Laser-driven pondermotive acceleration of electrons in vacuum has been considered using computer simulations. It is demonstrated that a low-energy free electron can be violently accelerated to final kinetic energy of GeV by a tightly focused ultra-short ultra-intense laser pulse. Suitable conditions that are crucial for this phenomenon to occur have been investigated. It is shown that selection of appropriate initial conditions like relative time delay between electron and the laser pulse, electron's incident angle and momentum, laser pulse duration and its focal spot size play important roles in the efficient acceleration scheme.
We have observed lasing on Ne-like 3s-3p line from titanium (32.4 nm), Ni-like 4p-4d line from silver (13.9 nm) and tin (11.9 nm) with the transient collisional excitation (TCE) scheme that uses combinations of a long pre-pulse (approximately ns) and a short main pulse (approximately ps) or a short pre-pulse (approximately ps) and a short main pulse (approximately ps). A gain coefficient of 24cm-1 have been measured for plasma length up to 4mm with silver slab targets and 14cm-1 up to 6 mm with tin slab targets. We have installed a step mirror in the focusing system to generate traveling wave on the target. The traveling speed on the target is measured to be 3.08 cm/s and very close to the traveling speed of light. The traveling wave system improves the gain coefficient to 35cm-1 from 24cm-1 for Ni-like Ag and to 30cm-1 from 14cm-1 for Ni-like Sn. The strong gain saturation has been observed for the Ni-like Ag and Ni-like Sn. The output energy of the N-like Sn x-ray laser is 20 (mu) J. Spatial beam profiles of propagating x-ray lasers through gain plasma have been measured and are indicating localization of very high gain area and x-ray laser refraction.
We proposed a method to generate highly spatial coherent x-ray laser, in which high order harmonics was used as a seed light of a laser-produced x-ray amplifier. In this case, the intensity and the spatial coherence of the output x-ray depended on the harmonic conversion efficiency and the spectral and spatial coupling efficiencies between the harmonics and the lasing line. Based on the present x-ray lasers using a transient collisional-excitation (TCE) scheme, we investigated the values of these efficiencies, which were needed to realize a high spatial coherence. For this purpose we constructed a Ti:Sapphire laser system in which the central wavelength and the spectral bandwidth were tunable, and we conducted a preliminary experiment. The neon-like Ti x-ray laser at a wavelength of 32.4 nm was taken as an example, and harmonics at the same wavelength was generated using Ar gas target under the conditions that the central wavelength of 810 nm and the pulse duration of 1 ps. The experimental result showed that the characteristics of the harmonics were good enough to use as a seed light of x-ray lasers.
High-density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized to the laser pulse. In the first part of this paper, the spectral and temporal properties of the x- ray emitted from plasma created on aluminum film by a femtosecond laser pulse are shown. The minimum pulse duration was < 3 ps as measured by an x-ray streak camera. The energy conversion efficiency, from laser pulse into soft x-ray at 14 +/- 0.05 nm, was 10-6 - 10-5. More than a 30-fold enhancement in soft x-ray emission was achieved by fabricating an array of nanoholes on an alumina surface. In the latter half, we demonstrate time-resolved absorption measurement in the soft x-ray region by means of pump-probe spectroscopy. Using a 10-ps x-ray pulse, we measured time-resolved absorption of optically-pumped silicon near its LII,III edge. We found that laser-pulse irradiation caused a more than 10% increase in soft x-ray absorption near the edge, which means that the transition of electrons in inner shells was rapidly modulated by excitation of valence electrons. The absorption change recovered within 20 ps.
Table-top x-ray lasers driven by a commercial terawatt (TW) femtosecond laser in gases such as N2, O2, SF6, and Kr are investigated. X-ray lasing to the ground state in low-charged nitrogen and oxygen ions is successfully demonstrated using a commercial TW femtosecond laser system by optical-field ionization. Small signal gain coefficients of 9.6 cm-1 for the NIII 3s(2S)-2p)2P) transition at 45.2 nm and 11.7 cm-1 for the OIII 2p3s(3P)-2p2(3P) transition at 37.4 nm x-ray lasers are achieved for a linearly-polarized 100-fs pump laser pulse of 25 mJ. Four models that includes the monopole collisional excitation and radiation trapping effects in neon-like S and nickel-like Kr ions were built and gains for neon-like S 3p-3s and nickel-like Kr 4d-4p transitions were calculated in SF6 and Kr gases with different gas pressures. Calculations yield positive prospects for table- top neon-like S and nickel-like Kr ion x-ray lasers driven by a 1-TW circularly-polarized 100-fs laser pulse.
Recent studies of soft-x-ray and XUV lasers in Ne- and Ni-like ions using the Asterix IV iodine laser as a pump are reviewed. Progress achieved includes the observation of new laser lines in a number of low to medium-Z Ne-like ions at wavelengths from 20 to 87 nm, the reduction of the driver energy for neon- like S to a value of 20 J and lasing in Ni-like Sn at 12 nm. We also present recent results in characterizing near- and far-field patterns and compare them with predictions of simulations, using a 2D hydrodynamic code combined with a ray tracing code.
