Ultrahigh Intensity Laser Research Articles

Scintillators in High-Power Laser-Driven Experiments

Nowadays, it is possible to accelerate bunches of particles in the interaction of ultrahigh intensity (UHI) laser pulses with matter. Electrons, protons, ions, and high-energy photon beams can be produced in experiments and reach kinetic energies close to hundreds of megaelectronvolts for protons and gigaelectronvolts for electrons and for the associated Bremsstrahlung photons. At these energies, these beams can induce a large variety of nuclear reactions, which can be detected and studied using $\gamma $ -ray spectroscopy techniques. At standard accelerator facilities, scintillator detectors are commonly used to perform prompt $\gamma $ -ray spectrometry studies. However, during laser–matter interactions, high fluxes of X-rays (mostly soft) are generated, which lead to instantaneous huge energy deposits (~1 $\mu \text{J}$ ) in these scintillators. Depending on the laser characteristics (energy and pulse duration), the detector recovery time after these X-ray flashes can reach several milliseconds, which makes any prompt or “in beam” measurement impossible. The origin of this long-duration signal is investigated in the case of a LaBr3 crystal coupled to different photodetectors. While it was impossible using standard photomultiplier tubes to detect $\gamma $ -ray emissions before a few milliseconds after a laser shot, we could, using a hybrid photodiode, resolve single $\gamma $ -ray emission a few tens of microseconds after the laser shot. Furthermore, we have also shown that the LaBr3 scintillator presents an unexpected long-lived light emission (afterglow). Directions are suggested for future studies in order to minimize the effects of this afterglow emission.

Simulation of internal stress waves generated by laser-induced damage in multilayer dielectric gratings.

Multilayer dielectric (MLD) gratings used in ultrahigh-intensity laser systems often exhibit a laser-induced damage performance below that of their constituent materials. Reduced performance may arise from fabrication- and/or design-related issues. Finite element models were developed to simulate stress waves in MLD grating structures generated by laser-induced damage events. These models specifically investigate the influence of geometric and material parameters on how stress waves can lead to degradation of material structural integrity that can have adverse effects on its optical performance under subsequent laser irradiation: closer impedance matching of the layer materials reduces maximum interface stresses by ~20% to 30%; increasing sole thickness from 50 nm to 500 nm reduces maximum interface stresses by ~50%.

Photon and positron generation by ultrahigh intensity laser interaction with electron beams

This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particle-in-cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.

Relativisitcally upshifted higher harmonic generation via relativistic flying mirrors

We have previously shown that laser light can be upshifted to higher frequencies by its reflection off relativistically moving mirrors in plasma. These mirrors were generated with ultra-high intensity laser pulses. However, the laser light which was reflected off the mirrors had relatively low intensity. We show via simulations that even high intensity light can be reflected off the mirrors and that this can generate relativistically upshifted harmonics.

Modelling the effects of the radiation reaction force on the interaction of thin foils with ultra-intense laser fields

The effects of the radiation reaction (RR) force on thin foils undergoing radiation pressure acceleration (RPA) are investigated. Using QED-particle-in-cell simulations, the influence of the RR force on the collective electron dynamics within the target can be examined. The magnitude of the RR force is found to be strongly dependent on the target thickness, leading to effects which can be observed on a macroscopic scale, such as changes to the distribution of the emitted radiation and the target dynamics. This suggests that such parameters may be controlled in experiments at multi-PW laser facilities. In addition, the effects of the RR force are characterized in terms of an average radiation emission angle. We present an analytical model which, for the first time, describes the effect of the RR force on the collective electron dynamics within the ‘light-sail’ regime of RPA. The predictions of this model can be tested in future experiments with ultra-high intensity lasers interacting with solid targets.

