PW Laser Research Articles

Ultra-bright γ-ray emission by using PW laser irradiating solid target obliquely

By using intense laser irradiating a micro plane target obliquely, an enhanced γ-ray source is generated. Due to the superposition of the incident and the reflected laser pulses, electron bunches with density of ∼300nc are extracted and accelerated. When these electron bunches separate from the edge of the target that the laser is leaving, they co-propagate with the laser field and emit dense γ-rays simultaneously. Simulation results show that the emitted γ photons are 253nc dense with an averaged energy of ∼12 MeV. The yield of γ photons is ∼7 × 1012, achieving a high brightness of ∼4 × 1023 . Influences of the laser intensity and the incident angle on the γ-rays emission are discussed. The γ-ray yield, the conversion efficiency from the laser to the γ-rays and the averaged γ-ray energy are increasing when irradiating a higher intensity laser. With the increasing of the incident angle, the peak photon density increases when the angle is smaller than 20° and then drops to a stable value, while the divergence decreases when the incident angle is smaller than ∼16° and then increases.

Collimated GeV attosecond electron–positron bunches from a plasma channel driven by 10 PW lasers

High-energy positrons and bright γ-ray sources are of great importance both in fundamental research and for practical applications. However, collimated GeV electron–positron pair jets and γ-ray flashes are still rarely produced in the laboratory. Here, we demonstrate that by irradiating a near-critical-density plasma channel with two 10 PW-scale laser pulses, highly directional GeV electron–positron pairs and bright γ-ray beams can be efficiently generated. Three-dimensional particle-in-cell simulations show the formation of GeV positron jets with high density (8×1021/ cm3), attosecond duration (400 as), and a divergence angle of 14°. Additionally, ultrabright [2×1025 photons s−1 mm−2 mrad−2 (0.1% bandwidth)−1] collimated attosecond (370 as) γ-ray flashes with a laser energy conversion efficiency of 5.6% are emitted. These features show the significant advantage of using a plasma channel as compared with a uniform plasma and thus open up new possibilities for a wide variety of applications.

Petawatt and exawatt class lasers worldwide

In the 2015 review paper ‘Petawatt Class Lasers Worldwide’ a comprehensive overview of the current status of high-power facilities of ${>}200~\text{TW}$ was presented. This was largely based on facility specifications, with some description of their uses, for instance in fundamental ultra-high-intensity interactions, secondary source generation, and inertial confinement fusion (ICF). With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification (CPA), which made these lasers possible, we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed. We are now in the era of multi-petawatt facilities coming online, with 100 PW lasers being proposed and even under construction. In addition to this there is a pull towards development of industrial and multi-disciplinary applications, which demands much higher repetition rates, delivering high-average powers with higher efficiencies and the use of alternative wavelengths: mid-IR facilities. So apart from a comprehensive update of the current global status, we want to look at what technologies are to be deployed to get to these new regimes, and some of the critical issues facing their development.

Polarized proton beams from laser-induced plasmas

We report on the concept of an innovative source to produce polarized proton/deuteron beams of a kinetic energy up to several GeV from a laser-driven plasma accelerator. Spin effects have been implemented into the particle-in-cell (PIC) simulation code VLPL (Virtual Laser Plasma Lab) to make theoretical predictions about the behavior of proton spins in laser-induced plasmas. Simulations of spin-polarized targets show that the polarization is conserved during the acceleration process. For the experimental realization, a polarized HCl gas-jet target is under construction using the fundamental wavelength of a Nd:YAG laser system to align the HCl bonds and simultaneously circularly polarized light of the fifth harmonic to photo-dissociate, yielding nuclear polarized H atoms. Subsequently, their degree of polarization is measured with a Lamb-shift polarimeter. The final experiments, aiming at the first observation of a polarized particle beam from laser-generated plasmas, will be carried out at the 10 PW laser system SULF at SIOM, Shanghai.

