We make a comparative study of hard x-ray emission from 15 $μ$m methanol microdroplets and a plain slab target of similar atomic composition at similar laser intensities. The hard X-ray yield from droplet plasmas is $\simeq$ 35 times more than that obtained from solid plasmas. A prepulse that is about 10ns and about 5% of the main pulse is essential for hard x-ray generation from the droplets.
A hot electron temperature of 36 keV is measured from the droplets at 8$\times10^{14}$ W cm$^{-2}$; three times higher intensity is needed to obtain similar hot electron temperature from solid plasmas with similar composition. We use 1D PIC simulation to obtain qualitative correlation to the experimental observations.
We demonstrate the high-efficiency generation of water-window soft x-ray emissions from polyethylene nanowire array targets irradiated by femtosecond laser pulses at the intensity of 4*10^19 W/cm^2. The experimental results indicate more than one order of magnitude enhancement of the water-window x-ray emissions from the nanowire array targets compared to the planar targets. The highest energy conversion efficiency from laser to water-window x-rays is measured as 0.5%/sr, which comes from the targets with the longest nanowires. Supported by particle-in-cell simulations and atomic kinetic codes…
We have developed a method for producing spatially stable micron-scale liquid targets of flexible shapes at kilohertz repetition rate for use in air and vacuum, by perturbing 5 and 30 μm diameter streams with femtosecond laser pulses and monitoring the temporal development of the perturbation. Using water, we have produced features such as 2.1 μm diameter droplet and 1.3 μm diameter neck with less than 0.3 μm shot-to-shot variation, with prospects for further reduction in size and variability. The use of such micron-scale targets can be expected to prevent conductive heat dissipation, enhance …
Using high-resolution, two-dimensional particle-in-cell simulations, we investigate numerically the mechanisms of terahertz (THz) emissions in submicron-thick carbon solid foils driven by ultraintense ($\sim 10^{20}\,\rm W\,cm^{-2}$), ultrashort ($30\,\rm fs$) laser pulses at normal incidence. The considered range of target thicknesses extends down to the relativistic transparency regime that is known to optimize ion acceleration by femtosecond laser pulses. By disentangling the fields emitted by longitudinal and transverse currents, our analysis reveals that, within the first picosecond after…
A theoretical model is developed for the interaction of intense femtosecond laser pulses with solid targets on the basis of the two-temperature equation of state for an irradiated substance. It allows the description of the dynamics of the plasma formation and expansion. Comparison of available experimental data on the amplitude and phase of the complex reflection coefficient of aluminum with the simulation results provides new information on the transport coefficients and absorption capacity of the strongly coupled Al plasma over a wide range of temperatures and pressures.
Laser-based sources of ionizing radiation have attracted a considerable attention in the last years for their broad potential applications. However the stability and robustness of such sources is still an issue that needs to be addressed. Aiming to solve such problems, we propose a source that uses a liquid jet -rather than a solid- as a target for the production of X-rays. Liquid jets offer always a clean surface for every laser shot which represent a clear advantage over solids. In this work, we present an experimental characterization of the X-ray emission of such targets, and study the eff…
We present an experimental demonstration of high conversion efficiency Betatron x-ray radiation from petawatt laser irradiated near critical plasmas. Direct laser acceleration serves as the dominant regime when laser pulse of ~5e20 W/cm2 intensity is focused into plasmas with electron density of 3e20 /cm3. Electron beam with a charge of ~35 nC is accelerated up to a maximum energy of 70 MeV and emit x-rays when oscillating in the laser field. The deduced energy conversion efficiency from laser to x-rays is up to 1e-4, orders of magnitude higher than other betatron regimes. Enhancement of accel…
In [I.R. Khairulin et al., submitted to Phys. Rev. Lett.] we propose a method for amplifying a train of sub-femtosecond pulses of circularly or elliptically polarized extreme ultraviolet (XUV) radiation, constituted by high-order harmonics of an infrared (IR) laser field, in a neon-like active medium of a plasma-based X-ray laser, additionally irradiated with a replica of a fundamental frequency laser field used to generate harmonics, and show the possibility of maintaining or enhancing the ellipticity of high-harmonic radiation during its amplification. In the present paper we describe this p…
The kinetic Boltzmann equation is used to model the non-equilibrium ionization phase that initiates the evolution of atomic clusters irradiated with single pulses of intense vacuum ultraviolet radiation. The duration of the pulses is < 50 fs and their intensity in the focus is < 10^{14} W/cm^2. This statistical model includes various processes contributing to the sample dynamics at this particular radiation wavelength, and is computationally efficient also for large samples. Two effects are investigated in detail: the impact of the electron heating rate and the effect of the plasma environment…
