全光汤姆逊散射X光源数值模拟

 使用蒙特卡罗数值模拟程序研究了不同电子束和激光强度下,尾场电子束与超短脉冲激光发生汤姆逊散射获得的X光子能谱、角分布等,初步探索了180°散射下高质量X射线光源所需要的电子束与激光强度。数值模拟结果显示,为了尽量提高光子产额并抑制非线性效应,激光归一化强度应为1的量级。为了获得方向性和单色性好、亮度高的X射线脉冲,需要较高的电子能量和较小的电子能散,电子束脉冲尽量短,且发散角尽量小。     

上海激光电子γ源

激光具有高强度、 高极化度等优异的性能。 用激光束轰击高能电子束就可以产生高强度、 高极化度的γ射线束。 上海激光电子γ源就是上海同步辐射装置上的这样一条束线站。 预计可以获得能量范围为1—22 MeV的准单色、 高强度（109—1011 s-1）和高极化度（线极化或圆极化）的γ射线束。 介绍了这条束线站目前的进展情况。 Shanghai Laser Electron Gamma Source (SLEGS) is a high intensity,short pulse and compact γ ray source which is based on inverse Compton scattering via interaction between pulsed high power laser beams and picosecond relativistic electron bunches. One of the attractive features of the laser Compton scattering is the easy control of polarization of the produced high energy photons that duplicates polarization of the applied laser beam. The γ ray with energy up to 22 MeV and intensity of 109—1011s 1 are expected to be produced by Compton backscattering of CO2 laser photons on the 3.5 GeV electrons bunches in the Shanghai Synchrotron Radiation Facility (SSRF). In this communication, we report same simulation results and the progressing status of SLEGS.     

Spatial beam profiles of femtosecond X-ray pulses generated by laser Compton scattering from a low-emittance electron beam

Polarization-dependent spatial beam profiles of femtosecond X-ray pulses generated by a laser Compton scheme were measured. The X-ray pulses were generated by the interaction at an angle of 90° between 100-fs laser light and a 3-ps, 3π-mm mrad electron beam. The polarization of the laser light was linear in two different directions, either parallel or perpendicular to the electron beam axis. The measured profiles showed good agreement with theoretical results. Received: 5 July 2002 / Revised version: 17 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +81-424/684477, E-mail: msf_yorozu@shi.co.jp     

The dependence of the Fe <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific K_α and K_β lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The K_α yield could be enhanced by a factor of 1.7 and reached 1.9×10⁸ Fe K_α photons/s in 4π with the laser pulse positively chirped, and 1.5×10⁸ Fe K_α photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse. PACS 42.65.Re; 52.38.Ph; 52.50.Jm     

超相对论强度激光与薄膜靶作用中0.4 nm以下X射线阿秒脉冲的产生

用一维粒子模拟程序对功率密度在10²² W/cm²以上的超强激光驱动薄膜靶产生的相对论电子层及其经过汤姆孙散射产生的阿秒X射线进行了研究. 结果表明, 在超相对论强度范围下增大驱动激光强度, 相应减小等离子体密度及厚度可使电子层获得更高纵向动量, 使汤姆孙散射光明显向更短波长移动. 优化相关参数得到了波长为 1.168 nm的阿秒脉冲. 经过对倍频探测光方案与驱动光以及薄膜靶参数进行综合考虑和优化, 得到的X射线相干辐射波长有效减小到0.4 nm以下, 产生的光子能量达到2 keV以上. 关键词： 超相对论强度激光 阿秒X射线 相对论电子层 汤姆孙后向散射     

Neutron production in picosecond laser-generated plasma on a be target

Experimental data on neutron production in a plasma generated on a Be target by a picosecond laser of intensity 2 × 10¹⁸ W/cm² are presented. In contrast to previous measurements, a Ta converter is not used in this study to generate γ rays. The neutron yield is equal to 2 × 10³ over a solid angle of 4π steradians per laser pulse. A simultaneous measurement of the maximum energy of hard x rays gave E _γmax ～ 6 MeV, the number of these photons being 5 × 10⁸ over an angle of 4π steradians per laser pulse. The energy distributions of fast electrons and photons are estimated theoretically.     

Traveling-wave Thomson scattering and optical undulators for high-yield EUV and X-ray sources

We present a novel high-yield Thomson scattering geometry that takes advantage of compact electron bunches, as available in advanced, low-emittance linear accelerators or laser wakefield accelerators. In order to avoid the restrictions on the X-ray photon yield imposed by the Rayleigh limit, we use ultrashort, pulse-front tilted laser pulses in a side-scattering geometry. Such a traveling-wave setup allows an overlap of electron and laser beams, even after propagating over distances much longer than the Rayleigh length. Experimental designs are discussed and optimized for different scattering angles. Specifically, to minimize group delay dispersion at large scattering angles >10°, we propose the use of varied-line spacing (VLS) gratings for spatio-temporal laser pulse shaping. Compared to head-on (180°) Thomson scattering, interaction lengths are in the centimeter to meter range and photon numbers for ultrashort X-ray pulses can increase by several orders of magnitudes.     

