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We report on the quantitative nondestructive analysis of the natural isotopic abundances of hafnium and tungsten elements using nuclear resonance fluorescence. Metallic samples of hafnium and tungsten were irradiated to six quasi-monochromatic γ -ray beams generated by laser Compton scattering in the energy range of 2.4–3.2 MeV. Multiple nuclei were simultaneously excited at each of the six γ -ray beam energies. A high-purity germanium detector array detected deexcitations of the nuclei. In total, 51 transitions were unprecedentedly employed to estimate the isotopic abundances of heavy elements nondestructively. The estimated abundances of three hafnium isotopes and three tungsten isotopes are consistent with standard known natural abundances within the experimental uncertainties. The deviation from the standard values ranges from 0.18 % to 1.36 %. [ABSTRACT FROM AUTHOR]

Accurate modeling is necessary to support precision experiments investigating strong-field QED phenomena. This modeling is particularly challenging in the transition between the perturbative and nonperturbative regimes, where the normalized laser amplitude a0 is comparable to unity and wavelength-scale interference is significant. Here, we describe how to simulate nonlinear Compton scattering, Breit–Wheeler pair creation, and trident pair creation in this regime, using the Monte Carlo particle-tracking code Ptarmigan. This code simulates collisions between high-intensity lasers and beams of electrons or γ rays, primarily in the framework of the locally monochromatic approximation. We benchmark our simulation results against full QED calculations for pulsed plane waves and show that they are accurate at the level of a few per cent, across the full range of particle energies and laser intensities. This work extends our previous results to linearly polarized lasers and arbitrary polarized γ rays. [ABSTRACT FROM AUTHOR]

In all-optical Compton scattering driven by a multi-petawatt laser, it is critical to have accurate spatiotemporal synchronization between the ultrarelativistic electron bunch and the ultrahigh-intensity laser beam. Such a synchronization was realized by using two complementary optical setups. The first setup, used for the initial synchronization, recorded the spatial interferogram between the two femtosecond lasers used for a GeV electron beam production and an ultrahigh scattering laser beam. The second one, consisting of spatial and spectral interferometers, measured the time delay between the two laser beams in the range of 0–200 fs in real time. These monitoring systems played an essential role in conducting Compton scattering experiments. [ABSTRACT FROM AUTHOR]

New laser facilities will reach intensities of 10 23 W cm - 2 . This advance enables novel experimental setups in the study of laser–plasma interaction. In these setups with extreme fields, quantum electrodynamic (QED) effects such as photon emission via nonlinear Compton scattering and Breit–Wheeler pair production become important. We study high-intensity lasers grazing the surface of a solid-state target by two-dimensional particle-in-cell simulations with QED effects included. The two laser beams collide at the target surface at a grazing angle. Due to the fields near the target surface, electrons are extracted and accelerated. Finally, the extracted electrons collide with the counter-propagating laser, which triggers many QED effects and leads to a QED cascade under a sufficient laser intensity. Here, the processes are studied for various laser intensities and angle of incidence and finally compared with a seeded vacuum cascade. Our results show that the proposed target can yield many orders of magnitude more secondary particles and develop a QED cascade at lower laser intensities than the seeded vacuum alone. [ABSTRACT FROM AUTHOR]

We propose to use a 10 petawatt (PW) laser irradiating onto a target with a concave surface, which can focus the laser beam and attain a more intense laser field, so as to increase both the yield and mean energy of emitted γ-rays. 2D particle-in-cell simulation results show that the peak electric field after the reflection of the laser from the target in this new scheme can reach ∼ 1.8 times as high as that in the plane target case. Such an enhanced laser field enables an increase in the number of hot electrons in front of the incident plane and raises the probability rate of generating high-energy γ-rays significantly by non-linear Compton scattering. As a result, 1.5 × 10 13 γ photons with a mean energy of 9.2 MeV are obtained. These photons mainly point toward two directions and distribute in pulses with the duration of each pulse of ∼ 400 as. A brightness as high as 3.6 × 10 25 photons / (mm 2 mrad 2 s 0.1 % BW) is obtained in one pulse. Compared to no positron observed for a plane target, a positron yield of 3.0 × 10 8 can be achieved in this new scheme. [ABSTRACT FROM AUTHOR]

