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A new approach is proposed to analyze Bremsstrahlung X-rays that are emitted from laser-produced plasmas (LPP) and are measured by a stack type spectrometer. This new method is based on a spectral tomographic reconstruction concept with the variational principle for optimization, without referring to the electron energy distribution of a plasma. This approach is applied to the analysis of some experimental data obtained at a few major laser facilities to demonstrate the applicability of the method. Slope temperatures of X-rays from LPP are determined with a two-temperature model, showing different spectral characteristics of X-rays depending on laser properties used in the experiments.

Optical generation of compact magnetized plasma structures is studied in the moderate intensity domain. A sub-ns laser beam irradiated snail-shaped targets with the intensity of about 1016 W/cm2. With a neat optical diagnostics, a sub-megagauss magnetized plasmoid is traced inside the target. On the observed hydrodynamic time scale, the hot plasma formation achieves a theta-pinch-like density and magnetic field distribution, which implodes into the target interior. This simple and elegant plasma magnetization scheme in the moderate-intensity domain is of particular interest for fundamental astrophysical-related studies and for development of future technologies.

Short-pulse x-ray/gamma-ray sources have become indispensable light sources for investigating material science, bio technology, and photo-nuclear physics. In past decades, rapid advancement of high intensity laser technology led extensive progresses in the field of radiation sources based on laser-plasma interactions - x-ray lasers, betatron radiation and Compton gamma-rays. Ever since the installation of a 100-TW laser in 2006, we have pursued the development of ultrashort x-ray/gamma-ray radiations, such as x-ray lasers, relativistic high-order harmonics, betatron radiation and all-optical Compton gamma-rays. With the construction of two PW Ti:Sapphire laser beamlines having peak powers of 1.0 PW and 1.5 PW in 2010 and 2012, respectively [1], we have investigated the generation of multi-GeV electron beams [2] and MeV betatron radiations. We plan to carry out the Compton backscattering to generate MeV gamma-rays from the interaction of a GeV electron beam and a PW laser beam. Here, we present the recent progress in the development of ultrashort x-ray/gamma-ray radiation sources based on laser plasma interactions and the plan for developing Compton gamma-ray sources driven by the PW lasers. In addition, we will present the applications of laser-plasma x-ray lasers to x-ray holography and coherent diffraction imaging. [references] 1. J. H. Sung, S. K. Lee, T. J. Yu, T. M. Jeong, and J. Lee, Opt. Lett. 35, 3021 (2010). 2. H. T. Kim, K. H. Pae, H. J. Cha, I J. Kim, T. J. Yu, J. H. Sung, S. K. Lee, T. M. Jeong, J. Lee, Phys. Rev. Lett. 111, 165002 (2013).

The interaction between two counter-streaming laser-produced plasmas is studied with shadowgraph and interferometry on the Shenguang II (SG-II) laser facility. Shockwaves and filaments are observed at different timing. The simulation and theoretical analysis indicate that these structures are probably induced by collisionless mechanisms.

Interactions of high-intensity femtosecond lasers with deuterium clusters leading to Coulomb explosions and subsequent production of fusion neutrons have attracted considerable attention in recent years. In order to maximize the neutron yield, finding the dependence of clusters size and their spatial distribution on the experimental conditions has become very important. In this paper, we analyze the possibility of measuring the spatial distributions of deuterium clusters experimentally by using complex interferometry diagnostics. For this purpose, close-to-reality computer-generated interferograms were produced, which included a small phase-shift disturbance modeling the clusters. Subsequent analysis of these interferograms provided results that identified this diagnostics as potentially suitable for such measurements.

We have characterized the optical parametric chirped-pulse amplification of femtosecond Ti:sapphire laser pulses by using a BBO crystal. It is numerically verified that a high gain and a broad gain bandwidth can be obtained with a 532-nm pump laser. The dependence of the gain profile of OPA on phase matching angles, pump intensity, and crystal length is numerically investigated. Experimental results shows that the temporal fluctuation of a pump laser causes the modulation of an amplified spectrum in OPCPA.

