Efficient Proton Acceleration Research Articles

Efficient laser-driven proton acceleration from a petawatt contrast-enhanced second harmonic mixed-glass laser system

Efficient laser-driven plasma acceleration of ion beams requires precision control of the target–plasma profile, which is crucial to optimize the laser energy transfer. Along the laser propagation direction, this can be achieved by tailoring the temporal structure of the laser pulse. We show for the first time that frequency-doubling of a short pulse (hundreds-femtosecond range) petawatt-class mixed-glass laser system, which results in temporal intensity contrast enhancement, enables surface and volumetric laser–energy coupling, and the acceleration of proton beams from few-nanometer-thick foil targets. Experimentally, maximum ion energies and laser-to-proton energy conversion efficiencies were found to be both maximized at optimum laser and target conditions manifested when the normalized target density nearly equalizes the normalized laser vector potential, which is in agreement with theory and simulations. These signatures are recognized as a unique indication of the interaction between ultra-intense laser pulses with high temporal intensity contrast and ultra-thin nanometer-scale targets. Transverse modulations of accelerated proton beams in the form of bubble- and ring-like structures measured in the thinnest targets provide additional evidence of volumetric laser-driven particle acceleration regimes and transitional features in ultra-thin foil targets specific to laser–plasma interactions characterized by a high temporal intensity contrast. These results open avenues in the generation of high contrast laser pulses from short-pulse-femtosecond petawatt mixed-glass laser systems and demonstrate the feasibility of this technique for applications requiring high laser intensity contrast with high efficiency.

Early-time γ-ray constraints on cosmic-ray acceleration in the core-collapse SN 2023ixf with the Fermi Large Area Telescope

Context. While supernova remnants (SNRs) have been considered the most relevant Galactic cosmic ray (CR) accelerators for decades, core-collapse supernovae (CCSNe) could accelerate particles during the earliest stages of their evolution and hence contribute to the CR energy budget in the Galaxy. Some SNRs have indeed been associated with TeV γ-rays, yet proton acceleration efficiency during the early stages of an SN expansion remains mostly unconstrained. Aims. The multi-wavelength observation of SN 2023ixf, a Type II supernova (SN) in the nearby galaxy M 101 (at a distance of 6.85 Mpc), opens the possibility to constrain CR acceleration within a few days after the collapse of the red super-giant stellar progenitor. With this work, we intend to provide a phenomenological, quasi-model-independent constraint on the CR acceleration efficiency during this event at photon energies above 100 MeV. Methods. We performed a maximum-likelihood analysis of γ-ray data from the Fermi Large Area Telescope up to one month after the SN explosion. We searched for high-energy, non-thermal emission from its expanding shock, and estimated the underlying hadronic CR energy reservoir assuming a power-law proton distribution consistent with standard diffusive shock acceleration. Results. We do not find significant γ-ray emission from SN 2023ixf. Nonetheless, our non-detection provides the first limit on the energy transferred to the population of hadronic CRs during the very early expansion of a CCSN. Conclusions. Under reasonable assumptions, our limits would imply a maximum efficiency on the CR acceleration of as low as 1%, which is inconsistent with the common estimate of 10% in generic SNe. However, this result is highly dependent on the assumed geometry of the circumstellar medium, and could be relaxed back to 10% by challenging spherical symmetry. Consequently, a more sophisticated, inhomogeneous characterisation of the shock and the progenitor’s environment is required before establishing whether or not Type II SNe are indeed efficient CR accelerators at early times.

High-energy Radiation and Ion Acceleration in Three-dimensional Relativistic Magnetic Reconnection with Strong Synchrotron Cooling

We present the results of 3D particle-in-cell simulations that explore relativistic magnetic reconnection in pair plasma with strong synchrotron cooling and a small mass fraction of nonradiating ions. Our results demonstrate that the structure of the current sheet is highly sensitive to the dynamic efficiency of radiative cooling. Specifically, stronger cooling leads to more significant compression of the plasma and magnetic field within the plasmoids. We demonstrate that ions can be efficiently accelerated to energies exceeding the plasma magnetization parameter, ≫σ, and form a hard power-law energy distribution, f i ∝ γ −1. This conclusion implies a highly efficient proton acceleration in the magnetospheres of young pulsars. Conversely, the energies of pairs are limited to either σ in the strong cooling regime or the radiation burnoff limit, γ syn, when cooling is weak. We find that the high-energy radiation from pairs above the synchrotron burnoff limit, ε c ≈ 16 MeV, is only efficiently produced in the strong cooling regime, γ syn < σ. In this regime, we find that the spectral cutoff scales as ε cut ≈ ε c (σ/γ syn) and the highest energy photons are beamed along the direction of the upstream magnetic field, consistent with the phenomenological models of gamma-ray emission from young pulsars. Furthermore, our results place constraints on the reconnection-driven models of gamma-ray flares in the Crab Nebula.

