremsstrahlung in the interaction of direct laser-accelerated (DLA) electrons with a several-millimeters-thick high-Z converter. A directed beam of direct laser-accelerated electrons with energies up to 100 MeV was produced in the interaction of a sub-ps laser pulse of moderate relativistic intensity with long-scale plasma of near-critical density obtained by irradiation of low-density polymer foam with an ns laser pulse. In the experiment, tantalum isotopes generated via photonuclear reactions with threshold energies above 40 MeV were observed. The Geant4 Monte Carlo code, with the measured electron energy and angular distribution as input parameters, was used to characterize the bremsstrahlung spectrum responsible for the registered yields of isotopes from 180 Ta to 175 Ta. It is shown that when the direct laser-accelerated electrons interact with a tantalum converter, the directed bremsstrahlung with an average photon energy of 18 MeV and ∼2⋅1011 photons per laser shot in the energy range of giant dipole resonance (GDR) and beyond (≥7.5 MeV) is produced. This results in an ultra-high photon flux of ∼6 × 10 22 sr −1 ·s −1 and a record conversion efficiency of 2% of the focused laser energy into high-energy bremsstrahlung.

A version is proposed of multistage electron acceleration based on a multichannel laser facility with a peak laser pulse power of up to 15 PW in one channel. In contrast to strongly nonlinear acceleration regimes with the presence of electron density cavitation on the radiation propagation axis, the proposed moderately nonlinear regime will make it possible to achieve not only sufficiently high energies (60 to 100 GeV at three to five accelerator stages), but also a high quality of accelerated electron bunches (relative energy spread of maximum 1

The Mu2e experiment at Fermilab will search for the neutrinoless μ−→e− conversion in the field of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I and Run II, separated by an approximately two-year-long shutdown. This paper presents an estimate of the expected Mu2e Run I search sensitivity and includes a detailed discussion of the background sources, uncertainties of their prediction, analysis procedures, and the optimization of the experimental sensitivity. The expected Run I 5σ discovery sensitivity is Rμe=1.2×10−15, with a total expected background of 0.11±0.03 events. In the absence of a signal, the expected upper limit is Rμe<6.2×10−16 at 90% CL. This represents a three order of magnitude improvement over the current experimental limit of Rμe<7×10−13 at 90% CL set by the SINDRUM II experiment.

We consider a possibility of generating high-current beams of ultrarelativistic electrons accelerated in the regime of direct laser acceleration and their application in interdisciplinary research. The key approaches are based on the results of simulations and performed experiments on the interaction of relativistically intense laser pulses with large-scale near-critical density plasma produced using low-density aerogels. The use of a set of XCELS laser pulses will make it possible to achieve the generation efficiency of particles (electrons, positrons, protons, and neutrons) and hard radiation quanta in the energy range of tens of megaelectronvolts, which is orders of magnitude higher than the existing record-high values.

Ultra-intense MeV photon and neutron beams are indispensable tools in many research fields such as nuclear, atomic and material science as well as in medical and biophysical applications. For applications in laboratory nuclear astrophysics, neutron fluxes in excess of 10 21 n/(cm 2 s) are required. Such ultra-high fluxes are unattainable with existing conventional reactor- and accelerator-based facilities. Currently discussed concepts for generating high-flux neutron beams are based on ultra-high power multi-petawatt lasers operating around 10 23 W/cm 2 intensities. Here, we present an efficient concept for generating γ and neutron beams based on enhanced production of direct laser-accelerated electrons in relativistic laser interactions with a long-scale near critical density plasma at 10 19 W/cm 2 intensity. Experimental insights in the laser-driven generation of ultra-intense, well-directed multi-MeV beams of photons more than 10 12 ph/sr and an ultra-high intense neutron source with greater than 6 × 10 10 neutrons per shot are presented. More than 1.4% laser-to-gamma conversion efficiency above 10 MeV and 0.05% laser-to-neutron conversion efficiency were recorded, already at moderate relativistic laser intensities and ps pulse duration. This approach promises a strong boost of the diagnostic potential of existing kJ PW laser systems used for Inertial Confinement Fusion (ICF) research.

Abstract The time-of-flight technique coupled with semiconductor detectors is a powerful instrument to provide real-time characterization of ions accelerated because of laser–matter interactions. Nevertheless, the presence of strong electromagnetic pulses (EMPs) generated during the interactions can severely hinder its employment. For this reason, the diagnostic system must be designed to have high EMP shielding. Here we present a new advanced prototype of detector, developed at ENEA-Centro Ricerche Frascati (Italy), with a large-area (15 mm × 15 mm) polycrystalline diamond sensor having 150 μm thickness. The tailored detector design and testing ensure high sensitivity and, thanks to the fast temporal response, high-energy resolution of the reconstructed ion spectrum. The detector was offline calibrated and then successfully tested during an experimental campaign carried out at the PHELIX laser facility ( ${E}_L\sim$ 100 J, ${\tau}_L = 750$ fs, ${I}_L\sim \left(1{-}2.5\right)\times {10}^{19}$ W/cm2) at GSI (Germany). The high rejection to EMP fields was demonstrated and suitable calibrated spectra of the accelerated protons were obtained.

