Ultrahigh Intensity Laser Pulses Research Articles

On the distribution function of electron spectra from hot laser plasmas

The emission of electrons from hot plasmas generated in the interaction of ultra-short (and ultra-high intensity) laser pulses with matter is often characterized by the so-called ‘hot electron temperature’. In this article it is shown that this number is not unambiguous. The reason is the following: to assign a temperature to an electron spectrum, it is necessary to describe the spectrum with a distribution function. However, different types of distribution functions are in use, e.g. the Boltzmann or Maxwell distribution, leading to different electron temperatures in spite of providing nearly the same form of the electron spectrum. For this reason, the main characteristics of all these distribution functions are presented in this article and compared. Depending on the distribution function used, the value of the hot electron temperature varies by up to 30% and in extreme cases by more than a factor of four. This fact should always be kept in mind when comparing values of hot electron temperatures. In addition, the reasons for using equilibrium distributions to describe the characteristics of laser-produced electrons—although probably no thermodynamic equilibrium is prevailing—are discussed.

Phase control of nonlinear Breit-Wheeler pair creation

Electron-positron pair creation occurs throughout the universe in the environments of extreme astrophysical objects, such as pulsar magnetospheres and black hole accretion disks. The difficulty of emulating these environments in the laboratory has motivated the use of ultrahigh-intensity laser pulses for pair creation. Here we show that the phase offset between a laser pulse and its second harmonic can be used to control the relative transverse motion of electrons and positrons created in the nonlinear Breit-Wheeler process. Analytic theory and particle-in-cell simulations of a head-on collision between a two-color laser pulse and electron beam predict that with an appropriate phase offset, the electrons will drift in one direction and the positrons in the other. The resulting current may provide a collective signature of nonlinear Breit-Wheeler, while the spatial separation resulting from the relative motion may facilitate isolation of positrons for subsequent applications or detection. Published by the American Physical Society 2024

Ion acceleration from the interaction of ultrahigh-intensity laser pulses with near-critical density, nonuniform gas targets

Using one-dimensional, long-timescale particle-in-cell simulations, we study the processes of ion acceleration from the interaction of ultraintense (1020 W cm−2), ultrashort (30 fs) laser pulses with near-critical, nonuniform gas targets. The considered initially neutral, nitrogen gas density profiles mimic those delivered by an already developed noncommercial supersonic gas shock nozzle: they have the generic shape of a narrow (20 μm wide) peak superimposed on broad (∼1 mm, ∼180 μm scale length), exponentially decreasing ramps. While keeping its shape constant, we vary its absolute density values to identify the interaction conditions leading to collisionless shock-induced ion acceleration in the gas density ramps. We find that collisionless electrostatic shocks (CES) form when the laser pulse is able to shine through the central density peak and deposit a few 10% of its energy into it. Under our conditions, this occurs for a peak electron density between 0.35 nc and 0.7 nc. Moreover, we show that the ability of the CES to reflect the upstream ions is highly sensitive to their charge state and that the laser-induced electron pressure gradients mainly account for shock generation, thus highlighting the benefit of using sharp gas profiles, such as those produced by shock nozzles.

Theoretical investigation of the interaction of ultra-high intensity laser pulses with near critical density plasmas

A theoretical model of energy transfer from laser to particles in the ultra-high intensity regime of laser plasma interaction is proposed, assuming that most of the laser energy will be transferred to hot electrons. Varying the target density and thickness, the optimal parameters for the maximum conversion efficiency of the laser energy to particles are studied. Through 2D particle-in-cell simulations, the model is validated for a near-critical density plasma between (where cm−3 is the critical density for a laser wavelength of m) irradiated by a laser pulse of intensity in the range 1020–1023 W cm−2 and the pulse duration in the range 6.5–100 fs. As an application to this model, laser ion acceleration is studied for a laser intensity of 1022 W cm−2 and a pulse duration of 20 fs. Based on the literature and new findings from our model, the optimum thickness for ion acceleration and the maximum ion energies for an expansion like mechanism are predicted. These results can be used for applications requiring high energy ions and for preparations of experiments at the Apollon and ELI laser facilities.