High-energy neutrinos from merging stellar-mass black holes in active galactic nuclei accretion disc

ABSTRACT A population of binary stellar-mass black hole (BBH) mergers are believed to occur embedded in the accretion disc of active galactic nuclei (AGNs). In this Letter, we demonstrate that the jets from these BBH mergers can propagate collimatedly within the disc atmosphere along with a forward shock and a reverse shock forming at the jet head. Efficient proton acceleration by these shocks is usually expected before the breakout, leading to the production of TeV−PeV neutrinos through interactions between these protons and electron-radiating photons via photon–meson production. AGN BBH mergers occurring in the outer regions of the disc are more likely to produce more powerful neutrino bursts. Taking the host AGN properties of the potential GW190521 electromagnetic (EM) counterpart as an example, one expects ≳1 neutrino events detectable by IceCube if the jet is on-axis and the radial location of the merger is R ≳ 105Rg, where Rg is the gravitational radius of the supermassive BH. Neutrino bursts from AGN BBH mergers could be detected by IceCube following the observation of gravitational waves (GWs), serving as precursor signals before the detection of EM breakout signals. AGN BBH mergers are potential target sources for future joint GW, neutrino, and EM multi-messenger observations.

Optimum design of double-layer target for proton acceleration by ultrahigh intense femtosecond lasers considering relativistic rising edge.

Advances in laser technology have led to ever-increasing laser intensities. As a result, in addition to the amplified spontaneous emission and pedestal, it has become necessary to accurately treat the relativistic rising edge component. This component has not needed much consideration in the past because of its not relativistic intensity. In the previous study, a thin contamination layer was blown away from the target by the rear sheath field due to the relativistic rising edge component, and the target bulk was accelerated by the sheath field due to the main pulse. These indicated that the proton acceleration is not efficient in the target normal sheath acceleration by the ultrahigh intense femtosecond laser if the proton-containing layer is as thin as the contamination layer. Here we employ a double-layer target, making the second (rear) layer thick enough not to be blown away by the rising edge, so that the second layer is accelerated by the main pulse. The first layer is composed of heavy ions to reduce the total thickness of the target for efficient proton acceleration. We investigate an optimal design of a double-layer target for proton acceleration by the ultrahigh intense femtosecond laser considering the relativistic rising edge using two-dimensional particle-in-cell simulations. We also discuss how to optimize the design of such a double-layer target and find that it can be designed with two conditions: the first layer is not penetrated by hole boring, and the second layer is not blown away by the rising edge.

Efficient ion acceleration driven by a Laguerre–Gaussian laser in near-critical-density plasma

Laser-driven ion accelerators have the advantages of compact size, high density, and short bunch duration over conventional accelerators. Nevertheless, it is still challenging to generate ion beams with quasi-monoenergetic peak and low divergence in experiments with the current ultrahigh intensity laser and thin target technologies. Here we propose a scheme that a Laguerre–Gaussian laser irradiates a near-critical-density (NCD) plasma to generate a quasi-monoenergetic and low-divergence proton beam. The Laguerre–Gaussian laser pulse in an NCD plasma excites a moving longitudinal electrostatic field with a large amplitude, and it maintains the inward bowl-shape for dozens of laser durations. This special distribution of the longitudinal electrostatic field can simultaneously accelerate and converge the protons. Our particle-in-cell (PIC) simulation shows that the efficient proton acceleration can be realized with the Laguerre–Gaussian laser intensity ranging from 3.9 × 1021 W⋅cm−2–1.6 × 1022 W⋅cm−2 available in the near future, e.g., a quasi-monoenergetic proton beam with peak energy ∼ 115 MeV and divergence angles less than 5° can be generated by a 5.3 × 1021 W⋅cm−2 pulse. This work could provide a reference for the high-quality ion beam generation with PWclass laser systems available recently.

