Picosecond laser-driven coded-source radiography with high resolution and contrast.

The X-ray sources for Compton radiography of ICF experiments are generated by using intense picosecond lasers to irradiate wire targets. The wire diameter must be designed thin enough, for example ∼ 10 µm in many published works, to comply a high spatial resolution. This results in a low laser-target interception, which limits the photon yield. We investigated a technique of coded-source radiography based on laser-driven annular sources via Monte Carlo and PIC simulations. The annular X-ray source is formed by laser irradiating tube target in which the effect of electron recirculation plays an important role. We proved that this technique has an increased spatial resolution and contrast than that using the Gaussian source produced by wire targets. Therefore, the diameter of the backlighter target can be significantly increased to uplift laser-target interception without compromising on spatial resolution. This contributes towards a reconciliation between the spatial resolution and photon yield for Compton radiography. The results predict the possibility of improving source photon yield by several times in future experiments.

Target Density Effects on Charge Transfer of Laser-Accelerated Carbon Ions in Dense Plasma.

We report on charge state measurements of laser-accelerated carbon ions in the energy range of several MeV penetrating a dense partially ionized plasma. The plasma was generated by irradiation of a foam target with laser-induced hohlraum radiation in the soft x-ray regime. We use the tricellulose acetate (C_{9}H_{16}O_{8}) foam of 2 mg/cm^{3} density and 1 mm interaction length as target material. This kind of plasma is advantageous for high-precision measurements, due to good uniformity and long lifetime compared to the ion pulse length and the interaction duration. We diagnose the plasma parameters to be T_{e}=17 eV and n_{e}=4×10^{20} cm^{-3}. We observe the average charge states passing through the plasma to be higher than those predicted by the commonly used semiempirical formula. Through solving the rate equations, we attribute the enhancement to the target density effects, which will increase the ionization rates on one hand and reduce the electron capture rates on the other hand. The underlying physics is actually the balancing of the lifetime of excited states versus the collisional frequency. In previous measurement with partially ionized plasma from gas discharge and z pinch to laser direct irradiation, no target density effects were ever demonstrated. For the first time, we are able to experimentally prove that target density effects start to play a significant role in plasma near the critical density of Nd-glass laser radiation. The finding is important for heavy ion beam driven high-energy-density physics and fast ignitions. The method provides a new approach to precisely address the beam-plasma interaction issues with high-intensity short-pulse lasers in dense plasma regimes.

Measuring fluence distribution of intense short laser based on the radiochromic effect.

This Letter demonstrates a novel, to the best of our knowledge, method to measure the fluence distribution of an intense short laser pulse based on the radiochromic effect. We discovered that an intense short laser pulse can induce the color reaction with a radiochromic film (RCF). Further, the net optical density of an irradiated RCF is proportional to the fluence of the incident laser pulse in a large range (${2 {-} 120}\;{{{\rm mJ}/{\rm cm}}^2}$). This method supports a large detection area up to near square-meter scale by splicing multi-pieces of RCFs (${8} \times 10\;{{\rm inch}^2}$ each). The spatial resolution reaches as high as 60 lines/mm. It offers a thin-film (${\sim}{100}\;{\unicode{x00B5}{\rm m}}$ thick), flexible, vacuum-compatible solution to intense short laser measurements, especially to laser facilities above petawatt, with beam sizes up to near square-meter scale, e.g., extreme light infrastructure.

Retrieving the excitation and polarization configurations in Coulomb explosion of a trimer driven by strong laser field.

Ultrafast imaging and manipulating transient molecular structures in chemical reactions and photobiological processes is a fundamental but challenging goal for scientists. Theoretically, the challenge originates from the complex multiple-time-scale correlated electron dynamics and their coupling with the nuclei. Here, we employ classical polyatomic models for this kind of study and take the Coulomb explosion of argon and neon trimers in strong laser fields as an illuminating example. Our results demonstrate that the degree of asymmetry on the kinetic energy release (KER) spectrum, together with a Dalitz plot, constitutes a powerful tool for retrieving the ionization, excitation, and polarization configurations (femtosecond-to-attosecond time-scale electron dynamics) of trimers under strong-field radiation.