PIConGPU modeling of nanoplasma formation in helium nanodroplets irradiated by intense femtosecond laser pulses
Max-Planck-Institut für Kernphysik（马克斯·普朗克核物理研究所）；Helmholtz-Zentrum Dresden - Rossendorf, Center for Advanced Systems Understanding (CASUS)（德累斯顿-罗森多夫亥姆霍兹中心，先进系统理解中心）；Helmholtz-Zentrum Dresden - Rossendorf（德累斯顿-罗森多夫亥姆霍兹中心）；Institute of Physics, University of Kassel（卡塞尔大学物理研究所）；Department of Physics and Quantum Center of Excellence for Diamond and Emerging Materials, Indian Institute of Technology Madras（马德拉斯印度理工学院物理系与钻石及新兴材料量子卓越中心）；ELI Beamlines facility, The Extreme Light Infrastructure ERIC（极亮基础设施欧洲利益协调实体 ELI光束线设施）
We study the excitation of electron currents in a transparent cell of sub-millimeter size filled by an atomic gas and illuminated by an intense two-color femtosecond laser pulse. The pulse consists of a strong fundamental component and its second harmonic of low intensity, both circularly polarized. We show that for sufficiently small $20μ$m cells the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous within the interaction volume. This coherent dipole plasma oscillation results in a remarkably efficient conversion of the electron energy into that of radiation …
Bright Ar K-shell x-ray with very little background has been generated using an Ar clustering gas jet target irradiated with an 800 mJ, 30 fs ultra-high contrast laser, with the measured flux of 1.1 x 10^4 photons/mrad^2/pulse. This intense x-ray source critically depends on the laser contrast and the laser energy and the optimization of this source with interaction is addressed. Electron driven by laser electric field directly via nonlinear resonant is proved in simulation, resulting in effective electron heating and the enhancement of x-ray emission. The x-ray pulse duration is demonstrated …
We show that ulta-dense Z-pinches with nanoscale dimensions can be generated by irradiating aligned nanowires with femtosecond laser pulses of relativistic intensity. Using fully three-dimensional relativistic particle-in-cell simulations we demonstrate that the laser pulse drives a forward electron current in the area around the wires. This forward current induces return current densities of $\sim$ 0.1 Giga-Amperes per $μ$m\textsuperscript{2} through the wires. The resulting strong, quasi-static, self-generated azimuthal magnetic field pinches the nanowires into hot plasmas with a peak electr…
Magnetized high energy density physics offers new opportunities for observing magnetic field-related physics for the first time in the laser-plasma context. We focus on one such phenomenon, which is the ability of a laser-irradiated magnetized plasma to amplify a seed magnetic field. We performed a series of fully kinetic 3D simulations of magnetic field amplification by a picosecond-scale relativistic laser pulse of intensity $4.2\times 10^{18}$ W/cm$^2$ incident on a thin foil. We observe axial magnetic field amplification from an initial 0.1 kT seed to 1.5 kT over a volume of several cubic …
As it is known from visible light experiments, silicon under femtosecond pulse irradiation can undergo the so-called 'nonthermal melting' if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called 'thermal melting'. This thermal melting can occur even if the excited-electron density is much too low to induc…
We investigate bulk ion heating in solid buried layer targets irradiated by ultra-short laser pulses of relativistic intensities using particle-in-cell simulations. Our study focuses on a CD2-Al-CD2 sandwich target geometry. We find enhanced deuteron ion heating in a layer compressed by the expanding aluminium layer. A pressure gradient created at the Al-CD2 interface pushes this layer of deuteron ions towards the outer regions of the target. During its passage through the target, deuteron ions are constantly injected into this layer. Our simulations suggest that the directed collective outwar…
We report the experimental results of simultaneous measurements on the electron and X-ray spectra from near-critical-density (NCD) double-layer targets irradiated by relativistic femtosecond pulses at the intensity of 5E19 W/cm^2. The dependence of the electron and X-ray spectra on the density and thickness of the NCD layer was studied. For the optimal targets, electrons with temperature of 5.5 MeV and X-rays with critical energy of 5 keV were obtained. 2D particle-in-cell simulations based on the experimental parameters confirm the electrons are accelerated in the plasma channel through direc…
We present a numerical study, based on two-dimensional particle-in-cell simulations, of the synchrotron emission induced during the interaction of femtosecond laser pulses of intensities $I=10^{21}-10^{23}\,\mathrm{Wcm}^{-2}$ with nanowire arrays. Through an extensive parametric scan on the target parameters, we identify and characterize several dominant radiation mechanisms, mainly depending on the transparency or opacity of the plasma produced by the wire expansion. At $I=10^{22}\,\mathrm{Wcm}^{-2}$, the emission of high-energy ($>10\,\mathrm{keV}$) photons attains a maximum conversion effic…
Bright, directional electron emission from nanowire coated targets under petawatt, femtosecond irradiation
Tata Institute of Fundamental Research（塔塔基础研究所）；Extreme Light Infrastructure - Nuclear Physics (ELI-NP)（极轻基础设施-核物理（ELI-NP））；“Horia Hulubei” National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH)（“霍里亚·胡卢贝伊”国家物理与核工程研发研究所（IFIN-HH））；National University of Science and Technology POLITEHNICA Bucharest（布加勒斯特理工大学国立科技大学）；University of Osaka（大阪大学）
