Ultrathin Targets Research Articles

A hybrid simulation integrating molecular dynamics and particle-in-cell methods for improved laser-target interaction

Ultra-thin targets (less than 10 nm), such as graphene, can be irradiated with relativistic intensity lasers to generate energetic ions. However, the laser prepulse can prematurely destroy these targets and significantly influence the final ion energies. Due to the limitations of the conventional hydrodynamic model, simulating the interaction between ultra-thin targets and a prepulse is infeasible. To overcome this issue, we propose a hybrid simulation technique in this study. This technique involves simulating the target-prepulse interaction using molecular dynamics (MD) simulation, which is then combined with the particle-in-cell simulation for the target-main pulse interaction, in order to accurately model the entire laser-target interaction dynamics. A realistic, experimentally measured laser intensity profile for the prepulse is used for the MD simulation, and the particle energies from this hybrid simulation are found to be in good agreement with the experiment.

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.

Numerical optimization of longitudinal collimator geometry for novel x-ray field

Objective. A novel x-ray field produced by an ultrathin conical target is described in the literature. However, the optimal design for an associated collimator remains ambiguous. Current optimization methods using Monte Carlo calculations restrict the efficiency and robustness of the design process. A more generic optimization method that reduces parameter constraints while minimizing computational load is necessary. A numerical method for optimizing the longitudinal collimator hole geometry for a cylindrically-symmetrical x-ray tube is demonstrated and compared to Monte Carlo calculations. Approach. The x-ray phase space was modelled as a four-dimensional histogram differential in photon initial position, final position, and photon energy. The collimator was modeled as a stack of thin washers with varying inner radii. Simulated annealing was employed to optimize this set of inner radii according to various objective functions calculated on the photon flux at a specified plane. Main results. The analytical transport model used for optimization was validated against Monte Carlo calculations using Geant4 via its wrapper, TOPAS. Optimized collimators and the resulting photon flux profiles are presented for three focal spot sizes and five positions of the source. Optimizations were performed with multiple objective functions based on various weightings of precision, intensity, and field flatness metrics. Finally, a select set of these optimized collimators, plus a parallel-hole collimator for comparison, were modeled in TOPAS. The evolution of the radiation field profiles are presented for various positions of the source for each collimator. Significance. This novel optimization strategy proved consistent and robust across the range of x-ray tube settings regardless of the optimization starting point. Common collimator geometries were re-derived using this algorithm while simultaneously optimizing geometry-specific parameters. The advantages of this strategy over iterative Monte Carlo-based techniques, including computational efficiency, radiation source-specificity, and solution flexibility, make it a desirable optimization method for complex irradiation geometries.

Development of an ultrathin liquid sheet target for laser ion acceleration at high repetition rates in the kHz-range

Development of an ultrathin liquid sheet target for laser ion acceleration at high repetition rates in the kHz-range

Quasi-monoenergetic heavy ion acceleration driven by sub-100 PW linearly polarized laser pulses in the radiation-dominated QED regime

Heavy ion acceleration from an ultrathin foil target irradiated by a p-polarized and spatially Gaussian laser pulse at intensity of 1023 W/cm2 is studied by using two-dimensional particle-in-cell simulations. We find that, in the extremely intense laser fields, the radiation reaction force from bright γ-rays radiated by radially oscillating electrons is large enough to match the Coulomb explosive force of foil electrons. The undesirable transverse expansion of the foil from the electron heating and inhomogeneous radial profile of the laser intensity is effectively suppressed. The foil maintains relatively good opacity in its central region stabilizing localized acceleration of heavy ions. With a laser of intensity 3.4 × 1023 W/cm2, duration of 33 fs, and power of 96 PW, a dense monoenergetic Au79+ ion bunch with a peak energy of ∼160 GeV can be obtained in the radiation-dominated QED regime. Such a high-quality heavy ion beam is useful for investigating nuclear matter equation of state and quantum chromodynamic phase transition in intermediate-energy heavy ion collisions.