Theoretical Discourse on Producing High Temporal Yields of Nuclear Excitations in Cosmogenic 26Al with a PW Laser System: The Pathway to an Astrophysical Earthbound Laboratory

The development of the 10 PW laser system at the Extreme Light Infrastructure is a crucial step towards the realization of an astrophysical Earthbound laboratory. The interaction of high-power laser pulses with matter results in ultrashort (fs-ps) pulses of 10s of MeV ions and radiation that can create plasma and induce nuclear reactions therein. Due to the high fluxes of reaction-driving beam pulses, high yields of radioactive target nuclei in their ground and excited states can be provided in situ on short time scales. Cosmogenic 26Al, which is of pronounced astrophysical interest, is a prime candidate for evaluating these new experimental possibilities. We describe how, for a short duration of Δ t ∼ 200 ps , laser-driven protons with energies above E p ∼ 5 MeV can induce the compound nucleus reaction 26Mg(p, n)26Al leading to high and comparable yields of the three lowest-lying states in 26Al including the short-lived, t 1 / 2 = 1.20 ns state at 417 keV. In the aftermath of the reaction, for a short duration of t ∼ ns , the yield ratios between the ground and the two lowest-lying excited states will resemble those present at thermodynamic equilibrium at high temperatures, thus mimicking high 26Al entropies in cold environments. This can be seen as a possible first step towards an investigation of the interplay between those states in plasma environments. Theory suggests an intricate coupling of the ground state 26Alg.s. and the first excited isomer 26mAl via higher-lying excitations such as the J = 3 + state at 417 keV resulting in a dramatic reduction of the effective lifetime of 26Al which will influence the isotope’s abundance in our Galaxy.

Wavefront evolution of the signal beam in Ti:sapphire chirped pulse amplifier**Project supported by the National Natural Science Foundation of China (Grant No. 61775223) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB1603).

We studied the evolution of wavefront aberration (WFA) of a signal beam during amplification in a Ti:sapphire chirped pulse amplification (CPA) system. The results verified that the WFA of the amplified laser beam has little relation with the change of the pump beam energies. Transverse parasitic lasing that might occur in CPA hardly affects the wavefront of the signal beam. Thermal effects were also considered in this study, and the results show that the thermal effect cumulated in multiple amplification processes also has no obvious influence on the wavefront of the signal beam for a single-shot frequency. The results presented in this paper confirmed experimentally that the amplification in a Ti:sapphire CPA system has little impact on the WFA of the signal beam and it is very helpful for wavefront correction of single-shot PW and multi-PW laser systems based on Ti:sapphire.

Analysis on the longitudinal field strength formed by tightly-focused radially-polarized femtosecond petawatt laser pulse.

Tight focusing of radially- or azimuthally-polarized electromagnetic waves becomes attractive because of the strong field generation in the longitudinal direction. In this paper, we investigate the strength of longitudinal electric field when a radially-polarized femtosecond PW laser pulse is tightly focused by a parabolic surface. From the calculation using the vector diffraction approach, it has been shown that the highest strength of 2.2 × 1013 V/cm can be reached for the longitudinal field with a radially-polarized 11.2-fs, 11.2-J uniform-beam-profile laser pulse. The difference in the strength of longitudinal field with different beam profile and the spectrum of a laser pulse has been also carefully examined. The propagation of a laser spot has been simulated under an extremely-tight-focusing condition (0.25 in terms of f-number) and an achievable field strength for a standing longitudinal field has been examined by colliding two radially-polarized fs PW-level laser pulses.

339 J high-energy Ti:sapphire chirped-pulse amplifier for 10 PW laser facility.

We report on the laser pulse output of 339J centered at 800nm from a chirped-pulse amplification (CPA) Ti:sapphire laser system at the Shanghai Superintense Ultrafast Laser Facility. The experimental results demonstrated that the parasitic lasing as well as the transverse amplified spontaneous emission of the homemade 235-mm-diameter Ti:sapphire final amplifier were suppressed successfully via the temporal dual-pulse pumped scheme and the index-matching liquid cladding technique. The maximum pump-to-signal conversion efficiency of 32.1% was measured for the final amplifier. With a compressor transmission efficiency of 64% and a compressed pulse duration of 21fs obtained for the sample light at a lower energy level, this laser system could potentially generate a compressed laser pulse with a peak power of 10.3PW. The experimental results represent significant progress with respect to the CPA laser.

