Ultra-intense Laser Interaction Research Articles

Ultra-intense femtosecond laser interactions with aligned nanostructures

The interaction of ultrafast laser pulses of relativistic intensity with high aspect ratio nanostructures can efficiently and volumetrically heat matter to an ultra-high-energy-density regime encountered in the center of stars and within the core of fusion capsules compressed by the world’s largest lasers. It also generates gigantic quasi-static electromagnetic fields that accelerate particles to very high energy. Here, we present an overview of the physics and applications of these dense relativistic plasmas that can be created with pulses of relatively modest energy from lasers that can operate at a high repetition rate. Recent nanowire array experiments produced near-solid density plasmas with an extreme degree of ionization (e.g., Au+72), converted ultrafast pulses of laser light into intense x-ray flashes with record efficiency, and accelerated ions to MeV energies, efficiently driving micro-scale fusion reactions that generate flashes of quasi-monoenergetic neutrons. These plasmas also serve as a platform for advancing the understanding of atomic processes in extreme environments and open a new pathway to laser-driven fusion energy. The irradiation of nanostructures at intensities of >1×1022Wcm−2 is predicted to lead to an extreme ultra-high energy density plasma regime characterized by terabar pressures that is virtually unexplored.

From linear to nonlinear Breit-Wheeler pair production in laser-solid interactions.

During the ultraintense laser interaction with solids (overdense plasmas), the competition between two possible quantum electrodynamics (QED) mechanisms responsible for e^{±} pair production, i.e., linear and nonlinear Breit-Wheeler (BW) processes, remains to be studied. Here, we have implemented the linear BW process via a Monte Carlo algorithm into the QED particle-in-cell (PIC) code yunic, enabling us to self-consistently investigate both pair production mechanisms in the plasma environment. By a series of two-dimensional QED-PIC simulations, the transition from the linear to the nonlinear BW process is observed with the increase of laser intensities in the typical configuration of a linearly polarized laser interaction with solid targets. A critical normalized laser amplitude about a_{0}∼400-500 is found under a large range of preplasma scale lengths, below which the linear BW process dominates over the nonlinear BW process. This work provides a practicable technique to model linear QED processes via integrated QED-PIC simulations. Moreover, it calls for more attention to be paid to linear BW pair production in near future 10-PW-class laser-solid interactions.

Enhancement of positron yields using multi-layer targets irradiated by laser-induced energetic electrons

Positrons with high energy and short pulse duration generated by the ultra-short and ultra-intense laser interaction with a two-target system (under-density plasma target and high-Z metal target) have wide applications. In this paper, we proposed an optimal scheme for enhancing positrons with multi-layer high-Z converters. Positrons with larger divergence escape from the target zone, reducing positron annihilation in the target, while secondary particles with smaller divergence react with the subsequent target to produce more positrons. The total positron yield and positron beam divergence increased obviously with the target number when using the thin converter, while the scenario was reversed for the recorded positrons. The total positrons produced by bilayer 5-mm targets and eight-layer 1-mm targets increased by 14% and 62%, respectively, compared to the outgoing positrons produced by an 8-mm monolayer target. Positron yields can be further enhanced by adjusting the thickness of the subsequent target and distance, according to the intensity and angular distribution of positrons emitted from the previous target.

Enhanced quasi-monoenergetic ions generation: Based on gold nanoparticles application in gas-filled nanosphere targets

