Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs
during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through
laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and
an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection
and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on
collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the
forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The
source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.
This paper gives an overview of recent progress of x-ray laser research in Japan Atomic Energy Research Institute (JAERI). In the development of high quality x-ray laser beam, the progress includes the improvement of output energy of fully spatial coherent x-ray laser beam at a wavelength of 13.9 nm and generation of temporally coherent x-ray laser at 26.9 nm by use of seed x-ray injection technique. Beam stability is greatly improved to be better than 0.5 mrad by introducing new designed target chamber and target alignment system. In the application of the 13.9 nm laser, an experiment by use of x-ray speckle technique reveals firstly the existence of polarization clusters in ferroelectric substance. For the purpose of further application experiments, 0.1 Hz-repetition rate x-ray laser driver is being developed, which is based on an OPCPA pre-amplifier and a Nd:glass zigzag slab amplifier with two beam lines, and each line provides 10 Joules 1 ps pulse on target.
We have succeeded in developing a laser-pumped x-ray laser with full spatial coherence at 13.9 nm. A highly directed x-ray laser beam with the divergence of 0.2 mrad was generated from the double target experiment, where a seeding light from the first laser medium was amplified in the second medium. The observed divergence is close to the diffraction limited value within a factor of two. The seeding light was amplified in the second medium without refraction influence and the gain coefficient was about 8 cm-1. The gain region of the second medium was far away from the target surface compared with that of the first medium and located in the considerably low density region. From the measurement of visibility, it was found that the spatial coherent length is longer than the beam diameter.
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.
An ultra-short pulse CPA laser system for x-ray laser driver has been developed with a combination of Ti:sapphire front end and Nd:glass rod amplifiers. This laser system has two beam outputs and each beam line produces 20J pre pulse and 20J main. This laser system is designed for x-ray laser pumping driver, especially for transient gain scheme. The new transient gain x-ray laser scheme with thin foil metal targets has been proposed. This scheme has higher laser energy efficiency and less x-ray laser refraction effect and makes possible to generate shorter x-ray wavelength with a compact table-top sized laser system. The electron temperatures of plasma heated with a short pre pulse and short main pulse have been calculated with 1D hydrodynamic code and obtained electron temperature higher than 1 keV with 20J laser energy. X-ray laser propagation is also calculated with gain guiding effect.
C60 vapor was irradiated with an intense femtosecond laser pulse. Multiply charged carbon ions, up to C4+, were detected in the time-of-flight spectra. We have measured both energy and angular distributions with respect to the laser polarization direction for ions with different charge number. The kinetic energy of ions are distributed above 1 keV. The angular distribution measurement shows that the highly charged ions of C3+ and C4+ are mostly distributed in the direction parallel to the laser polarization, and the C+ ions are slightly distributed in the direction perpendicular to it. These observations clearly indicate that an anisotropic explosion takes place. The average energy of ions with the different charge number is found to be proportional to the square of the charge number, while the maximum energy is proportional to the charge number. Classical molecular dynamics simulations have been successfully carried out reproducing not only the energy spectra but also the angular distributions of ions. The simulations suggest that the most crucial process for the anisotropic Coulomb explosion of C60 is not the electron impact ionization, but the cascade hopping of electrons.
To obtain high gain saturated amplification soft x-ray lasing output, uniform and long amplification medium must be created by focusing and target set-up technology. We report on the cylindrical lens array and multi target coupling system for creating uniform line focus. Through these methods, serial x-ray lasing with the wavelength towards the wavelength of water window are obtained on the GekkoXII Nd glass laser facility in ILE, Osaka, University.
An ultra-short pulse CPA laser system for x-ray laser driver has been developed with a combination of Ti:sapphire front end and Nd:glass rod amplifiers. This laser system has two beam outputs and each beam line produces 20J pre pulse and 20J main. This laser system is designed for x-ray laser pumping driver, especially for transient gain scheme. The new transient gain x-ray laser scheme with thin foil metal targets has been proposed. This scheme has higher laser energy efficiency and less x-ray laser refraction effect and makes possible to generate shorter x-ray wavelength with a compact table-top sized laser system. The electron temperatures of plasmas heated with a short pre pulse and short main pulse have been calculated with 1D hydrodynamic code and obtained electron temperature higher than 1 keV and 20 J laser energy. X-ray laser propagation is also calculated with gain guiding effect.
We report experimental results on Ni-like x-ray laser at the wavelength as short as 4.4 nm. The performance of x-ray lasing pumped by various types of pulse trains which were composed of 100 ps pulses was investigated with a double slab targets which were placed in series to double the gain length. Two opposing laser beams irradiated the double targets with a suitable time difference for quasi traveling wave pumping. The well collimated double target amplification was successfully demonstrated with two beam irradiation for Yb and Hf lasing at 5.0 nm and 4.7 nm whose gain length products were 11 and 6, respectively. The Ni- like lasing line of Ta have been observed at 4.5 nm. Based on these results, we will report the suitable pumping condition for the saturated water window x-ray lasers.
From 1995, ILE has started development of a high power chirp pulse amplification Nd:glass laser for investigating the fast ignitor concept as a new approach toward high gain inertial confinement fusion. The output of the new glass laser line is estimated to be 100 TW, 100 J, 1 ps and is focused on a high density compressed plasma by the GEKKO-XII laser for the additional fast heating.