Dose calculations in a cell monolayer for high-throughput irradiation with proton beams generated by PW lasers for space applications

One of the specific properties of laser-driven radiation is a broadband energy spectrum, which is also a feature of the space radiation fields. This property can be used in materials science studies or radiobiology experiments to simulate the energy spectrum of space radiation exposures in a ground-based laboratory. However, the differences in effects between the higher dose rates of laser generated radiation and the lower dose rates of space radiation have to be investigated in separate, prior studies. A design for a high-throughput irradiation experiment and the associated Monte Carlo dose calculations for a broadband energy proton beam depositing energy in a cell monolayer is presented. Dose control and dose uniformity in the cell monolayer was achieved in the simulations using a variable thickness Ni attenuator. A set of target doses from 0.2 Gy to 4 Gy was obtained and dose uniformity was optimized to less than 4% variability. This work opens the possibility of single or multiple exposures, controllable, high-throughput irradiation experiments on biological samples or materials, using broadband energy particle beams generated by lasers, with relevance for space applications.

Improvement of the focusing ability by double deformable mirrors for 10-PW-level Ti: sapphire chirped pulse amplification laser system.

Double deformable mirrors (DMs) with different actuator densities are cascaded to optimize the wavefront aberrations to improve the focus intensity of the Shanghai super-intense ultrafast laser facility (SULF), which plans to generate 10 PW laser pulse. The beam aberrations near the focal spot are corrected from 0.556 um to 0.112 um in RMS by a 300-mm DM with a large stroke installed after the compressor. After then, it is further optimized to 0.041 um using a 130-mm DM with a high spatial resolution working after the main amplifier. The corrected beam is focused to 2.75 × 2.87 um2 at the full width at half maximum (FWHM) with an f/2.5 off-axis parabolic mirror (OAP), which contains approximately 27.69% energy. A peak intensity of 2 × 1022 W/cm2 is achieved at the output of 5.4 PW, and it could exceed 1023 W/cm2 in the SULF 10 PW laser facility using an f/1.8 OAP.

Effects of radiation reaction on laser proton acceleration in the bubble regime

We investigated laser proton acceleration in the bubble regime when radiation reaction (RR) effects become non-negligible. By using particle-in-cell simulations, it is shown that in addition to the bubble field, the RR trapped electrons form another charge-separation field, leading to fast acceleration of the background protons at the early stage. However, for insufficient laser intensities, the reduced bubble field and acceleration length originating from the laser depletion by gamma-photon emission do not allow for trapping and further acceleration of the protons. The final proton energy is significantly smaller than the one when RR is not considered. This effect can be mitigated by tuning the peak laser intensity (hence the pulse duration) for certain laser energy and plasma density. By increasing the laser intensity (shortening the pulse duration), we found that protons pre-accelerated by the RR-induced charge separation field gain enough momenta such that they are picked-up by the bubble field and continuously accelerated. In this case, the bubble field is also strong enough to trap protons located in the bubble front. Eventually, two groups of protons are effectively accelerated, resulting in a bump in the proton spectrum. These results can be used to design the future proton acceleration experiments in upcoming 100 PW laser facilities.

Spectral pulse shaping of a 5 Hz, multi-joule, broadband optical parametric chirped pulse amplification frontend for a 10 PW laser system.