The scrutiny of the resonant laser-assisted bremsstrahlung (LAB) of ultrarelativistic electrons within the laser plasma ambience in the field of a nucleus is presented. The kinematics of the explored process can develop within two probable channels. Thus, for the first channel an electron radiates a spontaneous photon and subsequently scatters on a nucleus. In contrast, for the second channel an electron scatters on a nucleus and consequently a spontaneous photon emits. It is important to underline, that within the laser field plasma the ultrarelativistic electron in the intermediate state emerges to the mass shell and transforms into the real particle. Furthermore, the second order effect functionally transmits into two sequential first order procedures (implementing the laser-stimulated Compton process and the laser-assisted Mott scattering) with respect to the fine structure constant. We evaluated the fluctuation of the spontaneous photon resonant frequency for the various initial parameters criteria. As a result, in the investigation a specific diapason of the resonant frequency modification for the second reaction scheme was highlighted when the ultrarelativistic electron at first scatters on a nucleus and then radiates a spontaneous photon. The designated interval consists of three frequency levels in alternative to the first channel interaction pattern. Moreover, the study delineates the occurrence of the particles propagation within a narrow angle cone in the vector direction parallel to the initial electron momentum. To summarize, the article estimates that the magnitude of the differential cross-section in the resonant conditions substantially exceeds the cross-section of the process without the laser plasma ambience. In addition, pulsed laser radiation laboratories (SLAC, FAIR, XFEL, ELI, XCELS) maintain the ability to verify the project calculations. [ABSTRACT FROM AUTHOR]

Compact X‐ray sources based on inverse Compton scattering provide brilliant and partially coherent X‐rays in a laboratory environment. The cross section for inverse Compton scattering is very small, requiring high‐power laser systems as well as small laser and electron beam sizes at the interaction point to generate sufficient flux. Therefore, these systems are very sensitive to distortions which change the overlap between the two beams. In order to monitor X‐ray source position, size and flux in parallel to experiments, the beam‐position monitor proposed here comprises a small knife edge whose image is acquired with an X‐ray camera specifically designed to intercept only a very small fraction of the X‐ray beam. Based on the source position drift recorded with the monitor, a closed‐loop feedback stabilizes the X‐ray source position by adjusting the laser beam trajectory. A decrease of long‐term source position drifts by more than one order of magnitude is demonstrated with this device. Consequently, such a closed‐loop feedback system which enables stabilization of source position drifts and flux of inverse Compton sources in parallel to experiments has a significant impact on the performance of these sources. [ABSTRACT FROM AUTHOR]

The Extreme Light Infrastructure is a major European undertaking with the aim of constructing a set of facilities that can produce the worlds highest intensity laser beams as well as unique high-brilliance, narrowbandwidth gamma-ray beams using laser-based inverse Compton scattering. The latter will be one of the unique features of the facility in Bucharest- Magurele, Romania, where the scientific focus will be towards nuclear physics and nuclear photonics both with high intensity lasers and gamma beams individually, as well as combined. One of the main instruments being constructed for the nuclear physics and applications with high-brilliance gamma-beams research activity is the ELIADE -ray detector array. This array consists of eight segmented HPGe clover detectors as well as largevolume LaBr3 detectors. The nuclear physics topics are expected to cover a large range including, but not limited to, properties of pygmy resonance and collective scissors mode excitations, parity violation in nuclear excitations, and matrix elements for neutrinoless double-beta decay. However, the uniqueness of the environment in which ELIADE will operate presents several challenges in the design and construction of the array. Here, we discuss some of these challenges and how we plan to overcome them, as well as the current status of implementation. [ABSTRACT FROM AUTHOR]