Measurement of angularly dependent spectra of betatron gamma-rays radiated by GeV electron beams from laser wakefield accelerators (LWFAs) are presented. The angle-resolved spectrum of betatron radiation was deconvolved from the position dependent data measured for a single laser shot with a broadband gamma-ray spectrometer comprising four-quadrant sectored range filters and an unfolding algorithm, based on the Monte Carlo code GEANT4. The unfolded gamma-ray spectra in the photon energy range of 0.1–10 MeV revealed an approximately isotropic angular dependence of the peak photon energy and photon energy-integrated fluence. As expected by the analysis of betatron radiation from LWFAs, the results indicate that unpolarized gamma-rays are emitted by electrons undergoing betatron motion in isotropically distributed orbit planes.

We have developed and characterized a 1-kHz self-seeding-type pulsed Ti:sapphire laser oscillator pumped by a frequency-doubled Nd:YAG laser. It is a dual-cavity laser composed of a grazing-incidence cavity and a feedback mirror. We found that the narrow-linewidth lasing can be achieved even though the grazing-incidence cavity for seeding does not lase separately without the feedback mirror. The maximum average output power is 1 W (1 mJ/pulse) at a 9-W (9 mJ/pulse) pumping, and the linewidth is less than 380 MHz.

Summary form only given. A commonly used laser-based method of acceleration of dense plasma, particularly applied for compression and ignition of a fusion target in ICF research, is ablative acceleration (AA) employing the “rocket effect ”. However, the energetic efficiency of acceleration in the AA scheme is relatively small (≤10%), so significant efforts have been made to find more efficient schemes. Very recently, a novel highly efficient scheme of laser acceleration of dense matter called laser-induced cavity pressure acceleration (LICPA) has been proposed1. In the LICPA accelerator, a projectile placed in a cavity is irradiated by a laser beam introduced into the cavity through a hole and accelerated along a guiding channel by the thermal pressure created in the cavity by the laser-produced plasma or by the photon pressure of the ultraintense laser radiation trapped in the cavity.

A simple method of production of supersonic plasma jets with parameters relevant to laboratory astrophysics applications is proposed and demonstrated. The method consists in using a cylindrical channel for guiding and collimating the plasma generated from a laser-irradiated thin foil target. In the experiment, a 120 J, 1.315 um, 0.3 ns laser pulse irradiated a thin (10 or 20 um) CH foil placed at the entrance of the cylindrical channel (made in the aluminum cylinder) of diameter dCh = 0.3 mm and length of 2 mm. It was found that when the focal spot diameter (dL) is well matched to the channel diameter (dL ≈ (2/3)dCh), the channel can form a collimated, high-density (up to 1020 cm3) plasma jet of the Mach number ≥10. The method seems to be flexible in the generation of jets of various hydrodynamic parameters and atomic numbers and does not require high-energy lasers for the jet production.

We have analyzed the temporal characteristics of amplified spontaneous emission (ASE) in femtosecond terawatt Ti:sapphire lasers by using a simple method based on fast photodiodes. Instead of measuring ASE directly with fast photodiodes, we created a narrow gap in the spectrum of seed pulses and, after amplification, detected the pure ASE signal through the gap by using a fast photodiode covered with a bandpass filter with high transmission at the gap. Because the detected ASE signal was completely separated from amplified main pulses, preceding and even trailing ASEs could be characterized quantitatively in a single-shot measurement. We believe that our method is a good alternative or a complement to conventional methods for ASE measurements.

We have separated amplified spontaneous emission (ASE) from main pulses and investigated its temporal characteristics in a 10-TW Ti:sapphire laser. We made a small gap in the spectrum of seed pulses and, after amplification, detected the pure ASE signal through the gap by using a fast photodiode.

