Laser-produced plasma helium-like titanium Kα x-ray source and its application to Rayleigh-Taylor instability study

Several experiments are performed on the ShenGuang-II laser facility to investigate an x-ray source and test radiography concepts.X-ray lines emitted from laser-produced plasmas are the most practical means of generating these high intensity sources.By using a time-integrated space-resolved keV spectroscope and pinhole camera,potential helium-like titanium Kα x-ray backlighting (radiography) line source is studied as a function of laser wavelength,ratio of pre-pulse intensity to main pulse intensity,and laser intensity (from 7.25 to～11.3×10 15 W/cm 2).One-dimensional radiography using a grid consisting of 5 μm Au wires on 16 μm period and the pinhole-assisted point projection is tested.The measurements show that the size of the helium-like titanium Kα source from a simple foil target is larger than 100 μm,and relative x-ray line emission conversion efficiency ξ x from the incident laser light energy to heliumlike titanium K-shell spectrum increases significantly with pre-pulse intensity increasing,increases rapidly with laser wavelength decreasing,and increases moderately with main laser intensity increasing.It is also found that a gold gird foils can reach an imaging resolution better than 5-μm featured with high contrast.It is further demonstrated that the pinhole-assisted point projection at such a level will be a novel two-dimensional imaging diagnostic technique for inertial confinement fusion experiments.     

Optimization of 4.7-keV X-ray titanium sources driven by 100-ps laser pulses

An experiment with thin titanium foils irradiated by two pulses delayed in time is conducted on the Shenguang-II laser facility.A prepulse induces an underdense plasma,2-ns later a main pulse(λ L = 0.35 μm,E L ≈ 120 J,τ L ≈ 100 ps) is injected into the underdense plasma and produces strong line emission from the titanium K shell(i.e.,He α at 4.7 keV).Data show that the intensity of 4.7-keV X-ray emission with the prepulse is approximately twice more than without the prepulse,and can be used as a backlighting source satisfying the diagnostic requirements for dense plasma probing.Highquality plasma images are obtained with the backlighting 4.7-keV X-rays in a Rayleigh-Taylor hydrodynamic instability experiment.     

Experimental study of K-shell X-ray emission generated from nanowire target irradiated by relativistic laser pulses

K-shell X-ray emission from a Cu nanowire target irradiated by an ultraintense femtosecond laser pulse is studied using an elliptically bent quartz crystal and imaging plate. The designed bent crystal spectrometer has better spectral resolution, which is higher than 1 000. The absolute Kα radiation photon yields are obtained from the experimental results and the Monte-Carlo model. The conversion efficiency of the Cu Kα line is estimated to be 0.019% from the interaction of 4 J, 50-fs laser pulse irradiated on a Cu nanowire target. The high yield of K shell X-ray has important applications in X-ray emission source.     

Cavity-Defect Plasma Bragg Gratings

Plasma Bragg grating （PBG） is composed of periodic variations of plasma and dielectric or vacuum. The defect mode characteristic of the PBG with a cavity-defect is studied by one-dimensional particle-in-cell （1D PIC） simulation. It is shown that the laser pulse with the defect frequency can be localized around the defect partly and at the same time leak out of both sides of the grating slowly because of the few number of the grating period. This results in local high laser field intensity and high plasma density produced at the defect area, from which the third harmonic is enhanced by one order of magnitude. With the enhancement of the light coupled to the defect and the decrease of the light leaking out of the defect, the conversion efficiency of the third harmonic from the incident laser can be increased.     

Effect of sample temperature on laser-induced semiconductor plasma spectroscopy

We investigate the temperature dependence of the emission spectrum of a laser-induced semiconductor(Ge and Si) plasma. The change in spectral intensity with the sample temperature indicates the change of the laser ablation mass. The reflectivity of the target surface is reduced as the sample is heated, which leads to an increase in the laser energy coupled to the surface of the sample and eventually produces a higher spectral intensity.The spectral intensities are enhanced by a few times at high temperatures compared with the cases at low temperatures. The spectral intensity of Ge is enhanced by 1.5 times at 422.66 nm, and 3 times at589.33 nm when the sample temperature increases from 50°C to 300°C. We can obtain the same emission intensity by a more powerful laser or by less pulse energy with a higher sample temperature. Based on experimental observations we conclude that the preheated sample can improve the emission intensity of laser-induced semiconductor plasma spectroscopy.     

