Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

Synthesis of multi-color laser pulses has been developed as a promising way to improve low conversion efficiency of high-order harmonic generation(HHG). Here we systematically study the effect of laser focus in a two-color waveform on generation of macroscopic HHG in soft x-rays. We find that the dependence of HHG yields on laser focus at low or high gas pressure is sensitive to the characteristics of single-atom harmonic response, in which “short”-or “long”-trajectory emissions can be selectively controlled by changing the waveform of two-color synthesized laser pulse. We uncover the phase-matching mechanism of HHG in the gas medium by examining the propagation of the two-color waveform and the evolution of time-frequency emissions of high-harmonic field. We further reveal that the nonlinear effects, such as geometric phase, atomic dispersion, and plasma defocusing, are responsible for modification of two-color waveform upon propagation. This work can be used to find better macroscopic conditions for generating soft x-ray HHG by employing two-color optimized waveforms.

Yield enhancement of elliptical high harmonics driven by bicircular laser pulses

We theoretically investigate the yield enhancement of elliptical high harmonics in the interaction of molecules with bicircular laser pulses by solving the time-dependent Schrodinger equation.It is shown that by adjusting the relative intensity ratio of the two bicircular field components in specific ranges the yield of the molecular high harmonics for the plateau and cutoff regions can be respectively enhanced.To analyze this enhancement phenomenon,we calculate the weights of the electron classical trajectories.Additionally,we also study the ellipticity distribution of harmonics for different intensity ratios.We find that these enhanced harmonics are elliptically polarized,which we mainly attribute to the recombination dipole moment of the major weighted trajectories.These enhanced elliptical extreme ultraviolet and soft x-ray radiations may serve as essential tools for exploring the ultrafast dynamics in magnetic materials and chiral media.

Search for Laser Lines in Sodium like Fe Plasmas

Energy levels, transition probabilities and effective collision strength for 1s2 2s2 2p6 3l, 4l, 5l (l = 0, 1, 2, 3, 4) states of sodium like Fe are used in the determination of the reduced populations for 21 fine structure levels over a wide rang of electron density values (1018 to 1020 cm–3) and at against electron plasma temperatures. Gain coefficients are evaluated and plotted against the electron density.

Theoretical Study of Laser Emission for C-Like (Ar XIII), (Ti XVII) and (Fe XXI)

Energy levels, transition probability and oscillator strengths have been calculated for the Ar XIII, Ti XVII and Fe XXI. The configurations included in the calculations are 2s2 2p2, 2s2 2p 3l (l = s, p & d) and 4l (l = s, p, d, & f) of C-like Ar XIII, Ti XVII & Fe XXI which has 69 fine structures by using the fully relativistic flexible atomic code (FAC) program. These data are used in the determination of the reduced population and gain coefficients over a wide range of electron densities from (10+18 to 10+23) and at various plasmas temperatures. The results show that the transitions in Ar18+, Ti22+, and Fe26+ ions are the most promising laser emission lines in the XUV and soft X-ray spectral regions.

High-repetition-rate seeded free-electron laser enhanced by self-modulation

The spectroscopic methods for the ultrafast electronic and structural dynamics of materials require fully coherent extreme ultraviolet and soft X-ray radiation with high-average brightness.Seeded free-electron lasers(FELs)are ideal sources for delivering fully coherent soft X-ray pulses.However,due to state-of-theart laser system limitations,it is challenging to meet the ultraviolet seed laser’s requirements of sufficient energy modulation and high repetition rates simultaneously.The self-modulation scheme has been proposed and recently demonstrated in a seeded FEL to relax the seed laser requirements.Using numerical simulations,we show that the required seed laser intensity in the self-modulation is~3 orders of magnitude lower than that in the standard high-gain harmonic generation(HGHG).The harmonic self-modulation can launch a singlestage HGHG FEL lasing at the 30th harmonic of the seed laser.Moreover,the proof-of-principle experimental results confirm that the harmonic self-modulation can still amplify the laser-induced energy modulation.These achievements reveal that the self-modulation can not only remarkably reduce the requirements of the seed laser but also improve the harmonic upconversion efficiency,which paves the way for realizing high-repetitionrate and fully coherent soft X-ray FELs.

Control of soft X-ray high harmonic spectrum by using two-color laser pulses

We demonstrate the suppression of soft X-ray high harmonics generated by two-color laser pulses interacting with Ne gas in a gas cell. We show that harmonic suppression can occur at the proper combination of the propagation distance and gas pressure. The physical mechanism behind is the phase mismatch between ＂short＂-trajectory harmonics generated at the early and later times through the interplay of geo- metric phase, dispersion, and plasma effects. In addition, we demonstrate that the position and depth of harmonic suppression can be tuned by increasing the gas pressure. Furthermore, the suppression can be extended to other laser focusing configurations by properly scaling macroscopic parameters. Our investigation reveals a simple controlling soft X-ray Laser Press and novel experimental scheme purely relying on the phase mismatch for selectively tabletop light sources without adopting the filters for applications.