Laser Driven Electron Acceleration from Near-Critical Density Targets towards the Generation of High Energy γ-Photons

In this paper, we investigate the production of high energy gamma photons at the interaction between an ultra-high intensity laser pulse with an energetic electron beam and with a near-critical density plasma for the laser intensity varying between 1019–1023 W/cm2. In the case of the interaction with an electron beam, and for the highest laser intensities considered, the electrons lose almost all their energy to emit gamma photons. In the interaction with a near-critical density plasma, the electrons are first accelerated by the laser pulse up to GeV energies and further emit high energy radiation. A maximum laser-to-photons conversion coefficient of 30% is obtained. These results can be used for the preparation of experiments at the Apollon and ELI laser facilities for the investigation of the emission of high energy γ-photons and to study the electron-positron pair creation in the laboratory.

Single-shot spatiotemporal characterization of a multi-PW laser using a multispectral wavefront sensing method.

The single-shot spatiotemporal characterization of an ultrahigh intensity laser pulse was performed using a multispectral wavefront sensor. For the measurement of the spatio-spectral electric field, a femtosecond laser pulse was spectrally modulated and separated by a Fabry-Perot etalon coupled with a grating pair, and its spatio-spectral electric field was measured with a wavefront sensor. The spatiotemporal electric field was reconstructed from the measured spatio-spectral electric field of a multi-PW laser pulse. We found that the spatiotemporal distortion could reduce the focused laser intensity by 15%, compared to the case of a diffraction-limited and transform-limited laser pulse.

Photon and positron production by ultrahigh-intensity laser interaction with various plasma foils

The generation of γ photons and positrons using an ultrahigh-intensity laser pulse interacting with various plasma solid foils is investigated with a series of quantum electrodynamic particle-in-cell (PIC) simulations. When ultrahigh-intensity lasers interact with plasma foils, a large amount of the laser energy is converted into γ photon energy. The simulation results indicate that for a fixed laser intensity with different foil densities, the conversion efficiency of the laser to γ photons and the number of produced photons are highly related to the foil density. We determine the optimal foil density by PIC simulations for high conversion efficiencies as approximately 250 times the critical plasma density, and this result agrees very well with our theoretical assumptions. Four different foil thicknesses are simulated and the effects of foil thickness on γ photon emission and positron production are discussed. The results indicate that optimal foil thickness plays an important role in obtaining the desired γ photon and positron production according to the foil density and laser intensity. Further, a relation between the laser intensity and conversion efficiency is present for the optimal foil density and thickness.

Ultrashort high energy electron bunches from tunable surface plasma waves driven with laser wavefront rotation.

We propose to use ultrahigh intensity laser pulses with wave-front rotation (WFR) to produce short, ultraintense surface plasma waves (SPW) on grating targets for electron acceleration. Combining a smart grating design with optimal WFR conditions identified through simple analytical modeling and particle-in-cell simulation allows us to decrease the SPW duration (down to a few optical cycles) and increase its peak amplitude. In the relativistic regime, for Iλ_{0}^{2}=3.4×10^{19}W/cm^{2}μm^{2}, such SPW are found to accelerate high charge (few 10s of pC), high energy (up to 70 MeV), and ultrashort (few fs) electron bunches.

Simulation of Plasma Density Gradient Generation by Wakefield and the Effect of Probe Pulse Time-Delay on the Photon Acceleration

Laser Wakefield is produced by ultra-high intensity laser pulse interacting with underdense plasma with special conditions for the laser wavelength and plasma density. In this mechanism, nonlinear forces appear due to the very high amplitudes of the electromagnetic wave and these forces evacuate plasma electrons from the path of the laser pulse leading to very high electron plasma density gradients. Due to the electrostatic forces which result from these density perturbations, the electrons move very fast in oscillatory manner to restore neutrality creating a wake of electron density perturbations behind the laser pulse. Detailed investigation has been dealt with the time-delay between the driver laser pulse and the probe pulse which can affect the production of high plasma gradients needed for photon acceleration process.

Two-stage γ ray emission via ultrahigh intensity laser pulse interaction with a laser wakefield accelerated electron beam

In this study, we investigate the generation of twin γ ray beams in the collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam using a particle-in-cell simulation. We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil. The electrons in the preplasma are trapped and accelerated by the wakefield. When the laser pulse is reflected by the thin solid foil, the wakefield accelerated electrons continue to move forward and pass through the foil almost without influence from the reflected laser pulse or foil. Consequently, two groups of γ ray flashes, with tunable time delay and energy, are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter-propagating petawatt laser pulse behind the foil. Additionally, we study the dependence of the γ photon emission on the preplasma densities, driving laser polarization, and solid foil.