We have numerically and experimentally verified that the sequence of optical elements in a femtosecond Ti:sapphire laser affects the output spectrum. With the same net dispersion in a cavity, the width and the shape of output spectrum was significant changed when the sequence of dispersive elements was changed in a Kerr-lens mode-locked Ti:sapphire laser with fused-silica prisms. We also showed that the dynamics of pulse formation can be affected by the output coupler's position though the combined action of output coupling and self-phase modulation. Such phenomena can be explained based on pulse formation in a laser cavity with discrete distribution of dispersion and self-phase modulation.

We have numerically and experimentally investigated the characteristics of the optical parametric chirped-pulse amplification (OPCPA) of femtosecond Ti:sapphire laser pulses by using BBO crystals. Numerical calculations showed that a high gain and a broad gain bandwidth could be obtained with a 532-nm pump laser. The dependence of the gain profile of OPA on phase matching angles, pump intensity, and crystal length was numerically investigated. Experimental results showed that the broadband amplification of femtosecond Ti:sapphire laser pulses could be realized by OPCPA and that the temporal fluctuation of a pump laser resulted in the modulation of amplified spectra in OPCPA.

We report the experiment conducted on the Prague Asterix Laser System (PALS) laser facility dedicated to make a parametric study of the laser-plasma interaction under the physical conditions corresponding to shock ignition thermonuclear fusion reactions. Two laser beams have been used: the auxiliary beam, for preplasma creation on the surface of a plastic foil, and the main beam to launch a strong shock. The ablation pressure is inferred from the volume of the crater in the Cu layer situated behind the plastic foil and by shock breakout chronometry. The population of fast electrons is analyzed by Kα emission spectroscopy and imaging. The preplasma is characterized by three-frame interferometry, x-ray spectroscopy and ion diagnostics. The numerical simulations constrained with the measured data gave a maximum pressure in the plastic layer of about 90 Mbar. © 2014 The Royal Swedish Academy of Sciences.

Laser produced plasma is one of the brilliant x-ray source that has unique capabilities for use in a wide range of science. Here we describe two examples of laser-produced plasma x-ray source application; one is for the semiconductor device industry and the other is for the astronomy. Extreme ultraviolet (EUV) light sources for microlithography are receiving much attention as an industrial application of laser-produced x-ray source. High power and clean EUV light source, 13.5 nm of wavelength, is developed for mass-production of next generation semiconductor devices. Highest EUV conversion efficiency of 4% has been attained by using low-density and minimum-mass tin targets produced by laser-driven explosion of micro-droplet. In addition, it was recently demonstrated that laser-produced x-ray source is very useful to simulate x-ray astronomical phenomena in the laboratory. A 0.5-keV Planckian x-ray source was created with laser driven implosion for producing non-local-thermodynamical-equilibrium (non-LTE) photoionized plasmas, which is a key to understand astronomical compact objects. Laboratory experiment of non-LTE photoionized plasma offers novel test bed for validation and verification of computational codes used in x-ray astronomy.

A novel efficient scheme of acceleration and collimation of dense plasma is proposed and examined. In the proposed scheme, a target placed in a cavity at the entrance of a guiding channel is irradiated by a laser beam introduced into the cavity through a hole and accelerated along the channel by the pressure created and accumulated in the cavity by the hot plasma expanding from the target and the cavity walls. Using 1.315-um, 0.3-ns laser pulse of energy up to 200J and a thin CH target, it was shown that the forward accelerated dense plasma projectile produced from the target can be effectively guided and collimated in the 2-mm cylindrical guiding channel and the energetic efficiency of acceleration in this scheme is an order of magnitude higher than in the case of conventional ablative acceleration., Comment: 4 pages, 6 figures

We have numerically and experimentally verified that the sequence of optical elements in a femtosecond Ti:sapphire laser affects the laser’s characteristics. Even with the same net dispersion in a cavity, the output spectrum showed a significant change in width and shape when the sequence of dispersive elements was changed in a Kerr-lens mode-locked Ti:sapphire laser with fused-silica prisms for dispersion compensation. It was also shown that the output coupler’s position can affect the dynamics of pulse formation by the combined action of output coupling and self-phase modulation. We explain such phenomena based on pulse formation in a laser cavity with discrete distribution of dispersion and self-phase modulation.