Intense, quasi-monochromatic, polarized γ -ray beams with high and tunable energy produced by Compton scattering of laser photons against relativistic electrons are used for fundamental studies and applications. Following a series of photoneutron cross section measurements in the Giant Dipole Resonance (GDR) energy region performed at the NewSUBARU synchrotron radiation facility, we have developed the eliLaBr Monte Carlo simulation code for characterization of the scattered γ -ray photon beams. The code is implemented using Geant4 and is available on the GitHub repository (https://github.com/dan-mihai-filipescu/eliLaBr). Here we report the validation of the eliLaBr code on NewSUBARU LCS γ -ray beam flux and spectral distribution data and two applications performed with it for asymmetric transverse emittance profiles electron beams, characteristic for synchrotrons. The first application is based on a systematic investigation of transverse collimator offsets relative to the laser and electron beam axis. We show that the maximum energy of the LCS γ -ray beam is altered by vertical collimator offsets, where the edge shifts towards lower energies with the increase in the offset. Secondly, using the eliLaBr code, we investigate the effect of the laser polarization plane orientation on the properties of the LCS γ -ray beams produced with asymmetric emittance electron beams. We show that: 1. The use of vertically polarized lasers contributes to the preservation of the LCS γ -ray beam maximum energy edge by increasing the precision in the vertical collimator alignment. 2. Under identical conditions for the electron and laser beams phase-space distributions, the energy spectrum of the scattered LCS γ -ray beam changes with the laser beam polarization plane orientation. More precisely, the use of vertically polarized laser beams slightly deteriorates the LCS γ -ray beam energy resolution. [ABSTRACT FROM AUTHOR]

Electron–positron pair production by the Breit–Wheeler process embedded in a strong laser pulse is analyzed. The transverse momentum spectrum displays prominent peaks which are interpreted as caustics, the positions of which are accessible by the stationary phases. Examples are given for the superposition of an XFEL beam with an optical high-intensity laser beam. Such a configuration is available, e.g., at LCLS at present and at European XFEL in near future. It requires a counter propagating probe photon beam with high energy which can be generated by synchronized inverse Compton backscattering. [ABSTRACT FROM AUTHOR]

Previously, fixed angle Laser-Compton Scattering (LCS) experiments have been conducted at the terminal of the 100 MeV LINAC of the Shanghai Institute of Applied Physics, using SINAP-I and SINAP-II facility. Sub-MeV energy continuously tunable laser-Compton light source device (SINAP-III) is an updated facility that will allow the collision angle between the laser and electron beam continuously adjustable from 20^\circ to 160^\circ . This new feature will enable convenient control on the maximum energy of the generated X/\gamma ray in a wide energy region, especially when the energy of electrons cannot be momentarily adjusted, e.g. on the storage ring. Keeping the electron beam and laser beam waist coincident at arbitrary angle is crucial for LCS gamma-ray production, an interaction chamber containing a rotatable bracket that holds a series of plane mirrors and convex lens is presented. This work is a summary of its design. The simulation of photon production’s variation caused by the systematic errors is performed using a MC code. The accuracies of installation and adjustment of mirrors and lens are given according to the simulation results. The sizes of these optical devices are also optimized. [ABSTRACT FROM PUBLISHER]

Gamma ray sources with high flux and spectral densities are the main requirements for new nuclear physics experiments to be performed in several worldwide laboratories with dedicated facilities. The paper is focalized on a proposal of experiment on \gamma photons production using Compton collisions between the DA\Phi NE electron beam and a high average power laser pulse, amplified in a Fabry-Pérot optical resonator. The calculations show that the resulting \gamma beam source has extremely interesting properties in terms of spectral density, energy spread and \gamma flux comparable (and even better) with the last generation \gamma sources. The energy of the \gamma beam depends on the adopted laser wavelength and can be tuned changing the energy of the electron ring. In particular we have analyzed the case of a \gamma factory tunable in the 2-9 MeV range. The main parameters of this new facility are presented and the perturbation on the transverse and longitudinal electron beam dynamics is discussed. A preliminary accelerator layout to allow experiments with the \gamma beam is presented with a first design of the accelerator optics. [ABSTRACT FROM PUBLISHER]