Understanding the interaction of kilojoule, picosecond laser pulse with long-scale length preplasma or homogeneous near critical density (NCD) plasma is crucial for guiding experiments at national short-pulse laser facilities. Using full three-dimensional particle-in-cell simulations, we demonstrate that in this regime, cross-filament stochastic acceleration is an important mechanism that contributes to the production of superponderomotive, high-flux electron beams. Since the laser power significantly exceeds the threshold of the relativistic self-focusing, multiple filaments are generated and can propagate independently over a long distance. Electrons jump across the filaments during the acceleration, and their motion becomes stochastic. We find that the effective temperature of electrons increases with the total interaction time following a scaling like $T_{\rm eff}\propto\tau_{i}^{0.65}$. By irradiating a submillimeter thick NCD target, the space charge of electrons with energy above 2.5 MeV reaches tens of $\mu$C. Such high-flux electrons with superponderomotive energies significantly facilitate applications in high-energy-density science, nuclear science, secondary sources and diagnostic techniques.

Particle-in-Cell models are among the most demanding computational problems that require appropriate supercomputing hardware. In this paper we consider the solution of generic PIC problems on the Desmos supercomputer equipped with novel AMD MI50 GPUs and Angara interconnect. The open-source PIConGPU code is used. The acceleration limits and bottlenecks for this type of calculations are considered.

We consider the possibility of improving the superhigh-power laser pulse to superponderomotive electrons energy conversion efficiency by using porous targets of near-critical density. We report the results of numerical simulations based on the typical parameters of laser pulses of the PEARL laser facility built on the principles of parametric chirped pulse amplification (OPCPA). An original scheme for producing a controllable prepulse based on the use of a pump laser switched to a two-pulse regime is discussed. The prepulse is required to homogenise the submicron inhomogeneities of a porous target. Simulations show a significant increase in the laser-to-electron energy conversion efficiency in comparison with solid-state and gas targets. This interaction regime can be used to improve the efficiency of a broad class of laser-driven secondary radiation sources, such as a betatron source, bremsstrahlung, neutron source, etc.

The process of multistage acceleration of polarised electrons in the wakefield of a relativistic femtosecond laser pulse is investigated. The main mechanism of the growth of slice emittance for a moderately nonlinear regime of laser-plasma acceleration is demonstrated. The main conditions for setting the initial parameters of the electron bunch are determined, which allow minimising the mixing of the phases of betatron oscillations of particles in the bunch slice and the emittance growth during acceleration. The method of smooth input of an electron bunch into the accelerating stage and its smooth removal from this stage is studied, which allows the main characteristics of the bunch to be preserved, such as emittance and polarisation, for further particle transportation and acceleration. The effect of the radiation friction force and the radiation polarisation mechanism on the process of particle depolarisation in acceleration to energies of ∼4 TeV in model fields characteristic of moderately nonlinear and strongly nonlinear regimes of laser-plasma acceleration is considered.

Large-amplitude electromagnetic radiofrequency fields are created by the charge-separation induced in interactions of high-intensity, short-pulse lasers with solid targets and have intensity that decreases with the distance from the target. Alternatively, it was experimentally proved very recently that charged particles emitted by petawatt laser–target interactions can be deposited on a capacitor-collector structure, far away from the target, and lead to the rapid (nanosecond-scale) generation of large quasi-static electric fields ($\mathrm{MV}/\mathrm{m}$), over wide regions. We demonstrate here the generation of both these fields in experiments at the PHELIX laser facility, with approximately $20\;\mathrm{J}$ energy and approximately ${10}^{19}\;\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ intensity, for picoseconds laser pulses, interacting with pre-ionized polymer foams of near critical density. Quasi-static fields, up to tens of kV/m, were here observed at distances larger than $1\;\mathrm{m}$ from the target, with results much higher than the radiofrequency component. This is of primary importance for inertial-confinement fusion and laser–plasma acceleration and also for promising applications in different scenarios.

Direct laser acceleration (DLA) of electrons in a plasma of near-critical electron density (NCD) and the associated synchrotron-like radiation are discussed for moderate relativistic laser intensity (normalized laser amplitude a0 ≤ 4.3) and ps length pulse. This regime is typical of kJ PW-class laser facilities designed for high-energy-density (HED) research. In experiments at the PHELIX facility, it has been demonstrated that interaction of a 1019 W/cm2 sub-ps laser pulse with a sub-mm length NCD plasma results in the generation of high-current well-directed super-ponderomotive electrons with an effective temperature ten times higher than the ponderomotive potential [Rosmej et al., Plasma Phys. Controlled Fusion 62, 115024 (2020)]. Three-dimensional particle-in-cell simulations provide good agreement with the measured electron energy distribution and are used in the current work to study synchrotron radiation from the DLA-accelerated electrons. The resulting x-ray spectrum with a critical energy of 5 keV reveals an ultrahigh photon number of 7 × 1011 in the 1–30 keV photon energy range at the focused laser energy of 20 J. Numerical simulations of betatron x-ray phase contrast imaging based on the DLA process for the parameters of a PHELIX laser are presented. The results are of interest for applications in HED experiments, which require a ps x-ray pulse and a high photon flux.