The Role of Jet–Cocoon Mixing, Magnetization, and Shock Breakout in Neutrino and Cosmic-Ray Emission from Short Gamma-Ray Bursts

Abstract We perform general relativistic magnetohydrodynamic and relativistic magnetohydrodynamic simulations of weakly and highly magnetized gamma-ray burst (GRB) jets propagating in binary neutron star (BNS) merger ejecta. Using the simulations, we first find that mixing between the jet and cocoon, which is present in all types of jets, inhibits the formation of subphotospheric collisionless shocks. However, we show that a mild magnetization may lead to the formation of collisionless subshocks, which allow efficient proton acceleration. We consider shear acceleration and diffusive shock acceleration at collimation shocks, internal shocks, shock breakout, and external shocks to provide the first estimate for neutrino and cosmic-ray (CR) signals from self-consistent simulations of GRBs in BNS mergers. We find that short GRBs do not produce detectable neutrino signals with current-day facilities. Shock breakout yields ∼10 PeV neutrinos at viewing angles ∼20°, independent of the jet magnetization. However, a neutrino signal from shock breakout is well below the detection limits of current detectors. Such a signal would allow a coincident neutrino–γ-ray detection, providing a testable prediction for shock breakout as a neutrino production site. Using the numerical modeling that fits GW 170817 afterglow emission, we find that blast waves in BNS mergers can account for 5%–10% of the Galactic CR luminosity in the PeV–EeV energy range. Based on these estimates, the observed level of CR anisotropy places a constraint on the distance of the latest Galactic BNS merger to ≲3 kpc.

Shock Drift Acceleration of Ions in an Interplanetary Shock Observed by MMS

Abstract An interplanetary (IP) shock wave was recorded crossing the Magnetospheric Multiscale constellation on 2018 January 8. Plasma measurements upstream of the shock indicate efficient proton acceleration in the IP shock ramp: 2–7 keV protons are observed upstream for about three minutes (∼8000 km) ahead of the IP shock ramp, outrunning the upstream waves. The differential energy flux of 2–7 keV protons decays slowly with distance from the ramp toward the upstream region (dropping by about half within 8 Earth radii from the ramp) and is lessened by a factor of about four in the downstream compared to the ramp (within a distance comparable to the gyroradius of ∼keV protons). Comparison with test-particle simulations has confirmed that the mechanism accelerating the solar wind protons and injecting them upstream is classical Shock Drift Acceleration (SDA). This example of observed proton acceleration by a low-Mach, quasi-perpendicular shock may be applicable to astrophysical contexts, such as supernova remnants or the acceleration of cosmic rays.

High-energy neutrino flux from individual blazar flares

ABSTRACT Motivated by the recently reported evidence of an association between a high-energy neutrino and a γ-ray flare from the blazar TXS 0506+056, we calculate the expected high-energy neutrino signal from past, individual flares, from 12 blazars, selected in declinations favourable for detection with IceCube. To keep the number of free parameters to a minimum, we mainly focus on BL Lac objects and assume the synchrotron self-Compton mechanism produces the bulk of the high-energy emission. We consider a broad range of the allowed parameter space for the efficiency of proton acceleration, the proton content of BL Lac jets, and the presence of external photon fields. To model the expected neutrino fluence, we use simultaneous multiwavelength observations. We find that in the absence of external photon fields and with jet proton luminosity normalized to match the observed production rate of ultrahigh-energy cosmic rays, individual flaring sources produce a modest neutrino flux in IceCube, $N^{\mathrm{IC,10 \,yr}}_{\nu _{\mu },{\mathrm{\gt 100~TeV}}} \lesssim 10^{-3}$ muon neutrinos with energy exceeding 100 TeV, stacking 10 yr of flare periods selected in the &amp;gt;800 MeV Fermi energy range, from each source. Under optimistic assumptions about the jet proton luminosity and in the presence of external photon fields, we find that the two most powerful sources in our sample, AO 0235+164, and OJ 287, would produce, in total, $N^{\mathrm{IC \times 10,10 \,yr}}_{\nu _{\mu }, \rm all~flares, \gt 100~TeV} \approx 3$ muon neutrinos during Fermi flaring periods, in future neutrino detectors with total instrumented volume ∼10 times larger than IceCube, or otherwise, constrain the proton luminosity of blazar jets.