We present a broadband optical parametric chirped pulse amplification (OPCPA) system delivering 4J pulses at a repetition rate of 5Hz. It will serve as a frontend for the 1.5kJ, <150 fs, 10PW laser beamline currently under development by a consortium of National Energetics and Ekspla. The spectrum of the OPCPA system is precisely controlled by arbitrarily generated waveforms of the pump lasers. To fully exploit the high flexibility of the frontend, we have developed a 1D model of the system and an optimization algorithm that predicts suitable pump waveform settings for a desired output spectrum. The OPCPA system is shown to have high efficiency, a high-quality top-hat beam profile, and an output spectrum demonstrated to be shaped consistently with the theoretical model.

Accuracy measurements of large-aperture femtosecond laser pulses in multipetawatt laser facility

A model is presented to analyze the effect of chromatic aberration of the spatial filter lenses on the compressed pulse duration. The parameters of the SHENGUANG (SG) II 5 PW laser system are demonstrated as examples. The numerical simulation results show that the compressed pulse duration with full aperture is increased by 3 times compared with the ideal compressed pulse duration in the case of chromatic aberration of the lenses. The result of full-aperture measurement at the SG II 5 PW laser system is in good agreement with the theoretical calculation. In the case of full-aperture measurement, the shortest compressed pulse duration is obtained by adjusting the compressor to compensate for the phase aberration introduced by the chromatic aberration of the spatial filter lenses at the SG II 5 PW laser facility. These results are helpful in finding an optimized pulse duration, considering the chromatic aberration of lenses in the femtosecond laser facility, and in obtaining a deep understanding of the effect of chromatic aberration of lenses on the pulse contrast ratio and pulse duration in ultrashort laser systems.

Efficient injection of radiation-pressure-accelerated sub-relativistic protons into laser wakefield acceleration based on 10 PW lasers

We propose a hybrid laser-driven ion acceleration scheme using a combination target of a solid foil and a density-tailored background plasma. In the first stage, a sub-relativistic proton beam can be generated by radiation pressure acceleration in intense laser interaction with the solid foil. In the second stage, this sub-relativistic proton beam is further accelerated by the laser wakefield driven by the same laser pulse in a near-critical-density background plasma with decreasing density profile. The propagating velocity of the laser front and the phase velocity of the excited wakefield wave are effectively lowered at the beginning of the second stage. By decreasing the background plasma density gradually from near critical density along the laser propagation direction, the wake travels faster and faster, while it accelerates the protons. Consequently, the dephasing between the protons and the wake is postponed and an efficient wakefield proton acceleration is achieved. This hybrid laser-driven proton acceleration scheme can be realized by using ultrashort laser pulses at the peak power of 10 PW for the generation of multi-GeV proton beams.

High-contrast high-intensity repetitive petawatt laser.

We report the generation of 63J of broadband pulse energies at 0.1Hz from the J-KAREN-P laser, which is based on an OPCPA/Ti:sapphire hybrid architecture. Pulse compression down to 30fs indicates a peak power of over 1 PW. High temporal contrast of 1012 prior to the main pulse has been demonstrated with 10J output energy. High intensities of 1022 W/cm2 on target by focusing a 0.3 PW laser with an f/1.3 off-axis parabolic mirror have been achieved. Fundamental processes of laser matter interaction at over 1022 W/cm2 intensities belong to a new branch of science that will be the principal research task of our infrastructure.

The generation of collimated γ-ray pulse from the interaction between 10 PW laser and a narrow tube target

A scheme to radiate a highly collimated γ-ray pulse is proposed through the interaction between an ultra-intense laser pulse and a narrow tube target. The γ-ray pulse, with high conversion efficiency, can be generated as a result of electron acceleration in a longitudinal electric field. In a Particle-in-Cell simulation with a 10-PW laser, 18% of the laser energy is transferred into the forward γ-rays in a divergence angle less than 3°. It is also found that such a highly collimated γ-ray pulse can be produced with a large range of tube diameters and laser intensities. This scheme could be realized in experiment with the coming 10-PW class lasers in the near future.