Experimental Study of Short-Wavelength Li-like Soft X-ray Lasers

In this paper,the results of the X-ray laser-gain experiments of Li-like K and Ca ions conducted recently at LF12 Laser Facility of SIOFM with KC1 and CaFi slab targets,will be presented.Also presented will be the space-resolved time history of ASE emission in the Li-like X-ray lasers and the in-situ calibration for the X-ray film used in the experiments.

Clean source of soft X-ray radiation formed in supersonic Ar gas jets by high-contrast femtosecond laser pulses of relativistic intensity

In this work,we optimized a clean,versatile,compact source of soft X-ray radiation(Ex-ray∼3 keV)with an yield per shot up to 7×10^11 photons/shot in a plasma generated by the interaction of high-contrast femtosecond laser pulses of relativistic intensity(Ilas∼10^18-10^19 W/cm^2)with supersonic argon gas jets.Using high-resolution X-ray spectroscopy approaches,the dependence of main characteristics(temperature,density and ionization composition)and the emission efficiency of the X-ray source on laser pulse parameters and properties of the gas medium was studied.The optimal conditions,when the X-ray photon yield reached a maximum value,have been found when the argon plasma has an electron temperature of Te∼185 eV,an electron density of Ne∼7×10^20 cm^-3 and an average charge of Z∼14.In such a plasma,a coefficient of conversion to soft X-ray radiation with energies Ex-ray∼3.1(±0.2)keV reaches 8.57×10^-5,and no processes leading to the acceleration of electrons to MeV energies occur.It was found that the efficiency of the X-ray emission of this plasma source is mainly determined by the focusing geometry.We confirmed experimentally that the angular distribution of the X-ray radiation is isotropic,and its intensity linearly depends on the energy of the laser pulse,which was varied in the range of 50-280 mJ.We also found that the yield of X-ray photons can be notably increased by,for example,choosing the optimal laser pulse duration and the inlet pressure of the gas jet.

Optical beat notes assisted attosecond soft X-ray pulse generation in high-gain free electron lasers

Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena.In this paper,a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating.Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation,which helps to generate a gradually varied spacing electron pulse train.Subsequently,the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation.Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique.

Soft X-ray laser observation of femtosecond-laser-driven ablation of tungsten

Ablation dynamics of tungsten irradiated with a 70 fs laser pulse is investigated with X-ray interferometry and X-ray imaging using a 13.9 nm soft X-ray laser of 7 ps pulse duration. The evolution of high-density ablation front of tungsten （i.e., W） is presented. The ablation front expands to ~120 nm above the original target surface at 160 ps after femtosecond-laser irradiation with an expansion speed of approximately 750 m/s. These results will provide important data for understanding ablation properties of W, which is a candidate material of the first wall of magnetic confinement fusion reactors.

用于高温等离子体电子密度测量的摩尔偏折仪

摩尔偏析测量术通过测量探针光束经过不均匀介质后光线的偏折来估算介质 的折射率。基于这一理论，研制了实用的摩尔偏折仪，利用软Ｘ射线激光作为探针来探测高温 高密等离子体的电子密度，结合软Ｘ射线激光实验做了初步的演示。

Development of readout electronics for bunch arrival-time monitor system at SXFEL

A bunch arrival-time monitor(BAM) system,based on electro-optical intensity modulation scheme, is under study at Shanghai Soft X-ray Free Electron Laser.The aim of the study is to achieve high-precision time measurement for minimizing bunch fluctuations. A readout electronics is developed to fulfill the requirements of the BAM system. The readout electronics is mainly composed of a signal conditioning circuit, field-programmable gate array(FPGA), mezzanine card(FMC150), and powerful FPGA carrier board. The signal conditioning circuit converts the laser pulses into electrical pulse signals using a photodiode. Thereafter, it performs splitting and low-noise amplification to achieve the best voltage sampling performance of the dual-channel analog-to-digital converter(ADC) in FMC150. The FMC150 ADC daughter card includes a 14-bit 250 Msps dual-channel high-speed ADC,a clock configuration, and a management module. The powerful FPGA carrier board is a commercial high-performance Xilinx Kintex-7 FPGA evaluation board. To achieve clock and data alignment for ADC data capture at a high sampling rate, we used ISERDES, IDELAY, and dedicated carry-in resources in the Kintex-7 FPGA. This paper presents a detailed development of the readout electronics in the BAM system and its performance.