We discuss high resolution two-dimensional near-field images of the neon-like nickel and germanium x-ray laser obtained using the Asterix laser at the Max-Planck-Institute and the Nova laser at Lawrence Livermore National Laboratory. Our imaging diagnostic consisted of a concave multilayer mirror that imaged the output end of the x-ray laser line onto a backside illuminated x-ray CCD detector. A 25 micrometer thick wire positioned at the end of the target provided a spatial fiducial. With the Asterix iodine laser, a prepulse 5.23 ns before the main pulse, was used to irradiate slab targets. A great deal of structure was observed in the near field images, particularly in the J equals 0 - 1 emission. We observed a large difference in the spatial dependence of the J equals 0 - 1 and J equals 2 - 1 lines of germanium, with the J equals 2 - 1 emission peaking farther away from the original target surface. A larger prepulse moved the peak emission farther away from the target surface. For the Nova experiments we used a series of 100 ps pulses spaced 400 ps apart to illuminate a germanium target. We obtained high resolution images of both the J equals 0 - 1 and J equals 2 - 1 lines of Ge. these measurements are compared to hydrodynamic simulations coupled with atomic kinetics and including refraction effects.
A new pumping scheme named exploding pumping is proposed for realizing recombination x-ray lasers with high excitation efficiency. In the new scheme, a very thin membrane is employed as a target and it is heated instantaneously before the plasma starts to move by a high peak power sub- picosecond pumping laser. In the scheme, the plasma heating efficiency is improved by being free from heat conduction loss to bulk, and by suppressing the energy loss to hydrodynamic motion. Inertia of the mass delays the start of the plasma motion and gives sufficient time for full ionization. Owing to the extreme thinness of the initial high density plasma, only a few micrometers expansion leads to great reduction of the density and cools down rapidly to produce large gain. Efficient heating of a membrane plasma is confirmed in experimentally observed x-ray spectra and charge collector signals. It is discussed that the most serious problem for realizing water window x-ray lasers is conventional ablation pumping is density gradient which causes refraction of x-rays and limits gain length. The new pumping scheme can solve this refraction problem. The density profile of the expanding plasma in this scheme is fairly uniform because all material expand explosively and because no mass is supplied during the expansion. According to a numerical simulation, 3.34-nm water window of gain length product of 10 will be realized with the 6 J/ 0.3 ps laser irradiation. Longer wavelength x-ray lasers around 13- nm will be realized with a few J/ a few ps pulse pumping. The key technology in the new pumping scheme is suppression of pre-pulse. The effect of pre-pulse is experimentally observed, and means for pre-pulse suppression is discussed.
For the study of future nm devices, electronic and chemical states must be observed with nm resolution, and a photoelectron microscope is strongly desired to be developed. A compact system for in-house analysis can be realized with a laser-plasma x-ray source. Experimental results confirm that an x-ray photoelectron microscope with a laser-plasma can become a powerful tool with potential spectrum acquisition time comparable to that with a synchrotron source. Reported are high energy resolution photoelectron spectra for Si revealing the existence of surface states, and 2-D mapping with resolution of 60 micrometers demonstrating that simultaneous detection of multi-elements by the time-of-flight energy analysis is powerful in obtaining a reliable mapping. High electron detection efficiency of the system with a pulse source is discussed to be essentially important for nm resolution.
In this paper we present the results of the first experiments on x-ray laser using a laser- irradiated gas puff target. The gas puff targets were created by pulsed injection of gas from a high-pressure valve through a nozzle into a vacuum. An x-ray laser active medium in a form of an elongated plasma column was produced by the perpendicular irradiation of the gas puff target in a form of a long sheet of gas, created using the nozzle having a linear exit aperture, with the laser beam focused to a line. The x-ray laser experiments were carried out at Garching using the ASTERIX IV high-power iodine laser. Up to 3-cm-long hot and dense plasma columns were produced. Lasing in neon-like argon at 46.9 nm and in nickel-like xenon at 10.0 nm was demonstrated for the first time.
The general expression of the optical path function and the focusing conditions of a variable line-spacing concave grating with a spherical surface are given. The spectral imaging of our flat-field grating spectrograph with a HITACHI flat-field grating was investigated and the typical experimental results of the two modes of the spectrograph (that is, the astigmatic mode and the stigmatic mode) are presented.
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 KCl and CaF2 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.
The spatial distributions of lithiumlike SiXII ion and sodiumlike CuXIX ion recombination X- ray lasing gains are presented in this paper. It shows that the recombination X-ray lasing gain region is at hundreds of micrometers from the target surface and the electron density in the gain region is about 1019 cm-3 while there exists absorption near the target surface with higher electron density (> 1020 cm-3).
For the X-ray laser researching toward shorter wave length, we set up compact short pulse laser facility with high intensive output at the variable pulsewidth from 20 ps to 2.5 ns. Occupying a table smaller than 10 m2 and with output of 2.5 J in 20 ps this facility is reported with the emphasis on the use of X-ray lasers research and laser-plasma study.
Experimental study of soft X-ray laser for the sodiumlike recombination scheme, carried out at the LF12 Laser Facility of Shanghai Institute of Optics and Fine Mechanics, is reported. Soft X-ray amplification was observed in sodiumlike copper ions by line-focused laser irradiation of slab targets. Based on the spatially-resolved measurements, the spatial distribution of the laser gain for the 6g-4f (72.22 A) transition of sodiumlike copper ions was obtained. The maxima of gain coefficients and gain-length products at 550 microns from target surface are 2.0/cm and 3.6, respectively. The evolution of spectrally discriminated spatial uniformity of line-shaped plasmas as lasing gain media produced by uniform laser illumination in line-focus is also investigated. It is found that, under this experimental condition, the line-shaped plasma is very uniform after optical laser's heating.
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