We have constructed a new laser-Compton-scattering facility, called the LEPS2 beamline, at the 8 GeV electron storage ring, SPring-8. This facility provides a linearly polarized photon beam in a tagged energy range of 1.3–2.4 GeV. Thanks to a small divergence of the low-emittance storage-ring electrons, the tagged photon beam has a size (σ) suppressed to about 4 mm even after it travels about 130 m to the experimental building that is independent of the storage ring building and contains large detector systems. This beamline is designed to achieve a photon beam intensity higher than that of the first laser-Compton-scattering beamline at SPring-8 by adopting the simultaneous injection of up to four high-power laser beams and increasing a transmittance for the long photon-beam path up to about 77%. The new beamline is under operation for hadron photoproduction experiments. [ABSTRACT FROM AUTHOR]

In 1957, Agodi predicted that the neutron angular distribution in ( γ , n) reactions with a 100% linearly polarized γ -ray beam for dipole excitation should be anisotropic and universally described by the simple function of a + b ⋅ cos ⁡ ( 2 ϕ ) at the polar angle θ = 90 ° , where ϕ is the azimuthal angle. However, this prediction has not been experimentally confirmed in over half a century. We have verified experimentally this angular distribution in the ( γ , n) reaction for 197 Au, 127 I, and natural Cu targets using linearly polarized laser Compton scattering γ -rays. The result suggests that the ( γ → , n) reaction is a novel tool to study nuclear physics in the giant dipole resonance region. [ABSTRACT FROM AUTHOR]

Abstract: A quasi-monochromatic hard X-ray source via laser Compton scattering (LCS) was developed on the basis of an S-band compact electron linac at AIST. The total number of photons generated and the maximum X-ray energy were ∼107 photons/s and 41keV, respectively, when a 42-MeV electron beam was collided with an 800-nm Ti:sapphire laser at a 15° collision angle. We successfully demonstrated K-edge imaging using these LCS X-rays for the high-contrast observation of a human phantom with barium sulfate-concentrated blood vessels. In this paper, we describe the results of the K-edge imaging and analytical estimates of the contrast enhancement effect for the energy range ∼37keV, which corresponds to the K-edge energy of barium. [Copyright &y& Elsevier]

Using \gamma -ray beams produced in the inverse Compton scattering between \CO_2 laser photons and relativistic electrons, we have calibrated electron beam energies in the nominal energy range 550–974 MeV at the synchrotron radiation facility NewSUBARU. The laser Compton-scattering (LCS) \gamma -ray beams were produced at energies from 561  to 1728 keV and detected with a high-purity germanium detector. The electron beam energies were determined by reproducing the full energy peaks of the \gamma -ray beams by Monte Carlo simulations. The accuracy of the calibration is (5.5 - 9.4) \times 10^ - 5. The reproducibility of the electron beam energy is excellent in an independent injection and deceleration. The present energy calibration of the electron beams offers a standard for the energy calibration of high-energy LCS \gamma -ray beams produced with a \Nd:YVO_4 laser. As applications of the energy calibration, we investigated the energy linearity of a 3.5^\prime\prime \times 4.0^\prime\prime \LaBr_3(\Ce) detector in the response to \gamma -rays at energies up to 10 MeV and the energy profile of the high-energy LCS \gamma -ray beams. [ABSTRACT FROM AUTHOR]

Highlights: [•] This is an analytical investigation of the transmutation of 107Pd via bremsstrahlung and Compton backscattering mechanisms. [•] Dependence of transmutation rate on the laser characters, irradiation time and photon flux facility is characterized. [•] The number of reactions in the Compton backscattering method is better than the one for bremsstrahlung method. [•] Optimal parameters to produce maximum number of reactions are evaluated. [Copyright &y& Elsevier]