It was recently shown that nanostructured targets with largely spaced gold ultrasmall nanoparticles (NPs) show outstanding performances in enhancing the laser-driven ions' acceleration process due to the higher laser-to-target energy absorption [Vallières et al., Phys. Rev. Accel. Beams 22, 091303 (2019)]. Based on this structure, here, an alternative nanostructured design is proposed to promote light/heavy ions' acceleration quality. The scheme relies on using a gold NP layered nanosphere filled with a low-density argon gas. The nanosphere has an inner layer of vanadium and an outer layer of proton–carbon (1:1) mixture. The validity of this suggestion has been simulated by the two-dimensional particle-in-cell code (EPOCH). Simulation results indicate that the interaction of ultra-intense laser (∼4.61 × 1019 W/cm2) with a gas-filled gold NP layered nanosphere can positively decrease the aggregation of electrons stated inside the target, leading to higher Coulomb repulsion between charged ions. Therefore, we can expect the generation of quasi-monoenergetic H+, C6+, V20+, and Au49+, as well as Ar15+ (cutoff energy of ∼0.49 MeV/u and relative divergence angle of 2.9°) at the end of the interaction. From simulations, as the interaction terminates, for a gas-filled gold NP layered nanosphere with an optimal gap space of 80 nm, a cutoff energy increase of roughly 19% for H+, 16.4% for C6+, and rather equal percent of 15.9% for medium-heavy ions (V20+ and Au49+) is obtained with respect to a hollow gold NP layered nanosphere. Moreover, a relative divergence angle decrease of up to nearly 0.29–1.91 times will be calculated for the accelerated ions. Overall, the results verify that a gas-filled gold NP layered nanosphere can be regarded as a candidate for the generation of quasi-monoenergetic ions through the spherical Coulomb explosion regime.

Polarized proton acceleration in ultraintense laser interaction with near-critical-density plasmas.

The production of polarized proton beams with multi-GeV energies in ultraintense laser interaction with targets is studied with three-dimensional particle-in-cell simulations. A near-critical density plasma target with prepolarized proton and tritium ions is considered for the proton acceleration. The prepolarized protons are initially accelerated by laser radiation pressure before injection and further acceleration in a bubblelike wakefield. The temporal dynamics of proton polarization is tracked via the Thomas-Bargmann-Michel-Telegdi equation and it is found that the proton polarization state can be altered by both the laser field and the magnetic component of the wakefield. The dependence of the proton acceleration and polarization on the ratio of the ion species is determined and it is found that the protons can be efficiently accelerated as long as their relative fraction is less than 20%, in which case the bubble size is large enough for the protons to obtain sufficient energy to overcome the bubble injection threshold.

Demonstration of repetitive energetic proton generation by ultra-intense laser interaction with a tape target

High power laser systems are an attractive driver for compact energetic ion sources. We demonstrate repetitive acceleration at 0.1 Hz of proton beams up to 40 MeV from a reeled tape target irradiated by ultra-high intensities up to 5 × 1021 Wcm−2 and laser energies ≈ 15 J using the J-KAREN-P laser system. We investigate the stability of the source and its behaviour with laser spot focal size. We compare the scaling of proton energy with laser energy to a recently developed analytical model, and also demonstrate that it is possible to reach energies up to 50 MeV on a single shot with a lower laser energy ≈ 10 J by using a thinner target, motivating development of high repetition targetry suitable for thinner targets.

Parametric study of ultra-intense laser interaction with uniform and nano-porous near-critical plasmas

Responses of the uniform near-critical plasma (UNCP) and nano-porous near-critical plasma (NPNCP) upon interaction with a short-intense laser have been scrutinized using two-dimensional (2D) particle-in-cell simulations. Maximum proton energy variation by the deposition of uniform and nano-porous layers in front of a solid target for a wide range of laser intensities (normalized amplitude a0 = 5–25) and average densities of the front layer ne = 0.3 − 3nc (where nc is the critical density) has been parametrically studied. It is found that the proton maximum energy for the front layers with sub-10 µm thicknesses is independent of the target porosity and density. However, in the relatively thick targets, the nano-porous structure decreases the laser energy absorption and, subsequently, the maximum proton energy compared to the uniform one. The results indicate that by employing UNCPs instead of NPNCPs, at the moderate laser intensity, the maximum proton energy reveals a 23% enhancement. This increment could be explained by rapid self-focusing of the laser pulse and dominant direct laser electron acceleration regime on the well-formed plasma channel in the UNCP layer. However, in the case of NPNCPs, the laser scattering from the plasma structure makes it less intense and more disordered, which influences the efficient laser energy coupling to the electrons.