Efficient collinear frequency tripling of femtosecond laser with compensation of group velocity delay

This paper demonstrates an approach that negative uniaxial crystal has a relative anomalous dispersion effect which can compensate group velocity delay, and applies this approach to nonlinear frequency conversion of an ultrafast laser field. High efficiency of the third harmonic generation is experimentally fulfilled by adopting a collinear configuration of doubing-compensation-tripling system. Through finely adjusting the incident angle and optical axis direction of the compensation plate, it obtains ultraviolet (UV) output energy of 0.32 mJ centered at 270 nm with spectral bandwidth of 2 nm when input beam at 800 nm was 70 fs pulse duration and 6 mJ pulse energy which was extracted from Ti:sapphire laser system by a diaphragm, corresponding to an 800-to-270 nm conversion efficiency of 5.3% and a factor-of-1.6 improvement in the third harmonic generation of UV band in comparison with a general conventional configuration. Furthermore, when the full energy of 18 mJ from a Ti:sapphire laser system was used and optimized, the UV emission could reach 0.83 mJ.     

Performance of gain-switched all-solid-state quasi-continuous-wave tunable Ti:sapphire laser system

We have made a gain-switched all-solid-state quasi-continuous-wave （QCW） tunable Ti：sapphire laser system, which is pumped by a 532 nm intracavity frequency-doubled Nd：YAG laser. Based on the theory of gain-switching and the study on the influencing factors of the output pulse width, an effective method for obtaining high power and narrow pulse width output is proposed. Through deliberately designing the pump source and the resonator of the Ti：sapphire laser, when the repetition rate is 6 kHz and the length of the cavity is 220 mm, at an incident pump power of 22 W, the tunable Ti：sapphire laser from 700 to 950nm can be achieved. It has a maximum average output power of 5.6W at 800nm and the pulse width of 13.2 ns, giving an optical conversion efficiency of 25.5% from the 532 mn pump laser to the Ti：sapphire laser.     

Digital coherent detection research on Brillouin optical time domain reflectometry with simplex pulse codes

The X-ray spectrometer used in high-energy-density plasma experiments generally requires both broad X-ray energy coverage and high temporal, spatial, and spectral resolutions for overcoming the difficulties imposed by the X-ray back- ground, debris, and mechanical shocks. By using an elliptical crystal together with a streak camera, we resolve this issue at the SG-II laser facility. The carefully designed elliptical crystal has a broad spectral coverage with high resolution, strong rejection of the diffuse and/or fluorescent background radiation, and negligible source broadening for extended sources. The spectra that are Bragg reflected （23° 〈 θ 〈 38°） from the crystal are focused onto a streak camera slit 18 mm long and about 80 μm wide, to obtain a time-resolved spectrum. With experimental measurements, we demonstrate that the quartz（1011） elliptical analyzer at the SG-II laser facility has a single-shot spectral range of （4.64-6.45） keV, a typical spectral resolution of E/△E = 560, and an enhanced focusing power in the spectral dimension. For titanium （Ti） data, the lines of interest show a distribution as a function of time and the temporal variations of the He-α and Li-like Ti satellite lines and their spatial profiles show intensity peak red shifts. The spectrometer sensitivity is illustrated with a temporal resolution of better than 25 ps, which satisfies the near-term requirements of high-energy-density physics experiments.     

Broadband time-resolved elliptical crystal spectrometer for X-ray spectroscopic measurements in laser-produced plasmas

The X-ray spectrometer used in high-energy-density plasma experiments generally requires both broad X-ray energy coverage and high temporal, spatial, and spectral resolutions for overcoming the difficulties imposed by the X-ray background, debris, and mechanical shocks. By using an elliptical crystal together with a streak camera, we resolve this issue at the SG-II laser facility. The carefully designed elliptical crystal has a broad spectral coverage with high resolution, strong rejection of the diffuse and/or fluorescent background radiation, and negligible source broadening for extended sources.The spectra that are Bragg reflected(23?< θ < 38?) from the crystal are focused onto a streak camera slit 18 mm long and about 80 μm wide, to obtain a time-resolved spectrum. With experimental measurements, we demonstrate that the quartz(1011) elliptical analyzer at the SG-II laser facility has a single-shot spectral range of(4.64–6.45) keV, a typical spectral resolution of E/?E = 560, and an enhanced focusing power in the spectral dimension. For titanium(Ti) data, the lines of interest show a distribution as a function of time and the temporal variations of the He-α and Li-like Ti satellite lines and their spatial profiles show intensity peak red shifts. The spectrometer sensitivity is illustrated with a temporal resolution of better than 25 ps, which satisfies the near-term requirements of high-energy-density physics experiments.     