Table-top kHz hard X-ray source with ultrashort pulse duration for time-resolved X-ray diffraction

The interaction of ultrashort laser pulses with solid state targets is studied concerning the production of short X-ray pulses with photon energies up to about 10 keV. The influence of various parameters such as pulse energy, repetition rate of the laser system, focusing conditions, the application of prepulses, and the chirp of the laser pulses on the efficiency of this highly nonlinear process is examined. In order to increase the X-ray flux, the laser pulse energy is increased by a 2nd multipass amplifier from 750 μJ to 5 mJ. By applying up to 4 mJ of the pulse energy a X-ray flux of 4×10¹⁰ Fe K _α photons/s or 2.75×10¹⁰ Cu K _α photons/s are generated. The energy conversion efficiency is therefore calculated to η _Fe≈1.4×10⁻⁵ and η _Cu≈1.0×10⁻⁵. The X-ray source size is determined to 15×25 μm². By focusing the produced X-rays using a toroidally bent crystal a quasi-monochromatic X-ray point source with a diameter of 56 μm×70μm is produced containing ≈10⁴ Fe K _α1 photons/s which permits the investigation of lattice dynamics on a picosecond or even sub-picosecond time scale. The lattice movement of a GaAs(111) crystal is shown as a typical application.     

Active synchronization of two mode-locked lasers with optical cross correlation

Two mode-locked Ti:sapphire lasers of different wavelengths were precisely synchronized by a simple feedback system employing sum-frequency generation (cross correlation). When the timing error exceeded the pulse duration, the periodic bunch of the sum-frequency pulse was used for rough timing adjustment. Using cross correlation with a stretched pulse, we struck a balance between wide locking range and sensitive timing detection. When the two lasers were well-synchronized, we obtained a continuous cross-correlation pulse train for 3 min. The holding time of the laser synchronization was extended to over one hour by adding a motorized stage to the PZT-mounted cavity mirror. We estimated the rms timing jitter between the two lasers by a scanning cross-correlation measurement. We confirmed that the rms timing jitter of the two lasers during 1.8 s was 28 fs. Received: 30 January 2002 / Revised version: 14 June 2002 / Published online: 8 August 2002     

强激光驱动高能极化正负电子束与偏振伽马射线的研究进展

高能自旋极化正负电子束与偏振伽马射线在高能物理、实验室天体物理与核物理等领域有十分重要的应用.近年来随着超短超强激光脉冲技术的快速发展,利用强激光与物质相互作用的非线性康普顿散射和多光子Breit-Wheeler过程为制备高极化度、高束流密度的高能极化粒子束提供了新的可能.本文对基于强激光产生高能极化正负电子束与偏振伽马射线的研究成果进行简要回顾,并介绍了这些方法的基本物理原理和主要结果.     

Proposal for a Raman X-ray free electron laser

A scheme for an X-ray free electron laser is proposed, based on a Raman process occurring during the interaction between a moderately relativistic bunch of free electrons, and twin intense short pulse lasers interfering to form a transverse standing wave along the electron trajectories. In the high intensity regime of the Kapitza-Dirac effect, the laser ponderomotive potential forces the electrons into a lateral oscillatory motion, resulting in a Raman scattering process. I show how a parametric process is triggered, resulting in the amplification of the Stokes component of the Raman-scattered photons. Experimental operating parameters and implementations, based both on LINAC and Laser Wakefield Acceleration techniques, are discussed.     

Temporal evolution of the xenon laser pulse

An experimental investigation was made of the xenon excimer laser. Several laser cavities have been employed with excitation by electron beams of cross section (15×2) cm² and (55×4) cm² and current density 100–150 A cm^-2 injected transverse to the cavity axis. A numerical model of the xenon laser is compared with experimental results. The laser pulse width was found to be dependent on laser intensity and cavity mirrors. Early termination of the laser pulse was observed consistent with changing reflectivity of the cavity mirrors. An uncoated MgF₂ retroreflector produced a laser output of significantly longer pulse width. Quasi-cw laser action was observed.     

Laser wake field acceleration: the highly non-linear broken-wave regime

We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×10¹⁰ electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period. Received: 12 December 2001 / Published online: 14 March 2002     

Short-pulse, high-brightness X-ray production with the PLEIADES Thomson-scattering source

PLEIADES is a compact, tunable, high-brightness, ultra-short-pulse, Thomson-scattering X-ray source. Picosecond pulses of hard X-rays (10–200 keV) are created by colliding an ultra-relativistic (20–100 MeV), picosecond-duration electron beam with a high-intensity, sub-picosecond, 800-nm laser pulse. Initial operation of this source has produced 78-keV X-rays with 10⁶ photons per pulse using a 57-MeV, 0.3-nC, 50-m rms width electron beam and a 180-mJ, 15-m rms width laser pulse. The angular distribution, energy, and energy spectrum of the source are found to agree well with theory and simulations. Source optimization is expected to increase X-ray output to between 10⁷ and 10⁸ photons per pulse with a peak brightness approaching 10²⁰ photons/s/0.1% bandwidth/mm²/mrad². PACS 41.50.+h; 07.85.Fv; 41.75.Ht; 42.62.-b     