Radiation reaction induced harmonics generation in ultra-relativistic intense laser interaction with plasmas

A new mechanism of harmonics generation is found in ultraintense laser interactions with plasmas, where radiation reaction effects dominate. The radiation reaction results in trapped electrons which oscillate in the laser field and emit high energy photons. Such emission event happens with a frequency spectrum containing double or high order components of the drive laser, inducing second and higher order harmonics emission. Although the intensity of such harmonics is too weak for practical applications, they could be used as a specific diagnostic to detect the radiation reaction effects in the upcoming experimental studies of the extremely intense laser pulse interacting with plasmas.

Simultaneous observation of ultrafast electron and proton beams in TNSA

The interaction of ultra-intense high-power lasers with solid-state targets has been largely studied for the past 20 years as a future compact proton and ion source. Indeed, the huge potential established on the target surface by the escaping electrons provides accelerating gradients of TV/m. This process, called target normal sheath acceleration, involves a large number of phenomena and is very difficult to study because of the picosecond scale dynamics. At the SPARC_LAB Test Facility, the high-power laser FLAME is employed in experiments with solid targets, aiming to study possible correlations between ballistic fast electrons and accelerated protons. In detail, we have installed in the interaction chamber two different diagnostics, each one devoted to characterizing one beam. The first relies on electro-optic sampling, and it has been adopted to completely characterize the ultrafast electron components. On the other hand, a time-of-flight detector, based on chemical-vapour-deposited diamond, has allowed us to retrieve the proton energy spectrum. In this work, we report preliminary studies about simultaneous temporal resolved measurements of both the first forerunner escaping electrons and the accelerated protons for different laser parameters.

MeV photoelectron spectrometer for ultraintense laser interactions with atoms and molecules.

Traditional laser-matter spectroscopy techniques fail to accurately analyze photoelectrons and ions from ultrahigh intensity studies with terawatt and petawatt laser systems. We present a magnetic deflection, photoelectron spectrometer for ultrahigh intensity laser interactions with atoms and molecules in the single atom/molecule limit. Spectrometer fabrication and calibration, and noise background are presented as well as example photoelectron spectra for argon and chloromethane over an energy range from 20 keV to 2 MeV.

Numerical studies on pair production in ultra-intense laser interaction with a thin solid-foil

A theoretical and numerical model of photon and electron–positron pair production in strong-field quantum electrodynamics (QED) is described. Two processes are contained in our QED theoretical model, one is photon emission in the interaction of ultra-intense laser with relativistic electron (or positron), and the other is pair production by a gamma-ray photon interacting with the laser field. This model has been included in a PIC/MCC simulation code named BUMBLEBEE 1D, which is used to simulate the laser plasma interaction. Using this code, the evolutions of electron–positron pair and gamma-ray photon production in ultra-intense laser interaction with aluminum foil target are simulated and analyzed. The simulation results revealed that more positrons are moved in the opposite direction to the incident direction of the laser under the charge separation field.

Simulations of bremsstrahlung emission in ultra-intense laser interactions with foil targets

Bremsstrahlung emission from interactions of short ultra-intense laser pulses with solid foils is studied using particle-in-cell (PIC) simulations. A module for simulating bremsstrahlung has been implemented in the PIC loop to self-consistently account for the dynamics of the laser–plasma interaction, plasma expansion, and the emission of gamma ray photons. This module made it possible to study emission from thin targets, where refluxing of hot electrons plays an important role. It is shown that the angular distribution of the emitted photons exhibits a four-directional structure with the angle of emission decreasing with the increase of the width of the target. Additionally, a collimated forward flash consisting of high energy photons has been identified in thin targets. The conversion efficiency of the energy of the laser pulse to the energy of the gamma rays rises with both the driving pulse intensity, and the thickness of the target. The amount of gamma rays also increases with the atomic number of the target material, despite a lower absorption of the driving laser pulse. The angular spectrum of the emitted gamma rays is directly related to the increase of hot electron divergence during their refluxing and its measurement can be used in experiments to study this process.