Ferromagnetic insulating substrate for magnetic proximity studies: LaCoO3thin film.

Ferromagnetic insulators (FMIs) are intriguing not only due to their rare nature, but also due to their potential applications in spintronics and various electronic devices. One of its key promising applications is based on an FMI-induced magnetic proximity effect, which can impose an effective time-reversal symmetry breaking on the target ultrathin layer to realize novel emergent phenomena. Here, we conduct systematic studies on thin film LaCoO3, an insulator known to be ferromagnet under tensile strain, with varying thicknesses, to establish it as an FMI platform to be integrated in heterostructures. The optimal thickness of the LaCoO3layer, providing a smooth surface and robust ferromagnetism with large remanence, is determined. A heterostructure consisting of an ultrathin target layer (2 uc SrRuO3), the LaCoO3FMI layer, and the La0.5Sr0.5CoO3conducting layer has been fabricated and the angle-resolved photoemission spectroscopy measurement on the multi-layer system demonstrates a sharp Fermi edge and a well-defined Fermi surface without the charging effect. This demonstrates the feasibility of the proposed heterostructure using LaCoO3thin film as the FMI layer, and further lays a groundwork to investigate the magnetic proximity induced phases in quantum materials.

Hundreds-petawatt laser pulses shaping and heavy ion acceleration based on conical plasma channels

In this paper, the effects of conical plasma channels on the laser pulses shaping and the heavy ion acceleration under the extreme light field conditions of hundreds-petawatt are investigated using a particle simulation method. The law of influence of the conical plasma channel on the spatio-temporal waveform and intensity of the incident laser is analyzed, when the QED effect is taken into account. The reason for the shaping laser-enhanced heavy ion acceleration is given, and the role of the QED effect in the acceleration process is explained.<br>It is found that, due to the non-linear interference and focusing effects, the conical plasma channel can shape the spatio-temporal waveform of the laser pulse and enhance the laser intensity. A tightly focused (beam waist radius < 1 μm) and ultra-high intensity (enhanced 6 times) shaping laser is obtained for a linearly polarized laser with an intensity of 5.46 × 10<sup>22</sup> W/cm<sup>2</sup> and a waist radius of 10 μm at an incident angle of <i>θ</i> = 10°. In the simulation, the conical plasma channel is fully ionized high-Z gold plasma with the electron densities up to <i>n<sub>e</sub></i>=2626.5<i>n<sub>c</sub></i>. Therefore most of the laser energy in the channel is reflected by the channel wall, and the QED effect has less impact on laser focusing and shaping. Using this laser to accelerate an ultra-thin flat target placed at the end of the channel. It is found that, the radiation reaction force can effectively suppress the transverse expansion of the ultra-thin flat target caused by the electron heating and the transverse non-uniform of the laser intensity. The transparency time of the ultra-thin flat target is prolonged, which will allow the gold ions to be fully accelerated. Ultimately, gold ions with a cutoff energy of up to ~ 240 GeV can be obtained. The results are expected to provide theoretical references and technical support for the design of future hundreds-petawatt laser heavy ion acceleration experiments and their application research of high-quality ion source, such as nucleus-nucleus collisions.

A comprehensive diagnostic system of ultra-thin liquid sheet targets

Abstract To meet the demands of laser-ion acceleration at a high repetition rate, we have developed a comprehensive diagnostic system for real-time and in situ monitoring of liquid sheet targets (LSTs). The spatially resolved rapid characterizations of an LST’s thickness, flatness, tilt angle and position are fulfilled by different subsystems with high accuracy. With the help of the diagnostic system, we reveal the dependence of thickness distribution on collision parameters and report the 238-nm liquid sheet generated by the collision of two liquid jets. Control methods for the flatness and tilt angle of LSTs have also been provided, which are essential for applications of laser-driven ion acceleration and others.