Modeling the interaction of an ultra-high intensity laser pulse with an ultra-thin nanostructured foil target

New plastic nanostructured foil targets interacting with an ultra-high intensity laser pulse are investigated by performing particle-in-cell simulations. The thickness of the ultra-thin planar and nanostructured foil target is in the range of tens of nanometers. Complementary numerical simulations which use the finite-difference time-domain method without considering plasma generation have been performed. The laser pulse has the parameters of the two 10 PW lasers from ELI-NP. The circularly polarized laser pulse has the twice intensity of the linearly polarized (LP) laser pulse. We show a strong dependence of the maximum proton energy on the nanospheres diameter. But, the carbon ion maximum energy varies very weak with the nanospheres diameter. We prove that the acceleration regime of the protons is a combination of the radiation pressure acceleration and target normal sheath acceleration regimes. The carbon ions experience the target normal sheath acceleration. This study is a very useful tool for the preparation of laser-ion acceleration experiments with nanostructured foil targets at ELI-NP and other PW laser facilities.

Achieving the laser intensity of 5.5×1022 W/cm2 with a wavefront-corrected multi-PW laser.

The generation of ultrahigh intensity laser pulses was investigated by tightly focusing a wavefront-corrected multi-petawatt Ti:sapphire laser. For the wavefront correction of the PW laser, two stages of deformable mirrors were employed. The multi-PW laser beam was tightly focused by an f/1.6 off-axis parabolic mirror and the focal spot profile was measured. After the wavefront correction, the Strehl ratio was about 0.4, and the spot size in full width at half maximum was 1.5×1.8 μm2, close to the diffraction-limited value. The measured peak intensity was 5.5×1022 W/cm2, achieving the highest laser intensity ever reached.

Impact of ion dynamics on laser-driven electron acceleration and gamma-ray emission in structured targets at ultra-high laser intensities

We examine the impact of the ion dynamics on laser-driven electron acceleration in an initially empty channel irradiated by an ultra-high intensity laser pulse with I > 1022 W cm−2. The negative charge of the accelerated electrons inside the channel generates a quasi-static transverse electric field that causes gradual ion expansion into the channel. Once the ions fill the channel, the pinching force from the quasi-static magnetic field generated by the accelerated electrons becomes uncompensated due to the reduction of the quasi-static transverse electric field. As a result there are two distinct populations of accelerated electrons: those that accelerate ahead of the expanding ion front while moving predominantly forward and those that accelerate in the presence of the ions in the channel while performing strong transverse oscillations. The ions diminish the role of the longitudinal laser electric field, making the transverse electric field the dominant contributor to the electron energy. The ion expansion also has a profound impact on the gamma-ray emission, causing it to become volumetrically distributed while reducing the total emitted energy. We formulate a criterion for the laser pulse duration that must be satisfied in order to minimize the undesired effect from the ions and to allow the electrons to remain highly collimated. We demonstrate the predictive capability of this criterion by applying it to assess the impact of a given pre-pulse on ion expansion. Our results provide a guideline for future experiments at multi-PW laser facilities with ultra-high intensities.

Radiation friction force effects on electron dynamics in ultra-intensity laser pulse

The electron dynamics in the ultra-high intensity laser pulse with radiation friction force in the Landau-Lifshitz form are studied. It is demonstrated that widely used approximation, where only the term dominating the dissipation of electron kinetic energy is retained in the expression for the radiation friction, is incorrect for the case of diverging electron trajectories. As a matter of fact, for large friction force effects, all components of the radiation friction force in the Landau-Lifshitz form have the same order in the equation of motion for electron trajectories, being equally important for both electron trajectory and thus energy gain in the case of diverging electron trajectories (e.g., determined by the superposition of few electromagnetic waves).

Nonlinear absorption of ultrashort ultrahigh intensity laser pulses in fullerene

Non linear absorption of intense few cycle laser pulses in fullerenes is studied by treating it as a laser driven nonlinear oscillator, using a rigid shell model. Efficient collisionless absorption of intense (Intensity I < 1018W/cm2) ultrashort laser pulses in fullerene (C60) is estimated owing to its extraordinary anharmonicty which is quite different than the conventionally used solid gas clusters. Compared to gas clusters nonlinear resonance in C60 has unusual characteristics. Factors other than nonlinear resonance which contribute in energy absorption in C60 are outlined. Strong dephasing of electrons with the laser field both at onset and during the dynamical oscillations leads to the efficient energy gain by the electrons. It is found that fullerene has a distinct threshold intensity requirement depending on its charge for efficient absorption. These properties make fullerene unique for efficient absorption of high intensity few cycle laser pulses.

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.

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.

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.

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.

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.