Numerical modeling of laser-driven ion acceleration from near-critical gas targets

In the past two decades, laser-accelerated ion sources and their applications have been intensely researched. Recently, it has been shown through experiments that proton beams with characteristics comparable to those obtained with solid targets can be obtained from gaseous targets. By means of particle-in-cell simulations, this paper studies in detail the effects of a near-critical density gradient on ion and electron acceleration after the interaction with ultra high intensity lasers. We can observe that the peak density of the gas jet has a significant influence on the spectrum features. As the gas jet density increases, so does the peak energy of the central quasi-monoenergetic ion bunch due to the increase in laser absorption while at the same time having a broadening effect on the electron angular distribution.

Macroscopic laser–plasma interaction under strong non-local transport conditions for coupled matter and radiation

Reliable simulations of laser–target interaction on the macroscopic scale are burdened by the fact that the energy transport is very often non-local. This means that the mean-free-path of the transported species is larger than the local gradient scale lengths and transport can be no longer considered diffusive. Kinetic simulations are not a feasible option due to tremendous computational demands, limited validity of the collisional operators and inaccurate treatment of thermal radiation. This is the point where hydrodynamic codes with non-local radiation and electron heat transport based on first principles emerge. The simulation code PETE (Plasma Euler and Transport Equations) combines both of them with a laser absorption method based on the Helmholtz equation and a radiation diffusion scheme presented in this article. In the case of modelling ablation processes it can be observed that both, thermal and radiative, transport processes are strongly non-local for laser intensities of 1013W/cm2 and above. In this paper simulations for various laser intensities and different ablator materials are presented, where the non-local and diffusive treatments of radiation transport are compared. Significant discrepancies are observed, supporting importance of non-local transport for inertial confinement fusion related studies as well as for pre-pulse generated plasma in ultra-high intensity laser–target interaction.

The effect of pre-plasma formation under nonlocal transport conditions for ultra-relativistic laser-plasma interaction

Interaction of high-power lasers with solid targets is in general strongly affected by the limited contrast available. The laser pre-pulse ionizes the target and produces a pre-plasma which can strongly modify the interaction of the main part of the laser pulse with the target. This is of particular importance for future experiments which will use laser intensities above 1021 W cm−2 and which are subject to the limited contrast. As a consequence the main part of the laser pulse will be modified while traversing the pre-plasma, interacting with it partially. A further complication arises from the fact that the interaction of a high-power pre-pulse with solid targets very often takes place under nonlocal transport conditions, i.e. the characteristic mean-free-path of the particles and photons is larger than the characteristic scale-lengths of density and temperature. The classical diffusion treatment of radiation and heat transport in the hydrodynamic model is then insufficient for the description of the pre-pulse physics. These phenomena also strongly modify the formation of the pre-plasma which in turn affects the propagation of the main laser pulse. In this paper nonlocal radiation-hydrodynamic simulations are carried out and serve as input for subsequent kinetic simulations of ultra-high intensity laser pulses interacting with the plasma in the ultra-relativistic regime. It is shown that the results of the kinetic simulations differ considerably whether a diffusive or nonlocal transport is used for the radiation-hydrodynamic simulations.

Ultra-intense laser interaction with nanostructured near-critical plasmas

Near-critical plasmas irradiated at ultra-high laser intensities (I > 1018W/cm2) allow to improve the performances of laser-driven particle and radiation sources and to explore scenarios of great astrophysical interest. Near-critical plasmas with controlled properties can be obtained with nanostructured low-density materials. By means of 3D Particle-In-Cell simulations, we investigate how realistic nanostructures influence the interaction of an ultra-intense laser with a plasma having a near-critical average electron density. We find that the presence of a nanostructure strongly reduces the effect of pulse polarization and enhances the energy absorbed by the ion population, while generally leading to a significant decrease of the electron temperature with respect to a homogeneous near-critical plasma. We also observe an effect of the nanostructure morphology. These results are relevant both for a fundamental understanding and for the foreseen applications of laser-plasma interaction in the near-critical regime.