TIME-RESOLVED STUDIES OF SOFT X-RAY EMISSIONS AND SPATIAL UNIFORMITY OF LINEAR PLASMAS

Laser plasmas produced by short-pulse lasers (from femtoseconds to nanoseconds) irradiating on the solid surface are intense soft X-ray (SXR) sources in the range from 100 eV to 2 keV, which are of important applications in X-ray laser researches and structure anal-

Soft X-ray recombination lasing by Li-like TiXX ion

Li-like ion scheme of recombination pumping mechanism is one of the most promisingapproaches to compact and efficiently saturated amplifier in the 'Water Window' spectral re-gion. Recently, numerous important investigations have been made in a number of famouslaboratories worldwide and considerable progresses have been made. However, someissues on the lasing mechanism have not been quite understood yet. Especially When one at-

A single stage EEHG at SXFEL for narrow-bandwidth soft X-ray generation

The echo-enabled harmonic generation （EEHG） scheme holds promising prospects for efficiently generat- ing intense coherent radiation at very high harmonics of the initial seed laser. In this paper, we study the feasibility of operating the Shanghai X-ray free-electron laser test facility with a single stage EEHG setup. With a combina- tion of existing numerical codes, start-to-end simulations considering various three-dimensional effects have been carried out to show the possible performance of the EEHG FEL. Simulation results demonstrate that a single stage EEHG can generate high power soft X-ray radiation pulses with narrow bandwidth directly from UV seed lasers. With the help of the harmonic lasing technique or upgraded linac, we also show the possibility of generating coherent soft X-ray radiation in the ＂water window＂ with a single stage EEHG.

Preliminary analysis of in HL-2A ohmic impurity transport discharges

This paper describes the behaviour of impurity transport in HL-2A ohmic discharges. In 2005, small quantities of metallic impurities （A1, Ni and Ti） were successfully injected into HL-2A plasmas by laser blow-off technique, and their progression was followed by the soft x-ray cameras with good spatial and temporal resolutions. The impurity confinement time is estimated from the characteristic decay time of the soft x-ray signal of the injected impurities, and it is about 30-60 ms. The transport coefficients of impurities （including diffusion coefficient and convection velocity） in radial different region have been derived by using a one-dimenslonal impurity transport code, the results present that diffusion coefficient is much smaller in the central region of plasmas than the outside of it, and it is much larger than that of neoclassical theory predictions; namely, it is anomalous.

Analysis of electro-optical intensity modulator for bunch arrival-time monitor at SXFEL

A bunch arrival-time monitor(BAM) based on an electro-optical intensity modulation scheme is currently under development at Shanghai Soft X-ray Free-Electron Laser to meet the high-resolution requirements for bunch stability. The BAM uses a radio frequency signal generated by a pickup cavity to modulate the reference laser pulses in an electro-optical intensity modulator(EOM), and the bunch arrival-time information is derived from the amplitude change of the laser pulse after laser pulse modulation.EOM is a key optical component in the BAM system.Through the basic principle analysis of BAM, many parameters of the EOM are observed to affect the measurement resolution of the BAM system. Therefore, a systematic analysis of the EOM is crucial. In this paper, we present two schemes to compare and analyze an EOM and provide a reference for selecting a new version of the EOM.

Maximizing magnetic field generation in high power laser-solid interactions

In order to understand the transport of fast electrons within solid density targets driven by an optical high power laser,we have numerically investigated the dynamics and structure of strong self-generated magnetic fields in such experiments.Here we present a systematic study of the bulk magnetic field generation due to the ponderomotive current,Weibel-like instability and resistivity gradient between two solid layers.Using particle-in-cell simulations,we observe the effect of varying the laser and target parameters,including laser intensity,focal size,incident angle,preplasma scale length,target thickness and material and experimental geometry.The simulation results suggest that the strongest magnetic field is generated with laser incident angles and preplasma scale lengths that maximize laser absorption efficiency.The recent commissioning of experimental platforms equipped with both optical high power laser and X-ray free electron laser(XFEL),such as European XFEL-HED,LCLS-MEC and SACLA beamlines,provides unprecedented opportunities to probe the self-generated bulk magnetic field by X-ray polarimetry via Faraday rotation with simultaneous high spatial and temporal resolution.We expect that this systematic numerical investigation will pave the way to design and optimize near future experimental setups to probe the magnetic fields in such experimental platforms.

Dispersion effects on performance of free-electron laser based on laser wakefield accelerator

In this study, we investigate a new simple scheme using a planar undulator(PU) together with a properly dispersed electron beam(e beam) with a large energy spread(～1%) to enhance the free-electron laser(FEL) gain. For a dispersed e beam in a PU, the resonant condition is satisfied for the center electrons, while the frequency detuning increases for the off-center electrons, inhibiting the growth of the radiation. The PU can act as a filter for selecting the electrons near the beam center to achieve the radiation. Although only the center electrons contribute, the radiation can be enhanced significantly owing to the high-peak current of the beam. Theoretical analysis and simulation results indicate that this method can be used for the improvement of the radiation performance, which has great significance for short-wavelength FEL applications.

软X射线激光偏折法测量激光等离子体电子密度分布

利用脉宽约为 5 0ps的类镍 银 13 9nm软X射线激光作为探针 ,探测由脉宽 80ps的驱动激光打C8H8靶产生的等离子体在 1ns后的电子密度分布信息 ,获得了清晰的莫尔条纹图像 .对结果的处理 ,给出了峰值电子密度为 1 1× 10 2 1 cm- 3 。