Abstract: We combine a simulation program for Laser Compton Scattering (LCS) with another for deuteron photodisintegration to simulate an experiment to search for parity violation in the reaction using a LCS source. We extract the differential cross-section for near threshold from fits to all relevant available experimental data. We use this information along with theoretical estimates of parity violation in this reaction to calculate neutron yields and PV asymmetries in near threshold using the weight method. This work lays a foundation for us to study the feasibility of this experiment, the optimization of experimental conditions, and systematic effects and false asymmetries in detail. [Copyright &y& Elsevier]

Abstract: This paper describes a design and performance of a three-dimensional 4-mirror optical cavity for development of intense photon sources by laser-Compton scattering. We achieved the finesse of 4000 and average power of 2.6kW in the cavity with the vertical laser beam spot size of in at the laser–electron interaction point. As a result, we observed 124±1 photons with average energy of 24MeV per beam crossing, which corresponds to the generation of 2.7×108 photons per second. [Copyright &y& Elsevier]

A setup of a unique x-ray source is put forward employing a relativistic electron beam interacting with two counterpropagating laser pulses in the nonlinear few-photon regime. In contrast to Compton scattering sources, the envisaged x-ray source exhibits an extremely narrow relative bandwidth of the order of 10-4, comparable with an x-ray free-electron laser. The brilliance of the x rays can be an order of magnitude higher than that of a state-of-the-art Compton source. By tuning the laser intensities and the electron energy, one can realize either a single peak or a comblike x-ray source of around keV energy. The laser intensity and the electron energy in the suggested setup are rather moderate, rendering this scheme compact and tabletop size, as opposed to x-ray free-electron laser and synchrotron infrastructures. [ABSTRACT FROM AUTHOR]

Abstract: A gamma-ray beam produced by Compton scattering of a laser beam with a relativistic electron beam has been used for nuclear physics research at the high intensity -ray source () facility at Duke University. The success of many experiments using the gamma-ray beam critically depends on the accurate knowledge of the gamma-ray beam energy distribution which is typically obtained by unfolding a measured energy spectrum. Conventionally, the detector response function used in the spectrum unfolding is simulated by a simple Monte Carlo code in which an isotropic gamma-ray event generator is used. However, for a Compton scattered gamma-ray beam, this simulation method could lead to an inaccurate detector response function, since it neglects the spatial and energy distributions of the gamma-ray beam. In this paper, we will present a novel end-to-end gamma-ray spectrum reconstruction method by completely modeling the process of the Compton gamma-ray beam production, collimation, transportation and detection. Using this method, we have successfully reconstructed the energy distribution of beams for nuclear physics research with a high degree of accuracy. [Copyright &y& Elsevier]

Abstract: A two-step positron facility using a multi-GeV electron beam is under study for electron–positron colliders as ILC or the SuperB factory. In the first step, the electron beam impinges on an oriented tungsten crystal. Channelling radiation provides a large number of γ-rays and consequently creates a large number of pairs in an amorphous target placed after the crystal. Such a method has been successfully tested at CERN and KEK and is now operating at KEK. As a second step and as an upgrade of the system, the electron beam, instead of being sent to the crystal, collides with a circularly polarized laser beam, creating circularly polarized Compton backscattered γ-rays, which impinge on a thin amorphous target where they generate longitudinally polarized positrons. Simulation results are provided for the two steps and the choice of the parameters is discussed for ILC and for the SuperB factory. Particularly, the choice of an ERL instead of a classical linac is recommended for the ILC. [Copyright &y& Elsevier]

Abstract: The conversion efficiency of ultra short-pulse laser radiation to K-α X-rays has been measured for various chlorine-containing targets to be used as X-ray scattering probes of dense plasmas. The spectral and temporal properties of these sources will allow spectrally-resolved X-ray scattering probing with picosecond temporal resolution required for measuring the plasma conditions in inertial confinement fusion experiments. Simulations of X-ray scattering spectra from these plasmas show that fuel capsule density, capsule ablator density, and shock timing information may be inferred. [Copyright &y& Elsevier]

