Matter and Radiation at Extremes

2023, 8(3): 034401

Diagnosis of ultrafast ultraintense laser pulse characteristics by machine-learning-assisted electron spinDownload：4060次

The rapid development of ultrafast ultraintense laser technology continues to create opportunities for studying strong-field physics under extreme conditions. However, accurate determination of the spatial and temporal characteristics of a laser pulse is still a great challenge, especially when laser powers higher than hundreds of terawatts are involved. In this paper, by utilizing the radiative spin-flip effect, we find that the spin depolarization of an electron beam can be employed to diagnose characteristics of ultrafast ultraintense lasers with peak intensities around 10^{20}–10^{22} W/cm^{2}. With three shots, our machine-learning-assisted model can predict, simultaneously, the pulse duration, peak intensity, and focal radius of a focused Gaussian ultrafast ultraintense laser (in principle, the profile can be arbitrary) with relative errors of 0.1%–10%. The underlying physics and an alternative diagnosis method (without the assistance of machine learning) are revealed by the asymptotic approximation of the final spin degree of polarization. Our proposed scheme exhibits robustness and detection accuracy with respect to fluctuations in the electron beam parameters. Accurate measurements of ultrafast ultraintense laser parameters will lead to much higher precision in, for example, laser nuclear physics investigations and laboratory astrophysics studies. Robust machine learning techniques may also find applications in more general strong-field physics scenarios.

A method for measuring the intensity of focused high-power laser pulses based on numerical simulation of high-harmonic generation in the laser peeler regime is proposed. The dependence of the efficiency of high-harmonic generation on the laser pulse intensity and the spatial parameters during interaction with solid targets is studied numerically. The simulation clearly shows that the amplitude of the generated harmonics depends on the laser pulse parameters. The proposed method is simpler than similar intensity measurement techniques and does not require complex preparation.

Ultra-intense laser-driven fast electron beam propagation in a silicon target is studied by three-dimensional hybrid particle-in-cell–fluid simulations. It is found that the transverse spatial profile of the fast electron beam has a significant influence on the propagation of the fast electrons. In the case of a steep spatial profile (e.g., a super-Gaussian profile), a tight fast electron beam is produced, and this excites more intense resistive magnetic fields, which pinch the electron beam strongly, leading to strong filamentation of the beam. By contrast, as the gradient of the spatial profile becomes more gentle (e.g., in the case of a Lorentzian profile), the resistive magnetic field and filamentation become weaker. This indicates that fast electron propagation in a solid target can be controlled by modulating the spatial gradient of the laser pulse edge.

Laser–plasma instability (LPI) is one of the main obstacles to achieving predictable and reproducible fusion at high gain through laser-driven inertial confinement fusion (ICF). In this paper, for the first time, we show analytically and confirm with three-dimensional particle-in-cell simulations that angular incoherence provides suppression of the instability growth rate that is additional to and much stronger than that provided by the well-known temporal and spatial incoherence usually used in ICF studies. For the model used in our calculations, the maximum field ratio between the stimulated Raman scattering and the driving pulses drops from 0.2 for a Laguerre–Gaussian pulse with a single nonzero topological charge to 0.05 for a super light spring with an angular momentum spread and random relative phases. In particular, angular incoherence does not introduce extra undesirable hot electrons. This provides a novel method for suppressing LPI by using light with an angular momentum spread and paves the way towards a low-LPI laser system for inertial fusion energy with a super light spring of incoherence in all dimensions of time, space, and angle, and may open the door to the use of longer-wavelength lasers for inertial fusion energy.

We estimate the parameters of a Cu plasma generated by an X-pinch by comparing experimentally measured x-rays with synthetic data. A filtered absolute extreme ultraviolet diode array is used to measure time-resolved x-ray spectra with a spectral resolution of ∼1 keV in the energy range of 1–10 keV. The synthetic spectra of Cu plasmas with different electron temperatures, electron densities, and fast electron fractions are calculated using the FLYCHK code. For quantitative comparison with the measured spectrum, two x-ray power ratios with three different spectral ranges are calculated. We observe three x-ray bursts in X-pinch experiments with two Cu wires conducted on the SNU X-pinch at a current rise rate of ∼0.2 kA/ns. Analysis of the spectra reveals that the first burst comprises x-rays emitted by hot spots and electron beams, with characteristics similar to those observed in other X-pinches. The second and third bursts are both generated by long-lived electron beams formed after the neck structure has been completely depleted. In the second burst, the formation of the electron beam is accompanied by an increase in the electron density of the background plasma. Therefore, the long-lived electron beams generate the additional strong x-ray bursts while maintaining a plasma channel in the central region of the X-pinch. Moreover, they emit many hard x-rays (HXRs), enabling the SNU X-pinch to be used as an HXR source. This study confirms that the generation of long-lived electron beams is crucial to the dynamics of X-pinches and the generation of strong HXRs.