Producing High Intense Attosecond Pulse Train by Interaction of Three-Color Pulse and Overdense Plasma

Amplifying the attosecond pulse by the chirp pulse amplification method is impossible. Furthermore, the intensity of attosecond pulse is low in the interaction of laser pulse and underdense plasma. This motivates us to propose using a multi-color pulse to produce the high intense attosecond pulse. In the present study; the relativistic interaction of a three-color linearly-polarized laser-pulse with highly overdense plasma is studied. We show that the combination of ω_1, ω_2 and ω_3 frequencies decreases the instance full width at half maximum reflected attosecond pulse train from the overdense plasma surface. Moreover,we show that the three-color pulse increases the intensity of generated harmonics, which is explained by the relativistic oscillating mirror model. The obtained results demonstrate that if the three-color laser pulse interacts with overdense plasma, it will enhance two orders of magnitude of intensity of ultra short attosecond pulses in comparison with monochromatic pulse.     

Spectral distribution of X-Ray yield of the M-shell emission fromlaser produced gold plasma

Experimental measurements of the spectral distribution of keV X-ray energy conversion of the M-shell emission from gold plasma produced by a 4 GW, 5 nS Nd:glass laser are presented. At a laser intensity of 2×10¹³ W/cm², the overall X-ray yield for 2.0 ⩽hv⩽3.6 keV is determined to be 0.1% of the laser energy. The effect of the keV X-ray spectral distribution on radiation preheat is discussed     

飞秒激光-物质相互作用产生Kα X射线

 用无色散X射线谱仪分别在靶前后测量了飞秒激光辐照铜箔产生的Kα X射线,获得了能量转换效率。入射激光脉冲宽度33 fs,能量在50 mJ～5 J,强度10₁₇～10₁₉ W/cm₂。靶后发射的Kα X射线强度随入射激光能量的增加而增加,其单色性较靶前好。采用100 μm厚靶,其能量转换率为2.2×10_-5。     

钛空腔靶高强度keV X射线源的实验研究

利用神光-Ⅲ原型激光装置实验研究了8束ns激光脉冲从一端注入钛空腔靶产生的keV X射线源的辐射特征,发现keV X射线主要产生于腔轴附近;钛空腔靶内径过大时keV X射线能流的峰值强度较低,内径过小时keV X射线能流的持续时间较短。为了在4π空间内使钛空腔靶获得最大的X射线(4~7keV能段)转换效率,腔内径的最优值在1000~1300μm附近,此时的keV X射线转换效率为4.7%,是相同激光参数下钛平面靶的2倍左右。激光单端注入有底部钛膜和无底部钛膜的空腔靶对比实验显示,底膜能够增强keV X射线的发射。     

罗斯滤片法测量激光打靶X射线辐射通量

利用一套罗斯滤片系统测量激光轰击固体金属Ti平面靶产生的X射线辐射通量,系统包括两个相同的X射线探测器及相应滤膜。罗斯滤片法的优点在于利用相邻核素对X射线相似的阻止率,可滤出一个窄的能带并去除高能部分的干扰,获得了Ti平面靶K壳层X光辐射产额。实验结果表明:硬X射线能段在4.5~4.9keV之间的K壳特征辐射占优,连续谱所占份额较低(与晶体谱仪一致);随着激光能量的增加,特征辐射增加;激光强度接近2×1015 W/cm2时,能量转换效率出现峰值。     

X-Rays emission by high-intensity pulsed lasers irradiating thin foils at PALS laboratory

X-rays and forward ion emission from laser-generated plasma in the Target Normal Sheath Acceleration regime of different targets with 10-μm thickness, irradiated at Prague Asterix Laser System (PALS) laboratory at about 10¹⁶ W/cm² intensity, employing a 1,315 nm-wavelength laser with a 300-ps pulse duration, are investigated. The photon and ion emissions were mainly measured using Silicon Carbide (SiC) detectors in time-of-flight configuration and X-ray streak camera imaging. The results show that the maximum proton acceleration value and the X-ray emission yield growth are proportional to the atomic number of the irradiated targets. The X-ray emission is not isotropic, with energies increasing from 1 keV for light atomic targets to about 2.5 keV for heavy atomic targets. The laser focal position significantly influences the X-ray emission from light and heavy irradiated targets, indicating the possible induction of self-focusing effects when the laser beam is focalized in front of the light target surface and of electron density enhancement for focalization inside the target.     

