Femtosecond X-ray generation through relativistic electron beam–laser interaction

Methods for generating ultra-short X-rays using the interaction of intense laser pulses with relativistic electron beams, and their application to measuring ultra-fast phenomena in solid state materials, are reviewed. Two different methods that use a long electron bunch and short laser pulse are discussed: Thomson scattering and optical slicing which have been implemented on linac and storage ring beams, respectively. The possibility of generating ultrashort electrons bunches from laser-plasma injectors is discussed.     

Plasma-based X-ray laser at 21 nm for multidisciplinary applications

An overview of recent advances in applications of currently the most energetic X-ray laser at 21 nm is given. The unique parameters of this half-cavity based X-ray laser such as record output energy of 10 mJ, highly symmetric beam, robustness and reproducibility, have made it possible to carry out a number of multidisciplinary scientific projects featuring novel applications of intense coherent X-ray radiation. Selected results obtained in these experiments are reviewed, including X-ray laser probing of dense plasmas, measurements of transmission of focused soft X-ray radiation at intensities of up to 10¹² W cm^-2, measurements of infrared laser ablation rates of thin foils, and ablative microstructuring of solids.     

X-ray diffraction from the ripple structures created by femtosecond laser pulses

In this paper, we present the investigation and characterization of the laser-induced surface structure on an asymmetrically cut InSb crystal. We describe diffraction from the ripple surface and present a theoretical model that can be used to simulate X-ray energy scans. The asymmetrically cut InSb sample was irradiated with short-pulse radiation centred at 800 nm, with fluences ranging from 10 to 80 mJ/cm². The irradiated sample surface profile was investigated using optical and atomic force microscopy. We have investigated how laser-induced ripples influence the possibility of studying repetitive melting of solids using X-ray diffraction. The main effects arise from variations in local asymmetry angles, which reduce the attenuation length and increase the X-ray diffraction efficiency.     

Tuning the high-order harmonic lines of a Nd:Glass laser for soft X-ray laser seeding

We report here more than 50% coverage of the XUV spectral range between 18 nm and 35 nm by tuning the high-order harmonics generated by a fixed frequency Nd:Glass laser system. The tuning range achieved is suitable to seed Ni-like Y, Zr and Mo soft X-ray lasers.     

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.     

Electron–positron pair production from vacuum in the field of high-intensity laser radiation

The works dealing with the theory of e⁺e^– pair production from vacuum under the action of highintensity laser radiation are reviewed. The following problems are discussed: pair production in a constant electric field E and time-variable homogeneous field E(t); the dependence of the number of produced pairs \({N_{{e^ + }{e^ - }}}\) on the shape of a laser pulse (dynamic Schwinger effect); and a realistic three-dimensional model of a focused laser pulse, which is based on exact solution of Maxwell’s equations and contains parameters such as focal spot radius R, diffraction length L, focusing parameter Δ, pulse duration τ, and pulse shape. This model is used to calculate \({N_{{e^ + }{e^ - }}}\) for both a single laser pulse (n = 1) and several (n ≥ 2) coherent pulses with a fixed total energy that simultaneously “collide” in a laser focus. It is shown that, at n ? 1, the number of pairs increases by several orders of magnitude as compared to the case of a single pulse. The screening of a laser field by the vapors that are generated in vacuum, its “depletion,” and the limiting fields to be achieved in laser experiments are considered. The relation between pair production, the problem of a quantum frequency-variable oscillator, and the theory of groups SU(1, 1) and SU(2) is discussed. The relativistic version of the imaginary time method is used in calculations. In terms of this version, a relativistic theory of tunneling is developed and the Keldysh theory is generalized to the case of ionization of relativistic bound systems, namely, atoms and ions. The ionization rate of a hydrogen-like ion with a charge 1 ≤ Z ≤ 92 is calculated as a function of laser radiation intensity (F and ellipticity ρ.     

An all-solid-state laser system for generation of 100 μJ femtosecond pulses near 625 nm at 1 kHz

Frequency doubling the output of a high-power femtosecond Cr:forsterite regenerative amplifier with >50% conversion efficiency in a temperature-tuned noncritically phase-matched LBO crystal produces femtosecond pulses of >100 μJ energy in the visible range near 625 nm at a pulse duration of about 200 fs or >65 μJ at <170 fs. Received: 29 March 1999 / Revised version: 27 April 1999 / Published online: 24 June 1999     