Mixing time of homogeneous/heterogeneous solutions in a micro-mixer with free impinging jets

Abstract We have developed a micro-mixer based on a free impinging liquid-sheet jet technique. We identified a mixing position where two different solutions were mixed uniformly and evaluated corresponding mixing times in the liquid-sheet jets, with a homogeneous combination (H2O and H2O) and heterogeneous combination (C2H5OH and H2O). A quenching reaction was observed and the mixing times were evaluated to be 36 μs for the homogeneous combination (H2O/H2O) and 46 μs for the heterogeneous combination (C2H5OH/H2O). To clarify the mixing mechanism in the liquid-sheet jet, the theoretical mixing times were calculated using two different models, assuming laminar and turbulence flows. The calculated mixing times based on energy dissipation in the turbulence flow were in agreement with the observed mixing times for both H2O/H2O and C2H5OH/H2O combinations. These results indicate that turbulent mixing is a dominant mixing mechanism in the liquid-sheet jet, and that no clear interface is formed between H2O solutions and between C2H5OH and H2O solutions. The liquid-sheet jet technique provides a windowless and ultra-thin target and would be useful to investigate intermediates in mixing-driven chemical reactions, such as oxidation in solution and a folding reaction of proteins proceeding in a microsecond time scale.

The femtosecond structure of extreme contrast, multi-terawatt second-harmonic laser pulses at 400 nm

Ultrahigh intensity contrast and short pulse laser–solid interactions offer an attractive platform for investigating high-energy-density matter, particularly in the context of structured and ultra-thin targets that form hot, dense plasma conditions. Harmonic generation can improve the contrast of laser pulses by several orders of magnitude. In this study, we present the characterization of extreme contrast, relativistic intensity second-harmonic pulses at 400 nm, using the self-diffraction frequency-resolved optical gating technique. The 400 nm pulses were generated at various input intensities using potassium dihydrogen phosphate and lithium triborate crystals. Our observations reveal the presence of spectral broadening, pulse compression, and complex structures at higher input intensities. We see that extreme contrast, few tens of femtosecond pulses can have multiple “prepulses” at the 100s femtosecond scale as large as ten percent of the peak value. These can preionize a solid significantly and may influence the interaction. Simulations based on nonlinear pulse propagation equations reinforce our findings.

Ultrathin Tape-Supercapacitor with High Capacitance and Durable Flexibility via Repeated In-situ Polymerizations of Polyaniline

Flexible supercapacitors (FSCs) are among the essential power suppliers of flexible electronics, while it is still challenging to achieve the high-performance, flexible, ultrathin and reliable targets concurrently. This work proposes the repeated in-situ chemical solution polymerizations (ICSPs) of polyaniline (PANI) on graphite-coated polyimide tape, and assembles the electrodes into self-encapsulated FSCs. Repeated ICSPs can increase the mass load of PANI and significantly influence its morphological, chemical and crystalline structures, as well as electronic and ionic transports, greatly increasing final energy-storage properties. The tape electrodes show maximum specific capacitances of 1955 mF cm−2 (1068F g−1) at 0.4 mA cm−2 and 1089 mF cm−2 (595F g−1) at 10 mA cm−2. Tape acts as substrates and packaging simultaneously. Consequently, the self-encapsulated FSCs show ultrathin thickness (∼200 μm), high areal and volumetric capacitances (481 mF cm−2 and 24F cm−3), durable flexibility (retaining a maximum of ∼ 97% after thousands of presses, bends, rolls, folds, twists, beats and even crumples for the FSC that was previously placed at open air over 2 months) and high device reliability. This work provides a prototype for reliable and advanced FSCs.

Fully Automated Continuous Centrifugal Microfluidics Isolates Natural Killer Cells with High Performance and Minimal Stress.