DD reaction enhancement and X-ray generation in a high-current pulsed glow discharge in deuterium with titanium cathode at 0.8–2.45 kV

The DD reaction yield (3-MeV protons) and the soft X-ray emission from a titanium (Ti) cathode surface in a periodic pulsed glow discharge in deuterium were studied at a discharge voltage of 0.8–2.45 kV and a discharge current density of 300–600 mA/cm². The electron screening potential U_e = 610 ± 150 eV was estimated in the range of deuteron energies 0.8 keV < E_d < 2.45 keV from an analysis of the DD reaction yield as a function of the accelerating voltage. The obtained data show evidence for a significant enhancement of the DD reaction yield in Ti in comparison to both theoretical estimates (based on the extrapolation of the known DD reaction cross section for E_d ≥ 5 keV to low deuteron energies in the Bosch-Halle approximation) and the results of experiments using accelerators at the deuteron energies E_lab ≥ 2.5 keV and current densities 50–500 μ A/cm². Intense emission of soft X-ray quanta (10¹³–10¹⁴ s^?1 cm^?2) was observed at an average energy of 1.2–1.5 keV. The X-ray emission intensity and the DD reaction yield enhancement strongly depend on the rate of deuterium diffusion in a thin subsurface layer of Ti cathode.     

黑腔中等离子体相互作用的流体力学现象观测

激光间接驱动惯性约束聚变实验中，黑腔内情况复杂，在激光烧蚀和辐射烧蚀等的驱动下，光斑区、冕区、纯辐射烧蚀区、射流区的多种等离子体以不同规律运动.发展了X光双能段窄能带的时间分辨成像方法，用以观测黑腔内多种等离子体的运动情况.在真空黑腔中观测到清晰的射流，分析了射流产生机制及其速度；在黑腔中充气，能有效消除射流和抑制冕区等离子体运动，但两种物质界面处可能会出现流体力学不稳定性等现象，分析了界面处的压力平衡关系和密度陡变情况.     

Formation of Ti and TiN ultra-thin films on Si by ion beam sputter deposition

Ultra-thin titanium and titanium nitride films on silicon substrate were obtained by ion beam sputtering of titanium target in vacuum and nitrogen atmosphere, using argon ions with energy of 5 keV and 15 μA target current. Elemental composition and chemical state of obtained films were investigated by X-ray photoelectron spectroscopy with using Mg-Kα X-ray radiation (photon energy 1253.6 eV). It was shown that it is possible to form both ultra-thin titanium films (sputtering in vacuum) and ultra-thin titanium nitride films (sputtering in nitrogen atmosphere) in the same temperature conditions. Photoelectron spectra of samples surface, obtained in different steps of films synthesis, detailed spectra of photoelectron emission from Si 2p, Ti 2p, N 1s core levels and also X-ray photoelectron spectra of Auger electrons emission are presented.     

激光加热Cu和NaF靶产生的1.2keV区X射线转换效率的测量

介绍了激光加热Cu靶和NaF靶发射的在1.2keV区X射线转换效率的测量方法和实验结果。结果表明,在激光辐照功率密度为1×10¹³—1×10¹⁴W·cm^-2条件下,激光波长为1.06μm或0.53μm时,Cu等离子体发射的1.2KeV区X射线的转换率为NaF等离子体的4—5倍;对此两种等离子体,激光波长为0.53μm的X射线转换效率是波长为1.06μm的2倍左右。 关键词：     

Nuclear Reactions under Irradiation of Deuterated Structures by X Rays

Measurements of emission from nuclear reaction products (neutrons and protons) have been carried out appearing in the deuterated structures of textured CVD diamond, palladium, titanium, and zirconium under irradiation with a beam of X rays using independent methods (neutron detector based on He-3 counters, Si surface-barrier semiconductor detectors and CR-39 track detector). The possibility of enhancement of both DD reaction and multi-particle deuterium fusion by the beam of X rays with energy ranging 20–30 keV in solid deuterated targets has been established. Analysis of X-ray fluorescence spectra of the target bombarded by beams of ions has revealed “additional” peaks, the occurrence of which cannot be related to any of the known elements, and requires separate study.