Spatial characteristics of Kα radiation from weakly relativistic laser plasmas

The spatial dependence of K _α emission generated from laser-produced hot electrons is investigated experimentally and theoretically. In addition, the conversion efficiency of K _α production as a function of laser intensity is measured and compared with modeling results. We use the terawatt Ti:sapphire laser at MPQ and vary the peak intensity from 10¹⁵ to 10¹⁸ W/cm² with a pulse duration of 200 fs. A solid Cu target is placed at various positions in the laser focus, which allows one to vary the intensity but keep the total energy on the target constant. When the target is near best focus, the FWHM of the K _α emission, measured using a knife-edge, is considerably larger than the FWHM of the laser intensity. In measuring the efficiency of K _α production using the fundamental wavelength of the laser, a clear maximum of K _α emission is observed at a position away from best focus, where the peak intensity is down by more than an order of magnitude from the value at best focus. When the second harmonic of the laser is used, the K _α emission is peaked near best focus. The K _α emission from layer targets is used to obtain an estimate of the temperature of the hot electrons. Modeling of K _α production, using a Monte Carlo electron/photon transport code, shows the relationship between incident electron energy and the emitted K _α emission. Efficient K _α generation from the low-intensity wings of the laser pulse contributes to the large spot size of the K _α emission. The lower electron temperatures that are expected for the second harmonic explain the differences in the location of maximum K _α emission for the two wavelengths. We discuss the use of K _α emission in photoionizing inner-shell electrons with the goal of achieving X-ray lasing at short wavelengths. Received: 6 April 1999 / Revised version: 31 May 1999 / Published online: 11 August 1999     

In situ observation of dynamics of plasma self-channeling and bulk modification in silica glasses induced by a high-intensity femtosecond laser

The time-resolved dynamics of plasma self-channeling and refractive index bulk modification in silica glasses were first observed in situ using a high-intensity femtosecond (110 fs) Ti:sapphire laser (λ_p=790 nm). Plasma channeling is induced in silica glass at an irradiation higher than an input intensity of 1.5×10¹² W/cm² and photoinduces either the refractive-index modification or optical crack modification. In the domain of refractive-index modification, the lifetime of induced plasma self-channeling was 20 ps and the structural transition time for forming the refractive-index change was 10 ps. In the domain of optical cracks, however, the lifetime of induced plasma formation was 30 ps and the structural transition time for forming the optical cracks was 40 ps. According to electron spin resonance spectroscopic (ESP) measurement, it was found that the defect concentration of the SiE^′ center increased significantly in the refractive index modification region. A maximum value of the refractive-index change Δn was measured to be 1.6×10^-2. The intensity profile of the output beam transmitted through the refractive-index modification showed that the bulk modification produced a permanent optical waveguide. Received: 8 April 2002 / Accepted: 12 April 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: shcho@riken.go.jp     

A simple method for timing an XFEL source to high-power lasers

We propose a technique for timing an X-ray free-electron laser (XFEL) to a high-power conventional laser with femtosecond accuracy, yielding the relative jitter between pump and X-ray probe, and allowing sorting of experimental results over a certain time window. The same electron bunch is used to produce both an XFEL pulse and an ultrashort optical pulse by means of an optical radiator downstream of the X-ray undulator. Being produced by the same electron bunch, these pulses are perfectly synchronized. Cross-correlation techniques will allow to determine relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser. Technical realization of the proposed timing scheme uses an optical-replica synthesizer setup to be installed after the final bunch-compression stage of the XFEL for electron bunch diagnostics purposes. A number of critical issues are quantitatively analyzed.     

Control of parameters of micropinches formed in current-carrying plasma jet

The temporal evolution and spatial pattern of X-ray emission from a laser-induced vacuum discharge of moderate power has been investigated. It was found that micropinches in the initial stage of the cathode jet expansion into the vacuum ambient were formed. They generated a soft X-ray radiation and beams of accelerated electrons; therewith these phenomena occurred just when both amplitude of the discharge current and energy of the initiating laser pulse lied in the specified ranges of values. Parameters of the micropinch, namely, its position within the interelectrode gap and also, intensity of the X-ray radiation and beams of the accelerated electrons emitted from the micropinch are variable over a wide range of values through changes of energy of the laser pulse and/or amplitude of the discharge current.     

超强激光与Ar团簇相互作用中X射线的研究

本文主要研究了超强超短激光与Ar团簇相互作用过程中X射线能谱、K壳层光子产额、能量转换效率以及激光对比度对X射线光子产额的影响.实验中得到K壳层的光子产额约为1× 10¹¹/发,能量转换效率约为2.8× 10^-5．同时观测到较强预脉冲离化团簇会导致预电离,产生膨胀等离子体,然而主脉冲与膨胀的等离子体相互作用的强度较未膨胀时降低了,从而导致K壳层光子产额降低,而使用高对比度的激光能增加X射线光子产额.