We consider the interaction of a high-intensity short laser pulse with an argon gas during optical field ionisation. Modelling in a three-dimensional cylindrical symmetric geometry is performed at various moderately relativistic intensities and positions of the focal plane of the laser beam in the case of both optical gas ionisation and a pre-ionised plasma. The influence of the processes occurring during optical field ionisation of the gas on the generation of wake waves is investigated, and the conditions are found under which intense wake fields are produced in a plasma formed from an inhomogeneous argon gas jet. The possibility of using a gas with a large number of electrons on the outer shell (argon) to excite intense wake waves and accelerate electrons is demonstrated. Despite significant ionisation refraction of the laser pulse on a radially inhomogeneous density profile of plasma electrons formed during optical field ionisation, the region of the parameters of the laser pulse and the gas target is determined at which ionisation refraction leads to the excitation of an intense wake wave. It is found that in a pre-ionised plasma a wake wave is not generated even at the same laser radiation intensity.

We report a theoretical analysis and numerical simulation of the dynamics of transverse emittance of an electron bunch during its acceleration in wake fields generated by a laser pulse in a weakly nonlinear mode. Analytical expressions are obtained for the main factors affecting the emittance growth during acceleration and the case is considered when the characteristic transverse size of the injected bunch exceeds the matched radius determined by the focusing force at the injection point, the initial emittance and the electron bunch energy, and the resulting value of emittance is much larger than the initial one. The dynamics of the emittance growth during acceleration as a function of the length of the electron bunch is described, and the length of the bunch is found at which there occurs a complete phase mixing of betatron oscillations of electrons and the emittance increases to its maximum value determined by the bunch parameters and the focusing force at the injection point. Analytical expressions are in good agreement with the results of numerical simulation.

M. M. Günther,1, ∗ O. N. Rosmej,1, 2, 3 P. Tavana,2 M. Gyrdymov,2 A. Skobliakov,4 A. Kantsyrev,4 S. Zähter,1, 2 N. G. Borisenko,5 A. Pukhov,6 and N. E. Andreev7, 8 GSI-Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64291 Darmstadt, Germany Goethe-Universität Frankfurt am Main, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany Helmholtz Forschungsakademie Hessen für FAIR (HFHF), Campus Frankfurt am Main, Germany Institute for Theoretical and Experimental Physics named by A.I. Alikhanov of NRC «Kurchatov Institute», B. Cheremuschkinskaya 25, 117218 Moscow, Russia P. N. Lebedev Physical Institute, RAS, Leninsky Prospekt 53, 119991 Moscow, Russia Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, Gebäude 25.32 Etage 01, 40225 Düsseldorf, Germany Joint Institute for High Temperatures, RAS, Izhorskaya st. 13, Bldg. 2, 125412 Moscow, Russia Moscow Institute of Physics and Technology (State University), Institutskiy Pereulok 9, 141700 Dolgoprudny Moscow region, Russia

We report on new findings in a laser driven enhanced electron beam generation in the multi MeV energy range at moderate relativistic laser intensities and their applications. In our experiment, an intense sub-picosecond laser pulse propagates through a plasma of a near critical electron density (NCD) and direct laser acceleration (DLA) of electrons takes place. The breakthrough toward high current relativistic electron beams became possible due to application of low density polymer foams of sub-mm thickness. In foams, the NCD-plasma was produced by a mechanism of super-sonic ionization. Compared to NCD-plasmas generated by laser irradiation of conventional foils, the DLA acceleration path in foams was strongly enhanced. Measurements resulted into 11÷13 MeV of the effective electron temperature and up to 100 MeV maximum of the electron energy measured in the laser pulse propagation direction. The growth of the electron energy was accompanied by a strong increase of the number of super-ponderomotive electrons and a well-defined directionality of the electron beam that propagates in a divergence cone with a half angle of 12°. For the energy range above 7.5 MeV that is relevant for gamma-driven nuclear reactions, we estimate a charge carried by these well-directed electron beams as high as 50 nC and a corresponding efficiency of the laser energy conversion into electrons of 6%. The electron spectra generated by the DLA-mechanism in NCD-plasma at 1019 Wcm-2 laser intensity were compared with those measured in shots onto conventional metallic foils at ultra-relativistic laser intensities of 1021 Wcm-2 . In the last case, the twice lower effective electron temperature and the twice lower maximum of the electron energy were registered. The substantial difference in the electron spectra for these two cases presented itself in the isotope production yield.