The suppression of radiation reaction and laser field depletion in laser-electron beam interaction

The effects of radiation reaction (RR) have been studied extensively by using the interaction of ultraintense lasers with a counter-propagating relativistic electron. At the laser intensity at the order of 1023 W/cm2, the effects of RR are significant in a few laser periods for a relativistic electron. However, a laser at such intensity is tightly focused and the laser energy is usually assumed to be fixed. Then, the signal of RR and energy conservation cannot be guaranteed. To assess the effects of RR in a tightly focused laser pulse and the evolution of the laser energy, we simulated this interaction with a beam of 109 electrons by means of a Particle-In-Cell method. We observe that the effects of RR are suppressed due to the ponderomotive force and accompanied by a non-negligible amount of laser field energy reduction. This is because the ponderomotive force prevents the electrons from approaching the center of the laser pulse and leads to an interaction at the weaker field region. At the same time, the laser energy is absorbed through ponderomotive acceleration. Thus, the kinetic energy of the electron beam has to be carefully selected such that the effects of RR become obvious.

Efficient monoenergetic proton beam from ultra-fast laser interaction with nanostructured targets

The broad energy spectrum of laser-accelerated proton beams is the most important difficulty associated with such particle sources on the way to future applications such as medical therapy, proton imaging, inertial fusion, and high-energy physics. The generation of proton beams with enhanced monoenergetic features through an ultra-intense laser interaction with optimized nanostructured targets is reported. Targets were irradiated by 40 fs laser pulses of intensity 5.5×1020 W cm−2 and wavelength 1 μm. The results of multi-parametric Particle-in-Cell calculations showed that proton beams with considerably reduced energy spread can be obtained by using the proposed nanostructured target. At optimized target dimensions, the proton spectrum was found to exhibit a narrow peak at about 63 MeV with a relative energy spread of ΔE/Epeak∼ 5% which is efficiently lower than what is expected for unstructured double layer targets (∼70%).

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.

Photon emission by bremsstrahlung and nonlinear Compton scattering in the interaction of ultraintense laser with plasmas

By implementing the bremsstrahlung with Monte Carlo algorithm into the particle-in-cell code, the bremsstrahlung and nonlinear Compton scattering can be studied simultaneously in comparison way in the laser plasma interactions. The simulations are performed for the laser of different intensities interacting with either low-$Z$ or high-$Z$ target. The relative strength of the two photon emission from bremsstrahlung and nonlinear Compton scattering are compared. The result shows that when an ultrastrong intensity laser interacting with a thin and relative high $Z$ target the nonlinear Compton scattering is dominant, however, when the laser intensity $I < 10^{22} \mathrm{W/cm^2}$, the photon emission contributed by bremsstrahlung is comparable to that from nonlinear Compton scattering. In this case the usual ignorable of bremsstrahlung need to be reconsidered.

Beam Line Design of Compact Laser Plasma Accelerator**Supported by the National Natural Science Foundation of China under Grant No 11575011, and the National Grand Instrument Project under Grant No 2012YQ030142.

A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical simulations, a beam line combining the advantages of quadrupole and analyzing magnets is designed to deliver proton beams with energy ranging from 1 to 44 MeV, energy spread within ±5% and protons per pulse. It turns out that the existence of space charge force of protons can be ignored for the increase of transverse and longitudinal envelopes even in the case of 10 protons in one pulse. To cope with the challenge to obtain a uniform distribution of protons at the final experiment target in laser acceleration, we manipulate the envelope beam waist in the Y direction to a proper position and obtain a relatively good distribution uniformity of protons with an energy spread of 0±5%.