Optimization of laser-nanowire target interaction to increase the proton acceleration efficiency

We report on improved proton acceleration from the interaction of a short pulse high intensity laser (>1020 Wcm−2) with nano-engineered targets. Planar targets (from 7 to 20 μm) with protruding gold nanowires having different total areal densities, lengths, and diameters, ranging from 3% to 60% of the size of the laser focal spot were used during an experimental campaign at the 3 J, 30 fs HERCULES laser facility. The results show the importance of the average number of nanowires per focal spot, N, on laser energy absorption. We show that the proton acceleration is significantly improved by using 1 nanowires per focal spot. Detailed analysis indicates that 1 nanowire per focal spot optimizes the interaction between laser pulse and nanowires, in which the wings of the pulse pull out electrons from the wires forming a plasma with density that allows for deep penetration of the laser pulse into the array. When moving away from this optimum in both directions, N ≪ 1 and N ≫ 1, the laser pulse-nanowire coupling is either too weak or unfavorable for obtaining maximum proton energy. Proton spectra are compared to simulations using 2D-3V particle-in-cell code which reproduces the experimental data with good agreement.

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.

Efficient proton acceleration from a 3 TW table-top laser interacting with submicrometric mass-produced solid targets

Thin layer membranes with controllable features and material arrangements are often used as target materials for laser driven particle accelerators. Reduced cost, large scale fabrication of such membranes with high reproducibility, and good stability are central for the efficient production of proton beams. These characteristics are of growing importance in the context of advanced laser light sources where increased repetition rates boost the need for consumable targets with design and properties adjusted to study the different phenomena arising in ultra-intense laser-plasma interaction. We present the fabrication of sub-micrometric thin-layer gold or aluminum membranes in a silicon wafer frame by using nano/micro-electro-mechanical-system (N/MEMS) processing which are suitable for rapid patterning and machining of many samples at the same time and allowing for high-throughput production of targets for laser-driven acceleration. Obtained targets were tested for laser-proton acceleration through the Target Normal Sheath Acceleration mechanism (TNSA) in a series of experiments carried out on a purpose-made table-top Ti:Sa running at 3 TW peak power and 10 Hz diode pump rate with a contrast over ASE of 108.

Near-100 MeV protons via a laser-driven transparency-enhanced hybrid acceleration scheme

The range of potential applications of compact laser-plasma ion sources motivates the development of new acceleration schemes to increase achievable ion energies and conversion efficiencies. Whilst the evolving nature of laser-plasma interactions can limit the effectiveness of individual acceleration mechanisms, it can also enable the development of hybrid schemes, allowing additional degrees of control on the properties of the resulting ion beam. Here we report on an experimental demonstration of efficient proton acceleration to energies exceeding 94 MeV via a hybrid scheme of radiation pressure-sheath acceleration in an ultrathin foil irradiated by a linearly polarised laser pulse. This occurs via a double-peaked electrostatic field structure, which, at an optimum foil thickness, is significantly enhanced by relativistic transparency and an associated jet of super-thermal electrons. The range of parameters over which this hybrid scenario occurs is discussed and implications for ion acceleration driven by next-generation, multi-petawatt laser facilities are explored.

Dominance of hole-boring radiation pressure acceleration regime with thin ribbon of ionized solid hydrogen

Laser-driven proton acceleration from novel cryogenic hydrogen target of the thickness of tens of microns irradiated by multiPW laser pulse is investigated here for relevant laser parameters accessible in near future. It is demonstrated that the efficiency of proton acceleration from relatively thick hydrogen solid ribbon largely exceeds the acceleration efficiency for a thinner ionized plastic foil, which can be explained by enhanced hole boring (HB) driven by laser ponderomotive force in the case of light ions and lower target density. Three-dimensional particle-in-cell (PIC) simulations of laser pulse interaction with relatively thick hydrogen target show larger energies of protons accelerated in the target interior during the HB phase and reduced energies of protons accelerated from the rear side of the target by quasistatic electric field compared with the results obtained from two-dimensional PIC calculations. Linearly and circularly polarized multiPW laser pulses of duration exceeding 100 fs show similar performance in terms of proton acceleration from both the target interior as well as from the rear side of the target. When ultrashort pulse (∼30 fs) is assumed, the number of accelerated protons from the target interior is substantially reduced.