Efficient ion acceleration and dense electron–positron plasma creation in ultra-high intensity laser-solid interactions

The radiation pressure of next generation ultra-high intensity (>1023 W cm−2) lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these laser pulses with matter. Here we show that these effects may lead to the production of an extremely dense (∼1024 cm−3) pair-plasma which absorbs the laser pulse consequently reducing the accelerated ion energy and laser to ion conversion efficiency by up to 30%–50% and 50%–65%, respectively. Thus we identify the regimes of laser-matter interaction, where either ions are efficiently accelerated to high energy or dense pair-plasmas are produced as a guide for future experiments.

Analysis of wavefront effects for large-aperture tiled-grating compressor

AbstractMonolithic large-aperture diffraction grating tiling is desired to increase the output capability of multi-kilojoule petawatt laser facilities. However, the wavefront errors of input pulse and gratings will degrade the focal spot quality and the compressibility of the output pulse. In this work, the effects of wavefront error of input pulse, deformation and wave aberration of the grating for the large-aperture tiled-grating compressor are investigated theoretically. A series of numerical simulations are presented to discuss the changing trends of focal spot energy caused by wavefront error of input pulse and obtain the error tolerance for specific goals. The influences of coating stress and the wave aberration of holographic exposure gratings on the diffraction wavefront are also discussed. Some advice is proposed for improving the performance of large-aperture tiled-grating. This work paves the way for the design of practical large-aperture tiled-grating compressor for ultrahigh intensity laser facilities in the future.

Prospects for femtosecond ultrahigh intensity laser system towards Exawatt level

The new generation system for ultrahigh-power laser running over Exawatt (EW, 10 18 W) level is emerging recently, which is under a mechanism called Raman backscattering (RBS) in plasma. The main advantage of using plasma is that it can tolerate much higher laser intensities, 10 17 W cm −2 , more than five order of solid-state devices limited, 10 12 W cm −2 . Although Petawatt (PW, 10 15 W) laser pulses have been realized by some groups based on the chirped pulse amplification scheme (CPA), many cutting-edge scientific researches and technical applications, such as inertial confinement fusion (ICF), plasma physics, astrophysics, plasma-based particle accelerators, and X-ray lasers, need even higher laser power than EW level. For these purposes, huge laser projects like the extreme light infrastructure (ELI) have been proposed to offer new paradigm in EW class laser power. However, such an ultrahigh intensity laser system could only be achieved by CPA using very large (beyond 1 m 2 ) and expensive compressor gratings. In addition, to extrapolate CPA to the EW power range, hundreds of such gratings would be required. Even if there is enough budget, the problem of energy restriction on the last grating is still under question. Therefore, it is crucial to find a new medium or new technology for generating femtosecond ultra-intense laser pulses. This paper introduces a solution for generating laser intensities many orders of magnitude higher than currently results. This technology of optical amplification that a process known as Raman backscattering amplification and compression could enable the generation of femtosecond pulses of 20000 times the original seed intensity in a column of plasma just a few millimeters in length and less than a millimeter in width, without stretcher and compressor. Such sufficient high intensity and lower frequency of the pulse amplification indeed have got into super-radiant amplification regime (SRA) or stimulated Raman backscattering (SRBS). It has revealed that an unprecedented large pulse intensity amplification could be realized. The seed pulse will be compressed to 25 fs and its unfocused intensity increased from 1×10 15 to 4×10 17 W cm −2 . Furthermore, it could be increased further to around 1×10 23 W cm −2 by focusing the resulting beam to 1 μm. Therefore, it might be possible to increase this power up to EW in a larger plasma and using more powerful optical pumping.

A pinhole camera for ultrahigh-intensity laser plasma experiments.

A pinhole camera is an important instrument for the detection of radiation in laser plasmas. It can monitor the laser focus directly and assist in the analysis of the experimental data. However, conventional pinhole cameras are difficult to use when the target is irradiated by an ultrahigh-power laser because of the high background of hard X-ray emission generated in the laser/target region. Therefore, an improved pinhole camera has been developed that uses a grazing-incidence mirror that enables soft X-ray imaging while avoiding the effect of hard X-ray from hot dense plasmas.