Theoretical investigation of the ultra-intense laser interaction with plasma mirrors in radiation pressure dominant regime

At laser intensity in the range ~ 1022 −1023W/cm2, the radiation pressure starts to play a key role in the interaction of an intense electromagnetic wave with a dense plasma foil. Depending upon the incident laser intensity, polarization of the incident beam and also on the density of the thin plasma layer the mirror motion may be assumed to be uniform, accelerated, or oscillatory. A solid dense plasma slab, accelerated in the radiation pressure dominant (RPD) regime, can efficiently reflect a counter-propagating relativistically strong source pulse consisting of up-shifted frequency and high harmonics. In this RPD regime we present our numerical results for the frequency and brightness of the reflected radiation from a uniformly moving plasma mirror. Our numerical results show that for the appropriate laser and plasma parameters in the case there are approximately 8.03 × 1042 photons / (mm2 - mrad2 - sec.-0.1% bandwidth) in the energy range ~ 10keV. In the case when for the same parameters and ad = 300, λd = 0.8 μm, the brightness is found to be 3.27 × 1034 photons / (mm2 - mrad2 - sec. - 0.1% bandwidth) in the energy range ~100 keV.

Relativistic ponderomtive force acceleration of electrons in weak inhomogenous underdense plasma

In the paper, relativistic ponderomotive force acceleration of electrons in weak inhomogenous underdense plasma is investigated. The results shown that, in interaction of ultraintense laser and the plasma, due to the influence of relativistic ponderomotive force, laser beam presents self-focusing, and electrons are accelerated obviously. Initially, electron energy presents a fluctuation variation along the propagation distance, then the fluctuation will weaken gradually, finally it trend to a steady state and the electrons acquire a high net energy. Compared with homogeneous plasma, electron energy would increase and its vibration length becomes short in weak inhomogeneous underdense plasma. Moreover, the high-energy electrons can be easily extracted by the relativistic ponderomotive force, the results may offer some useful theoretical proof for the design of new accelerators.

Helium ions acceleration by ultraintense laser interactions with foil-gas target

Laser-driven helium ion source with multi-MeV energy has an important application in the field of fusion reactor material irradiation damage. At present, the generating of high energy helium ions by relativistic ultraintense laser interacting with helium gas jet is the main scheme of laser-driven helium ion source. However, so far, this scheme has been hard to generate the helium ion beam with the characteristics, i.e., it is forward and quasi-monoenergetic and has multi-MeV in energy and high yield. These characteristics of helium ion beam are important for studying the material irradiation damage. In this paper, we propose a new scheme in which an ultraintense laser interacting with foil-gas complex target is used to generate helium ions. With this method, we perform an experiment on XingGuang III laser facility which has three laser beams with different laser durations (nanosecond, picosecond and femtosecond). In our experiment, we use a picosecond laser beam. The wavelength of this laser beam is 1054 nm and its duration is 0.8 ps. We use an off-axis parabola mirror to focus the 100 J energy of this laser beam onto a focal spot of 25 m far away. The laser intensity reaches 51018 W/cm2. The foil-gas target is composed of a copper foil with 7 m in thickness and a helium gas nozzle which is behind the copper foil. The helium gas nozzle can generate a helium gas jet with a full ionization electron density of 51019/cm3. We use the Thomson Parabola Spectrometer to record the helium ion signals and the Electron Magnetic Spectrometer to diagnose the hot electron temperature. In the experiment, the laser pulse interacts with the front surface of the copper foil and generates lots of hot electrons. These hot electrons result in the expansion of the rear surface of the copper foil. The expanding plasma accelerates the helium ions behind the copper foil. The experimental results show that the obtained helium ions are forward and quasi-monoenergetic (the peak energy is 2.7 MeV), and the total energy of the helium ions whose energies are all higher than 0.5 MeV is about 1.1 J/sr, and correspondingly the yield of helium ions is about 1013/sr. The helium ion spectrum and hot electron temperature given by particle in cell (PIC) simulation with using the experimental parameters are consistent with the experimental results. In addition, the PIC simulations also show that helium ions are accelerated by target normal sheath acceleration and collisionless shock acceleration-like mechanisms, and the maximum helium ion energy is proportional to the hot electron temperature.