A Broadband Infrared Laser Source (2.5–17 μm) for Plasma Diagnostics

This paper presents the results of studies aimed at the creation of a hybrid laser system which is composed of a gas lasers and a nonlinear crystal and appreciably broadens and enriches the radiation spectrum of these lasers. A highly efficient conversion (37%) is attained when generating the second harmonic in a ZnGeP₂ crystal owing to an increase in the peak power of CO laser radiation in the mode locking regime. The two-cascade conversion (generation of both sum and difference frequencies) of radiation of a broadband CO laser in the single sample of such nonlinear crystals as ZnGeP₂ and AgGaSe₂ is demonstrated. In this case, the radiation spectrum is broadened by nearly a factor of two, and the number of detected spectral lines grows by a factor of four. The use of a comparatively simple laser system of gas-discharge CO and CO₂ lasers to conversion in AgGaSe₂ results in laser radiation tunable over a set of narrow spectral lines within a range from 2.5 to 16.6 μm (more than two and a half octaves).     

A mid-infrared scanned-wavelength laser absorption sensor for carbon monoxide and temperature measurements from 900 to 4000 K

Mid-infrared laser absorption sensors based on quantum cascade laser (QCL) technology offer the potential for high-sensitivity, selective, and high-speed measurements of temperature and concentration for species of interest in high-temperature environments, such as those found in combustion devices. A new mid-infrared QCL absorption sensor for carbon monoxide and temperature measurements has been developed near the intensity peak of the CO fundamental band at 4.6 μm, providing orders-of-magnitude greater sensitivity than the overtone bands accessible with telecommunications lasers. The sensor is capable of probing the R(9), R(10), R(17), and R(18) transitions of the CO fundamental ro-vibrational band which are located at frequencies where H₂O and CO₂ spectral interference is minimal. Temperature measurements are made via scanned-wavelength two-line ratio techniques using either the R(9) and R(17) or the R(10) and R(18) line pairs. The high-speed (1–2 kHz) scanned-wavelength sensor is demonstrated in room-temperature gas cell measurements of CO and, to demonstrate the potential of the sensor for high-temperature thermometry, in shock-heated gases containing CO for a very wide range of temperature (950–3500 K) near 1 atm. To our knowledge, these measurements represent the first use of QCL-based absorption sensor for thermometry at elevated combustion-like temperatures. The high-temperature measurements of CO mole fraction and temperature agree with the post-reflected-shock conditions within ±1.5% and ±1.2% (1σ deviation), respectively.     

Multi-energy four-channel Kirkpatrick–Baez microscope for X-ray imaging diagnostics at the Shenguang-II laser facility

A four-channel Kirkpatrick–Baez microscope working at multiple energy bands is developed for multiframe X-ray imaging diagnostics at the Shenguang-II laser facility. The response to the multiple energy bands is realized by coating the double-periodic multilayers on the reflected surfaces of the microscope. Because of the limited size of the microstrips in the X-ray framing camera, the image separation is controlled by the conical angle of the reference cores during microscope assembly. This study describes the optical and multilayer design, assembly, and alignment of the developed microscope. The microscope achieves a spatial resolution of 4–5 mm in the laboratory and 10–20 mm at Shenguang-II laser facility within a 300 mm field of view. The versatile nature of the developed microscope enables the multiple microscopes currently installed in the laser facility to be replaced with a single, multipurpose microscope.     

Electron and Gamma Quantum Sources with Energy of 4–10 MeV for Calibration of Nuclear Detectors

Physics of Atomic Nuclei - A structural configuration and parameters of a modernized source of electrons and gamma quanta with energy of 4–10 MeV are presented. The source is designed on the...     

Laser pulses designed in time by adaptive hydrodynamic modeling for optimizing ultra-fast laser–metal interactions

Determining optimal temporal pulse shapes is an essential aspect for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultra-fast scales. In this respect, adaptive feedback loops based on temporal pulse manipulation have been inserted into a hydrodynamic code. The procedure enables us to reach the theoretical maximal temperature at a certain energy input. Several regimes have been considered with fluences ranging from the ablation threshold (F _th=0.34 J/cm²) up to 10 J/cm², proposing an optimal coupling for laser–solid and laser–plasma interactions in these fluence regimes. We determine shapes of optimal pulses on ultra-short and short scales (up to 42 ps) and forecast optimized interaction scenarios with fundamental control factors difficult to access experimentally. Simulations performed on aluminum reveal that ultra-short pulses are the natural better solution for localizing energy in space and time for F～F _th. For higher fluences, pulses spread over tens of picoseconds and ended by a final peak enable a better impulsive coupling with the nascent plasma, optimizing its maximal temperature.     