Radiation reaction studies in an all-optical set-up: experimental limitations

The recent development of ultra-high intensity laser facilities is finally opening up the possibility of studying high-field quantum electrodynamics in the laboratory. Arguably, one of the central phenomena in this area is that of quantum radiation reaction experienced by an ultra-relativistic electron beam as it propagates through the tight focus of a laser beam. In this paper, we discuss the major experimental challenges that are to be faced in order to extract meaningful and quantitative information from this class of experiments using existing and near-term laser facilities.

Modeling the ultra-high intensity laser pulse – cone target interaction for ion acceleration at CETAL facility

AbstractWe study the interaction of an ultra-high intensity laser pulse with plastic flat-top cone targets with curved walls and cone targets with straight walls. We find the appropriate type, dimensions of the cone target, and the ultra-high intensity laser pulse parameters for which the accelerated ions have the maximum energy and their number is the highest for a lower angular divergence and a better laser absorption. This numerical study will allow one to prepare and optimize first laser-ion acceleration experiments on CETAL using micro-cone targets.

Silicon nanowire based high brightness, pulsed relativistic electron source

We demonstrate that silicon nanowire arrays efficiently emit relativistic electron pulses under irradiation by a high-intensity, femtosecond, and near-infrared laser (∼1018 W/cm2, 25 fs, 800 nm). The nanowire array yields fluxes and charge per bunch that are 40 times higher than those emitted by an optically flat surface, in the energy range of 0.2–0.5 MeV. The flux and charge yields for the nanowires are observed to be directional in nature unlike that for planar silicon. Particle-in-cell simulations establish that such large emission is caused by the enhancement of the local electric fields around a nanowire, which consequently leads to an enhanced absorption of laser energy. We show that the high-intensity contrast (ratio of picosecond pedestal to femtosecond peak) of the laser pulse (10−9) is crucial to this large yield. We extend the notion of surface local-field enhancement, normally invoked in low-order nonlinear optical processes like second harmonic generation, optical limiting, etc., to ultrahigh laser intensities. These electron pulses, expectedly femtosecond in duration, have potential application in imaging, material modification, ultrafast dynamics, terahertz generation, and fast ion sources.

Charged paricle activation analysis for characterizing parameters of laser-accelerated protons

The protons accelerated by ultra-high intensity laser have been extensively studied. The most commonly used detectors for measuring laser-driven proton are Tomspon parabola ion energy analyser (TP) and filtered nuclear track detectors, such as radiochromic films (RCF). The TP uses a parallel magneto-electric field to distinguish ions. This conventional technique can precisely identify the species and energy spectra of ions. However, the strong electromagnetic field produced by the laser-plasma interaction has an effect on TP, which results in no spatial resolution of TP. The RCF can give the spatial integration spectrum of proton, but it is easy to be saturated and cannot be reused anymore. In this paper, we present a method based on the traditional charged particle activation analysis and the gamma-gamma coincidence measurement to measure the spectrum of protons accelerated by ultra intense lasers. In this method, a copper plate stack is placed in the proton emission direction. Colliding with MeV proton converts 63Cu in the copper plates into radionuclide 63Zn whose decay can be easily observed and measured. Proton spectrum is then recovered from 63Zn decay counts from layers in the copper stack. The layout of diagnostics and the method to solve proton spectrum are discussed in detail and a self-consistent test is given. This spectrum analysis method is used in a laser-driven proton acceleration experiment carried out on XG-Ⅲ laser facility. The results show that protons up to 18 MeV are obtained, and the spatial integrated spectrum and a laser-proton conversion efficiency of 1.07% are achieved. In conclusion, our method has some advantages as a laser-driven ion diagnostic tool. It has no saturation problem and is not affected by strong electromagnetic fields. The basic principle of charged particle activation analysis is based on nuclear reaction, and can be extended to the measuring of other charged particle beams besides protons, such as deuterons, helium ions produced by ultra-high intensity laser.