High Energy electron and proton acceleration by circularly polarized laser pulse from near critical density hydrogen gas target

Relativistic electron rings hold the possibility of very high accelerating rates, and hopefully a relatively cheap and compact accelerator/collimator for ultrahigh energy proton source. In this work, we investigate the generation of helical shaped quasi-monoenergetic relativistic electron beam and high-energy proton beam from near critical density plasmas driven by petawatt-circularly polarized-short laser pulses. We numerically observe the efficient proton acceleration from magnetic vortex acceleration mechanism by using the three dimensional particle-in-cell simulations; proton beam with peak energy 350 MeV, charge ~10nC and conversion efficiency more than 6% (which implies 2.4 J proton beam out of the 40 J incident laser energy) is reported. We detailed the microphysics involved in the ion acceleration mechanism, which requires investigating the role of self-generated plasma electric and magnetic fields. The concept of efficient generation of quasi-monoenergetic electron and proton beam from near critical density gas targets may be verified experimentally at advanced high power – high repetition rate laser facilities e.g. ELI-ALPS. Such study should be an important step towards the development of high quality electron and proton beam.

Proton acceleration in two-species targets irradiated by an ultra-intense femtosecond laser pulse

This paper presents results of two-dimensional particle-in-cell simulations of proton acceleration at the interactions of a 130 fs, linearly polarized laser pulse of intensity from the range 1021 –1023 W/cm2, predicted for the ELI (Extreme Light Infrastructure) lasers, with thin hydrocarbon (CH) or hydride (ErH3) targets. It is shown that for the targets of the areal density σ > 0.1 mg/cm2 and laser intensities below 1022 W/cm2 a higher efficiency of proton acceleration is achieved for hydride targets. However for the highest, ultra-relativistic laser intensities (~ 1023 W/cm2) considerably higher proton energies and proton beam intensities are achieved for thin (σ ≤ 0.1 mg/cm2) CH targets. In this case, at short distances from the irradiated CH target (< 50 um), the generated proton pulses are very short (< 20 fs), and the proton beam intensities and the proton current densities reach extremely high values, > 1021 W/cm2 and > 1012 A/cm2, respectively, which are much higher than those attainable in conventional accelerators. Such proton beams can open the door for new areas of research in nuclear physics and high energy-density physics as well as can be useful for materials research.

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions.

Hollow target for efficient generation of fast ions by ultrashort laser pulses

The efficiency of ion acceleration driven by high-power femtosecond laser pulses strongly depends on the target thickness and on the absorption of laser pulse energy into the ionized solid target. Enhanced absorption has been demonstrated for targets with submicrometer structures deposited on their front surface. However, increasing the overall thickness of the target by adding the layer with structures is undesirable. Here, microstructured hollow targets are proposed to enhance the absorption of the laser pulse energy while keeping the target thickness to minimum. It is demonstrated by full 3D particle-in-cell simulations that the efficiency of proton acceleration from hollow targets substantially exceeds the efficiency of the acceleration from flat foils of the same thickness. The fabrication of an ultrathin hollow target (prototype) by focused ion beam milling is also described.

Proton acceleration by irradiation of isolated spheres with an intense laser pulse.

We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3×10^{20}Wcm^{-2}. With a laser focal spot size of 10μm full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3μm. Maximum proton energies of ∼25 MeV are achieved for targets matching the focal spot size of 10μm in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.

On the compressibility effect in test particle acceleration by magnetohydrodynamic turbulence

The effect of compressibility in a charged particle energization by magnetohydrodynamic (MHD) fields is studied in the context of test particle simulations. This problem is relevant to the solar wind and the solar corona due to the compressible nature of the flow in those astrophysical scenarios. We consider turbulent electromagnetic fields obtained from direct numerical simulations of the MHD equations with a strong background magnetic field. In order to explore the flow compressibility effect over the particle dynamics, we performed different numerical experiments: an incompressible case and two weak compressible cases with Mach number M = 0.1 and M = 0.25. We analyze the behavior of protons and electrons in those turbulent fields, which are well known to form aligned current sheets in the direction of the guide magnetic field. What we call protons and electrons are test particles with scales comparable to (for protons) and much smaller than (for electrons) the dissipative scale of MHD turbulence, maintaining the correct mass ratio me/mi. For these test particles, we show that compressibility enhances the efficiency of proton acceleration, and that the energization is caused by perpendicular electric fields generated between currents sheets. On the other hand, electrons remain magnetized and display an almost adiabatic motion, with no effect of compressibility observed. Another set of numerical experiments takes into account two fluid modifications, namely, electric field due to Hall effect and electron pressure gradient. We show that the electron pressure has an important contribution to electron acceleration allowing highly parallel energization. In contrast, no significant effect of these additional terms is observed for the protons.