Compton scattering hard X-ray source which consists of an X-band (11.424 GHz) electron linear accelerator and YAG laser is under construction at Nuclear Professional School, the University of Tokyo (UTNS). Monochromatic hard X-rays are required for variety of medical and biological applications. Our scheme of the hard X-ray source is to produce a monochromatic hard X-ray via collision between 35 MeV electron beam and 2.5 J/10 nsec Nd:YAG laser. In order to increase the efficiency of the X-ray yield, we adopt a laser pulse circulation system. In our case, the laser pulse circulation system can increase the X-ray intensity of up to 50 times. Main features of our scheme are to produce monochromatic tunable hard (10-40 keV) X-rays with the intensities of 108-109 photons/sec. In addition, X-ray energy can be changed with rapidly by 40 ms by introducing two different wavelength lasers (YAG fundamental (1064 nm), 2nd harmonic (532 nm)) and optical switch. This quick energy change is indispensable to living specimens and very difficult by a large SR light source and others. We designed a laser pulse circulation system to increase the X-ray yield 10 times higher (up to 108 photons/RF pulse, 109 photons/sec). It can be proved that the laser total increases 10 times higher by principle experiment with lower energy laser (25 mJ/pulse). Dual-energy X-ray CT and subtraction X-ray CT are available to determine 3D distribution of atomicc number density and electron density, and specified atomic distribution, respectively. Here, the construction status of the X-band beam line and the application plan of the hard X-ray will be reported. [ABSTRACT FROM AUTHOR]

A tightly focused laser beam (laser wire) is used for measurement of transverse electron beam sizes in storage rings and linear colliders. It is assumed that the laser beam does nothing with the electron beam except Compton scatterings which happen with a rather small probability. In reality, electrons crossing the laser beam get kicks (with 100% probability) proportional to the square of the laser field and inversely proportional to the beam energy. In practical cases of beam diagnostics this effect is negligible. [Copyright &y& Elsevier]

In this paper, we report on the design and implementation of a compact and robust Compton spectrometer capable of measuring x-rays in the energy range 1 to 20 MeV. An iterative reconstruction method was employed to reconstruct the x-ray spectrum without assuming an initial distribution function. Spectral resolution was further optimized by judicious choice of composition and thickness of the electron-convertor, as well as angular acceptance into the spectrometer. A particle transport code was used to account for all instrument-related factors. Performance of the spectrometer was demonstrated experimentally by characterizing x-rays generated by high-order multiphoton Thomson scattering. [ABSTRACT FROM AUTHOR]

Relativistic spin-polarized positron beams are indispensable for future electron-positron colliders to test modern high-energy physics theory with high precision. However, present techniques require very large scale facilities for those experiments. We put forward a novel efficient method for generating ultrarelativistic polarized positron beams employing currently available laser fields. For this purpose, the generation of polarized positrons via multiphoton Breit-Wheeler pair production and the associated spin dynamics in single-shot interaction of an ultraintense laser pulse with an ultrarelativistic electron beam is investigated in the quantum radiation-dominated regime. The pair production spin asymmetry in strong fields, significantly exceeding the asymmetry of the radiative polarization, produces locally highly polarized particles, which are split by a specifically tailored small ellipticity of the laser field into two oppositely polarized beams along the minor axis of laser polarization. In spite of radiative de-polarization, a dense positron beam with up to about 90% polarization can be generated in tens of femtoseconds. The method may eventually usher high-energy physics studies into smaller-scale laser laboratories. [ABSTRACT FROM AUTHOR]