Natural killer (NK) cells are a part of the innate immune system, providing the first line of defense against cancer cells and pathogens at an early stage. Hence, they are attracting attention as a valuable resource for allogeneic cell immunotherapy. However, NK cells exist with limited proportion in blood, and obtaining sufficient clinical-grade NK cells with highly viable and minimal stress is critical for successful immune cell therapy. Conventional purification methods via immunoaffinity or density gradient centrifugation had several limitations in yield, purity, and cellular stress, which might cause an increased risk for graft versus host disease and reduced efficacy due to NK cell malfunction, exhaustion, and apoptosis. Moreover, reducing the variations of isolation performance caused by the manual process is another unmet need for uniform quality of the living drug. Here, an automated system using an NK disc (NKD) based on continuous centrifugal microfluidics (CCM) technology was developed to isolate NK cells from whole blood with high yield, purity, reproducibility, and low stress. The CCM technology, which operates fluidic manipulation under disc rotation, enabled precise extraction of the ultra-thin target fluid layer generated by blood centrifugation. Compared to the conventional manual method, the CCM-NKD isolated NK cells with higher yield (recovery rate) and purity, while maintaining better reproducibility. Furthermore, since the CCM-NKD maintained substantially milder centrifugation conditions (120 ×g for 10 min) compared to the conventional approach (1200 ×g for 20 min), it showed reduced cellular stress and increased antioxidant capacity in the isolated NK cells. Based on the results, the CCM-NKD is expected to be a useful tool to provide highly intact and viable cell weapons for successful immune cell therapy.

Low-energy nuclear reactions with stored ions: a new era of astrophysical experiments at heavy ion storage rings

Heavy ion storage rings are powerful tools to store and observe key nuclear properties of rare radioactive isotopes. Recent developments in ring physics and enhanced beam intensities have now opened up the possibility to carry out low-energy investigations of nuclear reactions at rings. Pure, intense, exotic beams of isotopes that are otherwise challenging to access can be impinged on pure, ultra-thin targets, allowing the study of long-standing nuclear astrophysical puzzles in a variety of stellar sites that have so far resisted traditional approaches. In this review paper, we will describe pioneering studies with decelerated beams at the ESR storage ring at GSI (Germany), as well as future exciting prospects at the ESR and CRYRING at GSI/FAIR.

Effects of electron heating and surface rippling on Rayleigh–Taylor instability in radiation pressure acceleration

The acceleration of ultrathin targets driven by intense laser pulses induces Rayleigh–Taylor-like instability. Apart from laser and target configurations, we find that electron heating and surface rippling, effects inherent to the interaction process, have an important role in instability evolution and growth. By employing a simple analytical model and two-dimensional particle-in-cell simulations, we show that the onset of electron heating in the early stage of the acceleration suppresses the growth of small-scale modes, but it has little influence on the growth of large-scale modes, which thus become dominant. With the growth of surface ripples, a mechanism that can significantly influence the growth of these large-scale modes is found. The laser field modulation caused by surface rippling generates an oscillatory ponderomotive force, directly modulating transverse electron density at a faster growth rate than that of ions and eventually enhancing instability growth. Our results show that when surface deformation becomes obvious, electron surface oscillation at 2ω0 (where ω0 is the laser frequency) is excited simultaneously, which can be seen as a signature of this mechanism.

Efficient Generation of Attopulses at the Interaction of Intense Laser Radiation with Ultrathin Targets

Efficient Generation of Attopulses at the Interaction of Intense Laser Radiation with Ultrathin Targets

Simulation and data analysis of a fission time projection chamber based on Monte Carlo method

In this study, we use Monte Carlo method to conduct simulation research on a time projection chamber (TPC) which used for fission cross-section measurement. Based on Garfield, SRIM, COMSOL and other platforms and software, we explore the physical process during its actual operation, and analyze the simulation data. We carry out α particle track measurement experiments with U-235 ultra-thin target, and the measured results are in good agreement with the simulation results. This paper focuses on the simulation and reconstruction of particle tracks at different emission angles, and introduces the concept of ballistic deficit to correct the experimental measurement results. The corrected 2D distribution of particle track length and energy is closer to the theoretical model than the uncorrected distribution.