Quasi-monoenergetic multi-GeV electron acceleration by optimizing the spatial and spectral phases of PW laser pulses

Generation of high-quality electron beams from laser wakefield acceleration requires optimization of initial experimental parameters. We present here the dependence of accelerated electron beams on the temporal profile of a driving PW laser, the density, and length of an interacting medium. We have optimized the initial parameters to obtain 2.8 GeV quasi-monoenergetic electrons which can be applied further to the development of compact electron accelerators and radiations sources.

Theory and simulations of radiation friction induced enhancement of laser-driven longitudinal fields

We consider the generation of a quasistatic longitudinal electric field by intense laser pulses propagating in a transparent plasma with radiation friction (RF) taken into account. For both circular and linear polarization of the driving pulse we develop a 1D analytical model of the process, which is valid in a wide range of laser and plasma parameters. We define the parameter region where RF results in an essential enhancement of the longitudinal field. The amplitude and the period of the generated longitudinal wave are estimated and optimized. Our theoretical predictions are confirmed by 1D and 2D PIC simulations. We also demonstrate numerically that RF should substantially enhance the longitudinal field generated in a plasma by a 10 PW laser such as ELI Beamlines.

Ultrashort PW laser pulse interaction with target and ion acceleration

We present the experimental results on ion acceleration by petawatt femtosecond laser solid interaction and explore strategies to enhance ion energy. The irradiation of micrometer thick (0.2–6.0 μm) Al foils with a virtually unexplored intensity regime (8 × 1019 W/cm2 – 1× 1021 W/cm2) resulting in ion acceleration along the rear and the front surface target normal direction is investigated. The maximum energy of protons and carbon ions, obtained at optimized laser intensity condition (by varying laser energy or focal spot size), exhibit a rapid intensity scaling as I0.8 along the rear surface target normal direction and I0.6 along the front surface target normal direction. It was found that proton energy scales much faster with laser energy rather than the laser focal spot size. Additionally, the ratio of maximum ion energy along the both directions is found to be constant for the broad range of target thickness and laser intensities.A proton flux is strongly dominated in the forward direction at relatively low laser intensities. Increasing the laser intensity results in the gradual increase in the backward proton flux and leads to almost equalization of ion flux in both directions in the entire energy range. These experimental findings may open new perspectives for applications.

The effect of combined pulsed wave low-level laser therapy and mesenchymal stem cell-conditioned medium on the healing of an infected wound with methicillin-resistant Staphylococcal aureus in diabetic rats.

This study aims to investigate the combined effects of Pulsed wave low-level laser therapy (PW LLLT) and human bone marrow mesenchymal stem cell-conditioned medium (hBM-MSC-CM) on the microbial flora and tensiometrical properties of an infected wound model with methicillin-resistant staphylococcal aureus (MRSA) in an experimental model for Type 1 diabetes mellitus (TIDM). TIDM was induced in rats by streptozotocin (STZ). One full-thickness excision was made on the backs of the rats. Next, the rats were divided into the following groups: Group 1 was the control (placebo) group; Group 2 received hBM-MSCs-CM four times; Group 3 were laser PWLLLT (890 nm, 80 Hz, 0.2 J/cm2 ); and Group 4 received hBM-MSCs-CM +LASER. Wounds were infected with MRSA. Microbiological examinations were performed on days 4, 7, and 15. Tensiometerical examinations were carried out on the 15th day. One-way analysis of variance showed that laser and CM alone and/or in combination significantly increases the tensiomerical properties of the repaired wounds compared with control wounds. A combination of PW laser and CM was statistically more effective than other treated groups. Two-way analysis of variance showed that laser and CM alone and/or in combination significantly decreases the colony-forming units (CFUs) compared with the control group. The application of hBM-MSC-CM and PWlaser alone and/or together significantly accelerates the wound-healing process in MRSA-infected cutaneous wounds in TI DM in rats. Additionally, a combined application of hBM-MSC-CM and PWlaser demonstrates a synergistic effect on the wound-healing process in MRSA-infected cutaneous wounds in Type I DM rats.