Ion acceleration in electrostatic field of charged cavity created by ultra-short laser pulses of 1020–1021 W/cm2

Ion acceleration resulting from the interaction of ultra-high intensity and ultra-high contrast (∼10−10) laser pulses with thin Al foil targets at 30° angle of laser incidence is studied. Proton maximum energies of 30 and 18 MeV are measured along the target normal rear and front sides, respectively, showing intensity scaling as Ib. For the target front bfront= 0.5–0.6 and for the target rear brear= 0.7–0.8 is observed in the intensity range 1020–1021 W/cm2. The fast scaling from the target rear ∼I0.75 can be attributed enhancement of laser energy absorption as already observed at relatively low intensities. The backward acceleration of the front side protons with intensity scaling as ∼I0.5 can be attributed to the to the formation of a positively charged cavity at the target front via ponderomotive displacement of the target electrons at the interaction of relativistic intense laser pulses with a solid target. The experimental results are in a good agreement with theoretical predictions.

Influence of linear chirp on the output characteristics of cross polarized wave with saturated power density

Ultra-short and ultra-intense laser is one of the hottest research spot of laser technology and strong field physics, due to its challenging and the frontier application research. As the key specification of ultrafast ultrahigh intensity laser pulse, the contrast ratio is very influential on the effect of laser-matter interaction. To perform the laser-matter interaction experiments at a high power level, the contrast is required to be as high as 1010 to prevent preplasma dynamics. To solve these problems, one has proposed many methods to improve the contrast of ultrafast laser, such as using the saturable absorbers, double chirped pulse amplification, plasma mirrors and the cross-polarized wave (XPW) generation. The XPW technology can not only enhance the contrast of the pulse by 3-4 orders of magnitude without introducing any space dispersion, but also extend the output spectrum to support shorter pulse duration. The XPW is a nonlinear filter technique in third-order nonlinear crystal with anisotropic susceptibility. Because of its simple and all-solid-state structure, the XPW technique has become one of the most effective methods to enhance the temporal pulse contrast and deliver shorter pulse duration in the field of high peak-power ultrafast lasers. This method has been used in many large laser facilities under construction or upgrades, such as the Apollon and ELI, the contrast ratio as high as 1010 has been achieved. It is known that the conversion efficiency and spectral characteristics of XPW have a strong dependence on the spatial and temporal magnitudes of the input driving pulse. In our experiment, it is found that the various changes of the driven pulse properties have different influences on the characteristics of XPW pulses. The relationship between the linear dispersion of driven pulse and temporal property of XPW is investigated theoretically. In addition, an experiment on verifying the theory is conducted by taking advantage of a programmable acousto-optic dispersion filter. The experimental results fit well to the theoretical results while some new phenomena emerge when the intensity in the BaF2 crystal reaches a saturation threshold. The spectral broadening capability of XPW becomes stronger and exceeds a theoretical upper limit. The pulse width can also be compressed to shorter than the theoretical limit. It is found that there are significant differences in spectral shape and conversion efficiency between the XPW signals by applying the opposite linear chirps to the driving pulse. A further analysis and theoretical explanation of these new phenomena are also presented.

Spectral phase effects and control requirements of coherent beam combining for ultrashort ultrahigh intensity laser systems.

Based on the premise that further improvements to the size and damage threshold of large-aperture optical components are severely limited, coherent beam combining (CBC) is a promising way to scale up the available peak power of pulses for ultrashort ultrahigh intensity laser systems. Spectral phase effects are important issues and have a significant impact on the performance of CBC. In this work, we analyze systematically factors such as spectral dispersions and longitudinal chromatism, and get the general spectral phase control requirements of CBC for ultrashort ultrahigh intensity laser systems. It is demonstrated that different orders of dispersion influence intensity shape of the combined beam, and high-order dispersions affect the temporal contrast of the combined beam, while the number of the channels to be combined has little impact on the temporal Strehl ratio (SR) of the combined beam. In addition, longitudinal chromatism should be controlled effectively since it has a detrimental effect on the combined beam at the focal plane, both temporally and spatially.