We present experimental measurements of the femtosecond time-scale generation of strong magnetic-field fluctuations during the interaction of ultrashort, moderately relativistic laser pulses with solid targets. These fields were probed using low-emittance, highly relativistic electron bunches from a laser wakefield accelerator, and a line-integrated $B$-field of $2.70 \pm 0.39\,\rm kT\,μm$ was measured. Three-dimensional, fully relativistic particle-in-cell simulations indicate that such fluctuations originate from a Weibel-type current filamentation instability developing at submicron scales …
We investigate the target normal sheath acceleration of protons in thin aluminum targets irradiated at relativistic intensity by two time-separated ultrashort (35 fs) laser pulses. For identical laser pulses and target thicknesses of 3 and 6 $μ$m, we observe experimentally that the second pulse boosts the maximum energy and charge of the proton beam produced by the first pulse for time delays below $\sim0.6-1$ ps. By using two-dimensional particle-in-cell simulations we examine the variation of the proton energy spectra with respect to the time-delay between the two pulses. We demonstrate that…
Theoretical study of heavy ion acceleration from ultrathin (<200 nm) gold foils irradiated by a short pulse laser is presented. Using two dimensional particle-in-cell simulations the time history of the laser bullet is examined in order to get insight into the laser energy deposition and ion acceleration process. For laser pulses with intensity , duration 32 fs, focal spot size 5 mkm and energy 27 Joules the calculated reflection, transmission and coupling coefficients from a 20 nm foil are 80 %, 5 % and 15 %, respectively. The conversion efficiency into gold ions is 8 %. Two highly collimated…
The paper reports the results of two-dimensional particle-in-cell simulations of proton beam acceleration at the interactions of a 130 fs laser pulse of intensity from the range of 10^21-10^23 W/cm^2, predicted for the Extreme Light Infrastructure (ELI) lasers currently built in Europe, with a thin hydrocarbon (CH) target. A special attention is paid to the effect of the laser pulse intensity and polarization (linear-LP, circular-CP) as well as the target thickness on the proton energy spectrum, the proton beam spatial distribution and the proton pulse shape and intensity. It is shown that for…
We review studies of superintense laser interaction with solid targets where the generation of propagating surface plasmons (or surface waves) plays a key role. These studies include the onset of plasma instabilities at the irradiated surface, the enhancement of secondary emissions (protons, electrons, and photons as high harmonics in the XUV range) in femtosecond interactions with grating targets, and the generation of unipolar current pulses with picosecond duration. The experimental results give evidence of the existence of surface plasmons in the nonlinear regime of relativistic electron d…
Extreme states of matter exist throughout the universe e.g. inside planetary cores, stars or astrophysical jets. Such conditions are generated in the laboratory in the interaction of powerful lasers with solids, and their evolution can be probed with femtosecond precision using ultra-short X-ray pulses to study laboratory astrophysics, laser-fusion research or compact particle acceleration. X-ray scattering (SAXS) patterns and their asymmetries occurring at X-ray energies of atomic bound-bound transitions contain information on the volumetric nanoscopic distribution of density, ionization and …
Plasma-state metasurfaces for ultra-intensive field manipulation
College of Physics, Sichuan University（四川大学物理学院）；School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）；School of Optics and Photonics, Beijing Institute of Technology（北京理工大学光电学院）；University of Hong Kong（香港大学）；Chinese Academy of Sciences（中国科学院）
Extreme-ultraviolet pulses can propagate through ionised solid-density targets, unlike optical pulses and, thus, have the potential to probe the interior of such plasmas on sub-femtosecond timescales. We present a synthetic diagnostic method for solid-density laser-generated plasmas based on the dispersion of an extreme-ultraviolet attosecond probe pulse, in a pump--probe scheme. We demonstrate the theoretical feasibility of this approach through calculating the dispersion of an extreme-ultraviolet probe pulse propagating through a laser-generated plasma. The plasma dynamics is calculated usin…
Petawatt class femtosecond lasers and x-ray free electron lasers (XFEL) open up a new page in research fields related to space and vacuum physics. One of fundamental principles can be explored by these new instruments is the equivalence principle, saying that gravitation and acceleration should be treated equivalently. If it is true this must lead to the appearance of the Unruh effect analogous to Hawking's black hole radiation. It says that a detector moving with a constant acceleration $w$ sees a boson's thermal bath with its temperature $T_w=\hbar w/2πk_B c$. Practical detection of the Unru…
Probing laser-driven surface and subsurface dynamics via grazing-incidence XFEL scattering and diffraction
European X-ray Free-Electron Laser Facility（欧洲X射线自由电子激光设施）；Department Physik, Universität Siegen（锡根大学物理系）；Helmholtz-Zentrum Dresden-Rossendorf（德累斯顿-罗斯多夫亥姆霍兹中心）；Department of Physics and Research Center OPTIMAS, RPTU University Kaiserslautern–Landau（凯泽斯劳滕-兰道理科大学物理系及OPTIMAS研究中心）