Si–LiNbO3–air–metal structure for concentrated terahertz emission from ultrashort laser pulses

A planar Si–LiNbO₃–air–metal structure is proposed as a further development of the highly efficient optical-to-terahertz conversion scheme in sandwich structures that was recently demonstrated. The new structure allows one to collect the terahertz emission into one spatial direction and to control its spectrum by varying an air gap between the metal substrate and the LiNbO₃ layer. While the overall increase in the terahertz generation can reach a factor of 2, the spectral density in the interesting for practical application interval 0.5–1.5 THz can be increased by a factor of 3.5–4.     

An iterative analytic–numerical method for scattering from a target buried beneath a rough surface

An efficiently iterative analytical-numerical method is proposed for two-dimensional （2D） electromagnetic scattering from a perfectly electric conducting （PEC） target buried under a dielectric rough surface. The basic idea is to employ the Kirchhoff approximation （KA） to accelerate the boundary integral method （BIM）. Below the rough surface, an iterative system is designed between the rough surface and the target. The KA is used to simulate the initial field on the rough surface based on the Fresnel theory, while the target is analyzed by the boundary integral method to obtain a precise result. The fields between the rough surface and the target can be linked by the boundary integral equations below the rough surface. The technique presented here is highly efficient in terms of computational memory, time, and versatility. Numerical simulations of two typical models are carried out to validate the method.     

Doping management for high-power fiber lasers: 100 W, few-picosecond pulse generation from an all-fiber-integrated amplifier

Thermal effects, which limit the average power, can be minimized by using low-doped, longer gain fibers, whereas the presence of nonlinear effects requires use of high-doped, shorter fibers to maximize the peak power. We propose the use of varying doping levels along the gain fiber to circumvent these opposing requirements. By analogy to dispersion management and nonlinearity management, we refer to this scheme as doping management. As a practical first implementation, we report on the development of a fiber laser-amplifier system, the last stage of which has a hybrid gain fiber composed of high-doped and low-doped Yb fibers. The amplifier generates 100?W at 100?MHz with pulse energy of 1 μJ. The seed source is a passively mode-locked fiber oscillator operating in the all-normal-dispersion regime. The amplifier comprises three stages, which are all-fiber-integrated, delivering 13?ps pulses at full power. By optionally placing a grating compressor after the first stage amplifier, chirp of the seed pulses can be controlled, which allows an extra degree of freedom in the interplay between dispersion and self-phase modulation. This way, the laser delivers 4.5?ps pulses with ～200 kW peak power directly from fiber, without using external pulse compression.     

Electron–positron momentum density distribution of Gd from 2D ACAR data via Maximum Entropy and Cormack’s methods

A successful application of the Maximum Entropy Method (MEM) to the reconstruction of electron–positron momentum density distribution in gadolinium out of the experimental of 2D ACAR data is presented. Formally, the algorithm used was prepared for two-dimensional reconstructions from line integrals. For the first time the results of MEM, applied to such data, are compared in detail with the ones obtained by means of Cormack’s method. It is also shown how the experimental uncertainties may influence the results of the latter analysis. Preliminary calculations, using WIEN2k code, of band structure and Fermi surface have been done as well.     

Electron impact total cross section calculations for CH3SH (methanethiol) from threshold to 5 keV

We report calculated total elastic cross sections Q_el, total ionisation cross sections, Q_ion, summed total excitation cross sections ∑Q_exc and total cross sections Q_T for CH₃SH upon electron impact for energies from ionisation threshold to 5 keV. We have employed Spherical Complex Optical Potential (SCOP) formalism to calculate total elastic cross section Q_el, and total inelastic cross section Q_inel and used Complex Scattering Potential – the ionisation contribution (CSP-ic) method to extract the ionisation cross sections, Q_ion, from the calculated Q_inel. The calculated total cross sections are examined as functions of incident electron energy and are compared with available data wherever possible and overall good agreement is observed. In this work Q_el, Q_ion, and ∑Q_exc are reported for the first time for CH₃SH in this energy range.     

Cross section for 160° elastic scattering of 1.4–2.4 MeV protons from nitrogen and titanium

Differential elastic scattering cross sections for 1·4–2·4 MeV protons from natural nitrogen and titanium were measured at laboratory scattering angle 160°. The present cross section data are tabulated for later use in backscattering analyses. Our results for nitrogen are in qualitative agreement with previous data taken at slightly different scattering angles. The cross sections of titanium agree with theoretical Rutherford values within experimental errors. Examples of recent analytical applications of proton backscattering are given.The authors wish to thank the members of NPI accelerator group for their assistance in the course of experiments.