Focusing single-order diffraction transmission grating with a focusing plane perpendicular to the grating surface.

By combining the single-order dispersion properties of quasi-sinusoidal single-order diffraction transmission gratings (QSTG) and the single-foci focusing properties of annulus-sector-shaped-element binary Gabor zone plate (ASZP), we propose a novel focusing single-order diffraction transmission grating (FSDTG). Different from the diffraction patterns of a normal transmission grating (TG), it has a focusing plane perpendicular to the grating surface. Numerical simulations are carried out to verify its diffraction patterns in the framework of Fresnel-Kirchhoff diffraction. Higher-order diffraction components of higher harmonics can be effectively suppressed by the FSDTG we designed. And we find that the focal depth and resolving power are only determined by the structure parameters of our FSDTG by theoretical estimations.

High direct drive illumination uniformity achieved by multi-parameter optimization approach: a case study of Shenguang III laser facility.

The uniformity of the compression driver is of fundamental importance for inertial confinement fusion (ICF). In this paper, the illumination uniformity on a spherical capsule during the initial imprinting phase directly driven by laser beams has been considered. We aim to explore methods to achieve high direct drive illumination uniformity on laser facilities designed for indirect drive ICF. There are many parameters that would affect the irradiation uniformity, such as Polar Direct Drive displacement quantity, capsule radius, laser spot size and intensity distribution within a laser beam. A novel approach to reduce the root mean square illumination non-uniformity based on multi-parameter optimizing approach (particle swarm optimization) is proposed, which enables us to obtain a set of optimal parameters over a large parameter space. Finally, this method is applied to improve the direct drive illumination uniformity provided by Shenguang III laser facility and the illumination non-uniformity is reduced from 5.62% to 0.23% for perfectly balanced beams. Moreover, beam errors (power imbalance and pointing error) are taken into account to provide a more practical solution and results show that this multi-parameter optimization approach is effective.

Single-order diffraction grating designed by trapezoidal transmission function.

Diffraction grating is a widely used dispersion element in spectral analysis from the infrared to the x-ray region. Traditionally, it has a square-wave transmission function, suffering from high-order diffraction contamination. Single-order diffraction can be achieved by sinusoidal amplitude transmission grating, but the fabrication is difficult. Here, we propose a novel idea to design a grating based on trapezoidal transmission function, which makes traditional grating a special case. Grating designed by this idea can not only suppress higher order diffraction by several orders of magnitude as sinusoidal amplitude grating does but also greatly reduce the fabrication difficulty to the level of processing for traditional grating. It offers a new opportunity for fabrication of grating with single-order diffraction and measurement of spectrum without contamination of high-order harmonic components. This idea can easily extend to varied-line-space grating, concave grating with single-order diffraction, or zone plates with single foci and will bring great changes in the field of grating applications.

Investigation of fragment sizes in laser-driven shock-loaded tin with improved watershed segmentation method.

Studying dynamic fragmentation in shock-loaded metals and evaluating the geometrical and kinematical properties of the resulting fragments are of significant importance in shock physics, material science as well as microstructural modeling. In this paper, we performed the laser-driven shock-loaded experiment on the Shenguang-Ш (SGШ) prototype laser facility, and employed X-ray micro-tomography technique to give a whole insight into the actual fragmentation process. To investigate the size distribution of the soft recovered fragments from Poly 4-methyl-1-pentene (PMP) foam sample, we further developed an automatic analysis approach based on the improved watershed segmentation. Comparison results of segmenting fragments in slices with different methods demonstrated that our proposed segmentation method can overcome the drawbacks of under-segmentation and over-segmentation, and has the best performance in both segmentation accuracy and robustness. With the proposed automatic analysis approach, other parameters such as the position distribution and penetration depth are also obtained, which are very helpful for understanding the dynamic failure mechanisms.

Blind deconvolution for spatial distribution of Kα emission from ultraintense laser-plasma interaction.