A large aperture deformable mirror for correcting the wavefront distortion and controlling the intensity profile of laser beam is presented. The mirror diameter and thickness are 400 mm and 8 mm, respectively. 37 actuators are glued hexagonally on the back surface of the mirror. The mechanical actuator has been designed and fabricated for producing strong force of 300 N in both pushing and pulling process. The initial reflective wavefront of the deformable mirror is (lambda) /6 ((lambda) is 633 nm) in rms value. The application to x-ray laser experiment of the deformable mirror is successful for a uniform line focus pattern by the wavefront control.
Results of the joint experiment between ILE Japan and NLHPLP/IAPCM China are reported. New optics for improved line focusing, a cylindrical lens array and a deformable mirror, were used in irradiation of x-ray laser targets. Double-pass amplification at 7.9 nm has been demonstrated. Strong lasing at 7.9 nm with approximately 100 (mu) J output energy has been obtained in Ni-like Nd ions by quasi-traveling wave pumping of double targets.
We measured laser channeling into an overdense plasma by using a 19.6 nm Ne-like Ge XUV laser. One micrometer/100 ps laser light at 1017 W/cm2 interacted with a long scale length plasma preformed on a CH slab target. Grid image refractometry (GIR) with the x-ray laser was applied to obtain the deflection information in the plasma, which provided two dimensional density profiles (2-D) of the overdense plasmas.
The physics of laser-plasma interaction and x-ray generation by laser have been extensively investigated as close relevance laser fusion. Efficient conversion to x-rays and controllability of wavelength, pulse width, and geometrical size have given us the high potentiality of applications to wide scientific and technical fields. An efficient high average power laser with a good beam quality is also under development as the key technology for the applications.
A high aspect ratio line focus on a target for the x-ray laser experiments is required for obtaining a high gain-length product. A new line focus system is developed to generate a uniform line focus. The system consists of a deformable mirror of a continuous faceplate type which provides an appropriate wavefront distribution for compensating an aberration of a line focus optic. The width and intensity distribution of 18.2 mm long line focus has been improved on 2 times diffraction limit. In another application, a rectangular beam shaping from a circular defocused beam is investigated by the experiment and the diffraction calculation. The controllability of intensity distribution of laser beam by deformable mirror has been demonstrated.
Experimental progress in Ne-like Ge x-ray lasers (19.6 nm and 23.6 nm) has been made by using a curved target and multiple-short pulse. The curved target was introduced to compensate x-ray laser beam refraction in a laser produced plasma. The pulse shape of the pumping laser was varied in search for the improvement in the pumping efficiency. For generation of shorter wavelength lasers, the scheme using a curved target pumped with a multiple-short pulse laser was extended to the amplification studies in Ni-like lanthanide elements (Nd, Sm, Gd, Tb and Dy) corresponding to the spectral range of 6 nm to 8 nm.
We have developed an advanced Kirkpatrick-Baez (AKB) microscope to diagnose a laser- produced-plasma. The AKB microscope optics are two pairs of hyperbolical and elliptical cylindrical-mirrors to avoid a spherical aberration and field obliquity. Ray trace calculation was applied to optimize the characterization. The microscope has attained a spatial resolution of less than 3 mm at 2.5-keV x-ray in the field of 800 mm from experiments.
X-ray emission spectra in the wavelength range of 2 - 13 nm from 21 kinds of material (carbon through tin) irradiated either by a 4 J/35 ns slab Nd:glass laser or by a 0.5 J/8 ns Nd:YAG laser were recorded with a grazing incidence spectrometer equipped with a microchannel plate detector. The absolute photon intensities of these spectra were determined by simultaneous measurement of the emission from a molybdenum plasma with the grazing incidence spectrometer and a transmission grating spectrometer. For the carbon plasma, the electron temperature and density are derived from the spectrum and the conditions for intense Lyman (alpha) line are discussed. Finally, we describe the application to an x-ray microscope with the laser-produced carbon plasma.
It is shown that curved slab targets are effective in compensating x-ray refraction due to electron density gradient in the expanding plasma. Significant improvement in the beam divergence and laser intensity has been observed. Soft x-ray laser of 1 mrad divergence has been generated in double- pass amplification of a collisionally-excited Ne-like Ge laser with the curved target. Generation of a polarized beam with a polarizing half cavity is described. Initial results of in-line holography as well as Fourier transform holography using the Ge laser as the light source are also presented.
We have successfully demonstrated double pass enhancement of amplified spontaneous emission of soft x rays, 23.2 and 23.6 nm of 3p - 3s transitions in Ne-like Ge, using an x-ray multilayer mirror. In this paper, we report on the fabrication of the mirror and analysis of its damage suffered during the experiments. The mirror used was a Mo-Si multilayer mirror with the reflectivity of 35% at the wavelength of 23.6 nm, deposited by an rf-sputtering system. In the damaged area of the mirror, only the multilayer was locally evaporated and the bare substrate underneath appeared. The size of the damaged area corresponded to the aperture size. We carried out the simulation on the spatial and temporal distribution of the mirror temperature during the experiment. Assuming that thermal x rays enter the mirror with the largest amount of energy among all the fluxes at the early stage of the enhancement, the result of the simulation can explain the damage feature and the temporal profile of the intensity of the amplified spontaneous emissions.
Basic characteristics of soft x-ray lasers generated as amplified spontaneous emission are described. Experimental results on soft x-ray amplification in neon-like germanium ions, obtained recently at the Institute of Laser Engineering, are reported. By comparing the experimental results with a simplified model on amplified spontaneous emission, basic parameters such as coherence and brightness of the Ge soft x-ray laser are evaluated.
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