Photoneutron cross sections were measured for 137Ba and 138Ba at energies below two-neutron threshold using quasimonochromatic γ-ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. The photoneutron data are used to constrain the γ-ray strength function on the basis of the Hartree-Fock-Bogolyubov plus quasiparticle random phase approximation using the Gogny D1M interaction. Supplementing the experimentally constrained γ-ray strength function with the zero-limit E1 and M1 contributions, which are unique to the deexcitation mode, we discuss radiative neutron capture cross sections relevant to the s-process nucleosynthesis of barium isotopes in the vicinity of the neutron magic number 82. [ABSTRACT FROM AUTHOR]

As an alternative to Compton backscattering and bremsstrahlung, the process of colliding high-energy electron beams with strong laser fields can more efficiently provide both a 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 multi-GeV electrons passing through the high-field region into photons, each of which carries more than half of the electron initial energy. [ABSTRACT FROM AUTHOR]

Electron–positron pair production at ultra-relativistic laser intensities has been one of the most interesting topics of high energy physics and plasma physics in the last several decades. Here we report simulations of copious electron–positron pair production via enhanced electron injection from a mass-limited foil driven by a circularly-polarized laser pulse focused to the forthcoming intensity of 1023 W cm–2. The laser pressure pushes the center foil forward as a whole with the lateral laser wave passing by the foil boundary and being reflected by the second thick carbon target. The accelerated electrons from the mass-limited foil are injected into the reflected laser wave in the hole-boring front of the carbon target, resulting in a significant enhancement of the γ-photon emission via nonlinear Compton scattering. Finally, we obtain 1014γ-photons with an average energy of 14.48 MeV and a conversion efficiency from the laser to the γ-ray energy of 18%. A dense beam of 1011 positrons is produced with an average energy of 160 MeV, which may have wide-range applications to astrophysics and fundamental physics. [ABSTRACT FROM AUTHOR]

Laser-driven gamma-ray source potentially offers a compact, cost-effective, ultra-short, and ultra-bright alternative to conventional gamma-ray sources based on large-scale particle accelerators. Based on the laser-driven approach, we use multidimensional particle-in-cell simulations to demonstrate that a nanostructured double-layer target, which consists of a nanostructured foam coated on top of a metal substrate, can absorb laser energy into high-energy electrons in the nanostructured foam, and then efficiently convert it into copious gamma photons via the nonlinear Compton scattering process enabled by the solid-density substrate, which acts as a plasma mirror to reflect the laser pulse. The effects of different nanostructures in the foam target and the oblique laser incidence are presented. It is shown that the conversion efficiency of gamma photons increases when the size of nanoparticles decreases or the filling factor of nanoparticles increases in nanostructured foam target, but decreases when the laser incidence angle increases. At realistic conditions with nanostructured foam and non-normal incidence, the double-layer target still exhibits an unprecedentedly high conversion efficiency in high-energy gamma-ray production due to the laser reflection by the plasma mirror, which can be two and even three orders of magnitude higher than that of the single-layer target without the substrate using currently available lasers with intensity of 1021 . [ABSTRACT FROM AUTHOR]

High-energy-density electron–positron pairs play an increasingly important role in many potential applications. Here, we propose a scheme for enhanced positron production by an ultra intense laser irradiating a gas-Al compound target via the multi-photon Breit–Wheeler (BW) process. The laser pulse first ionizes the gas and interacts with a near-critical-density plasma, forming an electron bubble behind the laser pulse. A great deal of electrons are trapped and accelerated in the bubble, while the laser front hole-bores the Al target and deforms its front surface. A part of the laser wave is thus reflected by the inner curved target surface and collides with the accelerated electron bunch. Finally, a large number of γ photons are emitted in the forward direction via the Compton back-scattering process and the BW process is initiated. Dense electron–positron pairs are produced with a maximum density of m−3. Simulation results show that the positron generation is greatly enhanced in the compound target, where the positron yield is two orders of magnitude greater than that in only the solid slab case. The influences of the laser intensity, gas density and length on the positron beam quality are also discussed, which demonstrates the feasibility of the scheme in practice. [ABSTRACT FROM AUTHOR]