The spatial distributions of the Kα emission from foil targets irradiated with ultra-intensity laser pulses have been studied using the x-ray coded imaging technique. Due to the effect of hard x-ray background contamination, noise as well as imperfection of imaging system, it is hard to determine the PSF analytically or measure it experimentally. Therefore, we propose a blind deconvolution method to restore both the spatial distributions of the Kα emission and the system's PSF from the coded images based on the maximum-likelihood scheme. Experimental restoration results from penumbral imaging and ring coded imaging demonstrated that both the structure integrity and the rich detail information can be well preserved.

Analysis on preferential free running laser wavelength and performance modeling of Tm³âº-doped YAP and YLF.

The preferential free running laser wavelength at room temperature for different axes cuts of Tm³âº-doped YAP and YLF is comparatively analyzed in this paper. The polarized gain spectrum of Tm:YAP and Tm:YLF with different product values of Tm³âº-doped concentration and crystal length is theoretically calculated under various cavity output mirror transmissions. From the gain spectrum, it straightforwardly determines the preferential free running laser wavelength for a given light polarization. In addition, a rate equation model is further used to model and compare the laser output performance for both the free running and some common artificially selected oscillating wavelengths, including 1.99 and 1.94 µm of Tm:YAP, and 1.89, 1.91, and 1.94 µm of Tm:YLF, respectively. To achieve an expected laser oscillating wavelength with acceptable output performance, our analysis presented here is very beneficial for one to choose the most suitable axis cut of crystal.

Realizing a Gabor zone plate with quasi-random distributed hexagon dots.

We propose a quasi-random-dot-array binary Gabor zone plate (QBGZP) with focusing properties of single order foci only. These features are verified with simulations and experiments in the visible light region. Moreover, we find that the performance of QBGZP, which is composed of hexagon patterns, is determined by the ratio of hexagon circumcircle diameter to the outermost zone width. The QBGZP offers a potential alternative for focusing and imaging in the soft x-ray and extreme ultraviolet region.

Measurement of the chirp characteristics of linearly chirped pulses by a frequency domain interference method.

A linear optical technique for chirp characteristics measurement based on frequency domain interference is developed. This technique can be applied to measure the temporal structure of linearly chirped pulses which have become increasingly important in ultrafast optics. To confirm this technique, an experiment is carried out to measure the chirp rate and duration of a picosecond chirped pulse with an imaging spectrometer.

Fractional spiral zone plates.

In this paper, we generalize the concept of classical spiral zone plates (SZPs) to fractional spiral zone plates (FSZPs). By using an SZP with a fractional topological charge and controlling the starting orientation, we can break down the symmetry of the focusing process to give orientation-selective anisotropic vortex foci. Numerical results show that its binary structure gives additional high-order foci on the optical axis and the intensities in the foci can be controlled by properly choosing the fractional topological charge. Our study reveals the feasibility to control the intensity in the foci by means of FSZPs.

Crystal structure prediction and non-superconductivity of N-doped LuH3at near ambient pressure.

Lanthanide polyhydrides, which have attracted the attention of researchers, are considered as a potential candidate material for high-temperature superconductivity. Especially, it is reported that N-doped LuH3exhibits near ambient superconductivity recently. It has attracted attention to room temperature superconductivity of ternary Lu-N-H systems at near ambient pressure. Here, we constructed a LuNH3(N-doped LuH3) compound to predict the crystal structural at relatively low pressures. We found a stable ternary LuNH3structure with a tetragonalP4mmphase under 5 GPa. In addition, ourTccalculations show that theP4mmLuNH3structure does not exhibit superconductivity down to 0.3 K at near ambient pressure due to the H atoms hardly contribute to acoustical phonons.

Selective generation of narrow-band harmonics by a relativistic laser pulse interaction with a detuned plasma grating.

The spectral characteristics of high-order harmonics generated by the interaction of a linearly polarized relativistic laser pulse with a plasma grating target are investigated. Through particle-in-cell simulations and an analytical model, it is shown that a plasma grating target with periodic structure can select special harmonics with integer multiples of the grating frequency, and that low-order harmonics with frequencies being integer times of the laser frequency are generated nearly parallel to the target surface from a Fresnel zone plate target with an aperiodic structure. Spectral control of the harmonics can be achieved by introducing a correction factor ß to the radius formula of the Fresnel zone plate, which can create a slightly detuned plasma grating, and then only the narrow-band harmonics can be selected nearly parallel to the target surface. The center order of the narrow-band harmonics can be tuned by adjusting the correction factor ß, while the bandwidth of the harmonics can be selected by adjusting the other parameter λ_{f} of the detuned plasma grating.