Effects of electron heating and surface rippling on Rayleigh–Taylor instability in radiation pressure accelerationDownload：633次

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 2*ω*_{0} (where *ω*_{0} is the laser frequency) is excited simultaneously, which can be seen as a signature of this mechanism.

Synthesis pressure and structural stability are two crucial factors for highly energetic materials, and recent investigations have indicated that cerium is an efficient catalyst for N_{2} reduction reactions. Here, we systematically explore Ce–N compounds through first-principles calculations, demonstrating that the cerium atom can weaken the strength of the N≡N bond and that a rich variety of cerium polynitrides can be formed under moderate pressure. Significantly, $P\stackrel{\u0304}{1}$ -CeN_{6} possesses the lowest synthesis pressure of 32 GPa among layered metal polynitrides owing to the strong ligand effect of cerium. The layered structure of $P\stackrel{\u0304}{1}$ -CeN_{6} proposed here consists of novel N_{14} ring. To clarify the formation mechanism of $P\stackrel{\u0304}{1}$ -CeN_{6}, the reaction path Ce + 3N_{2} → *trans*-CeN_{6} → $P\stackrel{\u0304}{1}$ -CeN_{6} is proposed. In addition, $P\stackrel{\u0304}{1}$ -CeN_{6} possesses high hardness (20.73 GPa) and can be quenched to ambient conditions. Charge transfer between cerium atoms and N_{14} rings plays a crucial role in structural stability. Furthermore, the volumetric energy density (11.20 kJ/cm^{3}) of $P\stackrel{\u0304}{1}$ -CeN_{6} is much larger than that of TNT (7.05 kJ/cm^{3}), and its detonation pressure (128.95 GPa) and detonation velocity (13.60 km/s) are respectively about seven times and twice those of TNT, and it is therefore a promising high-energy-density material.

Recently, room-temperature superconductivity has been reported in a nitrogen-doped lutetium hydride at near-ambient pressure [Dasenbrock-Gammon *et al.*, Nature 615 , 244 (2023)]. The superconducting properties might arise from *Fm*$\stackrel{\u0304}{3}$ *m*-LuH_{3-δ}N_{ε}. Here, we systematically study the phase diagram of Lu–N–H at 1 GPa using first-principles calculations, and we do not find any thermodynamically stable ternary compounds. In addition, we calculate the dynamic stability and superconducting properties of N-doped *Fm*$\stackrel{\u0304}{3}$ *m*-LuH_{3} using the virtual crystal approximation (VCA) and the supercell method. The *R*3*m*-Lu_{2}H_{5}N predicted using the supercell method could be dynamically stable at 50 GPa, with a *T*_{c} of 27 K. According to the VCA method, the highest *T*_{c} is 22 K, obtained with 1% N-doping at 30 GPa. Moreover, the doping of nitrogen atoms into *Fm*$\stackrel{\u0304}{3}$ *m*-LuH_{3} slightly enhances *T*_{c}, but raises the dynamically stable pressure. Our theoretical results show that the *T*_{c} values of N-doped LuH_{3} estimated using the Allen–Dynes-modified McMillan equation are much lower than room temperature.

动态信息 丨 2019-12-16

年终大喜：MRE被SCI正式收录！动态信息 丨 2019-01-02

The 4th ICMRE will be held in Hefei in May 2019动态信息 丨 2019-01-02

第四届ICMRE会议将于5月在合肥召开 动态信息 丨 2018-11-13

MRE将由AIP出版公司与中物院合作出版动态信息 丨 2018-09-12

MRE被ESCI收录