Ultrashort pulsed neutron source driven by two counter-propagating laser pulses interacting with ultra-thin foil

Neutron production via D(d, n)<sup>3</sup>He nuclear reaction during the interaction of two counter-propagating circularly polarized laser pulses with ultra-thin deuterium target is investigated by particle-in-cell simulation and Monte Carlo method. It is found that the rotation direction and initial relative phase difference of laser electric field vector have important effects on deuterium foil compression and neutron characteristics. The reason is attributed to the net light pressure and the difference in transverse instability development. The highest neutron yield can be obtained by choosing two laser pulses with a relative phase difference of 0 and the same rotation direction of the electric field vector. When the relative phase difference is 0.5π or 1.5π and the rotation direction of electric field vector is different, the neutrons have a directional spatial distribution and the neutron yield only slightly decreases. For left-handed circularly polarized laser pulse and right-handed circularly polarized laser pulse, each with an intensity of 1.23 × 10<sup>21</sup> W/cm<sup>2</sup>, a pulse width of 33 fs and a relative phase difference of 0.5π, it is possible to produce a pulsed neutron source with a yield of 8.5 × 10<sup>4</sup> n, production rate of 1.2 × 10<sup>19</sup> n/s, pulse width of 23 fs and good forward direction as well as tunable spatial distribution. Comparing with photonuclear neutron source and beam target neutron source driven by ultraintense laser pulses, the duration of neutron source in our scheme decreases significantly, thereby possessing many potential applications such as neutron nuclear data measurement. Our scheme offers a possible method to obtain a compact neutron source with short pulse width, high production rate and good forward direction.

Design of plasma shutters for improved heavy ion acceleration by ultra-intense laser pulses

In this work, we investigate the application of the plasma shutters for heavy ion acceleration driven by a high-intensity laser pulse. We use particle-in-cell and hydrodynamic simulations. The laser pulse, transmitted through the opaque shutter, gains a steep-rising front and its peak intensity is locally increased at the cost of losing part of its energy. These effects have a direct influence on subsequent ion acceleration from the ultrathin target behind the shutter. In our 3D simulations of silicon nitride plasma shutter and a silver target, the maximal energy of high-Z ions increases significantly when the shutter is included for both linearly and circularly polarized laser pulses. Moreover, application of the plasma shutter for linearly polarized pulse results in focusing of ions toward the laser axis in the plane perpendicular to the laser polarization. The generated high energy ion beam has significantly lower divergence compared to the broad ion cloud, generated without the shutter. The effects of prepulses are also investigated assuming a double plasma shutter. The first shutter can withstand the assumed sub-ns prepulse (treatment of ns and ps prepulses by other techniques is assumed) and the pulse shaping occurs via interaction with the second shutter. On the basis of our theoretical findings, we formulated an approach toward designing a double plasma shutter for high-intensity and high-power laser pulses and built a prototype.

High order modes of intense second harmonic light produced from a plasma aperture

Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles, plasma optical components are being developed to amplify, compress, and condition high-power laser pulses. We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated [Duff et al., Sci. Rep. 10, 105 (2020)]. Herein, we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes. The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness. The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse, which enables changing between a linearly polarized TEM01 mode and a circularly polarized Laguerre–Gaussian LG01 mode. This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Towards single-charge heavy ion beams driven by an ultra-intense laser

The acceleration of super-heavy ions from an ultra-thin lead target irradiated by a femtosecond laser pulse with an intensity in the range of ∼1022–1023 W cm−2 was investigated using an advanced 2D3V particle-in-cell code. It is shown that by properly selecting the laser pulse parameters, it is possible to produce a practically single-charge Pb ion beam with multi-GeV ion energies and the laser-to-ions energy conversion efficiency approaching 30%. At the laser intensity of 1023 W cm−2, Pb ions with the charge state Z = 72 carry over 90% of the total energy of all ions, while the peak intensity and peak fluence of the Pb+72 ion beam are at least two orders of magnitude higher than for other types of ions. In addition, the Pb+72 ion beam is more compact and has a smaller angular divergence than those for other types of ions. The above properties of the Pb+72 ion beam mean that further energy-efficient purification of the beam from other types of ions is possible, even in simple ion transport and selection systems.