Annulus-sector-element coded Gabor zone plate at the x-ray wavelength.

It is proposed in this paper that an x-ray Gabor zone plate can be realized by properly arranging annulus-sector-shaped nanometer structure apertures along each zone. This provides a new coding methodology which can be used to fabricate a binary zone plate with single order foci only. Numerical simulation results show good agreement with the physical design.

Observation of a high degree of stopping for laser-accelerated intense proton beams in dense ionized matter.

Intense particle beams generated from the interaction of ultrahigh intensity lasers with sample foils provide options in radiography, high-yield neutron sources, high-energy-density-matter generation, and ion fast ignition. An accurate understanding of beam transportation behavior in dense matter is crucial for all these applications. Here we report the experimental evidence on one order of magnitude enhancement of intense laser-accelerated proton beam stopping in dense ionized matter, in comparison with the current-widely used models describing individual ion stopping in matter. Supported by particle-in-cell (PIC) simulations, we attribute the enhancement to the strong decelerating electric field approaching 1 GV/m that can be created by the beam-driven return current. This collective effect plays the dominant role in the stopping of laser-accelerated intense proton beams in dense ionized matter. This finding is essential for the optimum design of ion driven fast ignition and inertial confinement fusion.

Quantum Mechanisms of Electron and Positron Acceleration through Nonlinear Compton Scatterings and Nonlinear Breit-Wheeler Processes in Coherent Photon Dominated Regime.

Electric force is presently the only means in laboratory to accelerate charged particles to high energies, corresponding acceleration processes are classical and continuous. Here we report on how to accelerate electrons and positrons to high energies using ultra intense lasers (UIL) through two quantum processes, nonlinear Compton scattering and nonlinear Breit-Wheeler process. In the coherent photon dominated regime of these two processes, the former can effectively boost electrons/positrons and the latter can produce high energy electrons and positrons with low energy γ photons. The energy needed for such quantum acceleration (QA) is transferred from large numbers of coherent laser photons through the two quantum processes. QA also collimate the generated high energy electrons and positrons along the laser axis and the effective acceleration distance is of microscopic dimensions. Proof of principle QA experiment can be performed on 100 petawatt (PW) scale lasers which are in building or planning.

Measurement of the injecting time of picosecond laser in indirect-drive integrated fast ignition experiments using an x-ray streak camera.

The injecting time of the picosecond laser in an indirect-drive integrated fast ignition experiment was measured by using an x-ray streak camera. Despite overlapping spatially and temporally in experiments, the soft x-ray signal from the nanosecond laser ablating the inner wall of an Au hohlraum and the hard x-ray signal from the bremsstrahlung radiation of hot electrons generated by a picosecond laser were separated by different image processes by filtering and collimating the two signals differently. The time sequence between the two x-ray signals was analyzed to extract the injection time of the picosecond laser relative to the hohlraum emission. By tracking the neutron yield as a function of the injection time of the picosecond laser, a clear positive correlation between the neutron yield enhancement and the derived injection times was exhibited. The heating effect of the picosecond laser was confirmed. It is concluded that this method could be used to measure the injecting time and validate the picosecond laser injection.

Nonlinear laser focusing using a conical guide and generation of energetic ions.

Using conventional methods, a laser pulse can be focused down to around 6-8 microm, but further reduction of the spot size has proven to be difficult. Here it is shown by particle-in-cell simulation that with a hollow cone an intense laser pulse can be reduced to a tiny, highly localized, spot of around 1 microm radius, accompanied by much enhanced light intensity. The pulse shaping and focusing effect is due to a nonlinear laser-plasma interaction on the inner surface of the cone. When a thin foil is attached to the tip of the cone, the cone-focused light pulse compresses and accelerates the ions in its path and can punch through the thin target, creating highly localized energetic ion bunches of high density.