Masataka Kado

X-ray laser beam with diffraction-limited divergence generated with two gain media

We demonstrate an x-ray laser at a wavelength of 13.9 nm with a beam divergence of 0.2 mrad, which is 1.8 times the diffraction limit. The x-ray laser is generated with two gain media; the seed x-ray pulse from the first medium is amplified in the second medium. The effect of refraction on x-ray propagation is reduced by spatially and temporally controlling the injection of the seed x-ray to the second medium.

Development of a pumping laser system for x-ray laser research

A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated.

The gain distribution of the transient collisional excited X-ray lasers

Abstract An atomic kinetics model of an electron collisional excited X-ray laser is developed, and the spatial and temporal evolution of the soft X-ray gain is investigated. The calculation of the gain agrees with experiment for the transient collisional excited (TCE) Ni-like Ag laser ( λ=139 A ) pumped by two 100 ps laser pulses. The mechanism of producing gain in the ionizing plasma is discussed. The calculation is applied to the optimization of the gain. It is found that higher gain can be obtained by pumping a thin foil target with 2 ps laser pulses. The saturation intensity of the X-ray lasers is also investigated through the analysis of the detailed atomic processes of the upper laser level.

Demonstration of a transient-gain nickel-like xenon-ion x-ray laser

We demonstrate a high-gain nickel-like xenon-ion x-ray laser, using a picosecond-laser-irradiated gas-puff target. The elongated x-ray laser plasma column was produced by irradiation of the gas-puff target with line-focused double picosecond laser pulses with a total energy of 18 J in a traveling-wave excitation scheme. Strong lasing at 9.98 nm was observed, and a high gain coefficient of 17.4 cm(-1) was measured on the transient collisionally excited 4d-4p , J=0-1 transition for nickel-like xenon ions with target lengths as great as 0.45 cm. A weak nickel-like lasing line at a shorter wavelength of 9.64 nm was also observed, with a gain coefficient of 5.9 cm(-1) .

Demonstration of a fully spatial coherent X-ray laser at 13.9 nm

We have recently reported the successful development of a fully coherent X-ray laser (XRL) at 13.9 nm by an oscillator-amplifier configuration with two targets. In the experiment, a seed XRL beam from the first target is injected into a plasma amplifier at the second target. The observed XRL beam has full spatial coherence and 0.2 mrad of nearly diffraction-limited divergence. In order to improve the output fluence, the amplification properties of the XRL beam have been investigated using various plasma lengths of the second amplifier target. The output energy has been improved by a factor of ten, increasing the length of the gain region to 10 mm, resulting in about 0.2 /spl mu/J of output energy.

Numerical analysis of high-gain transient collisional x-ray lasers

We have developed a collisional radiative model of electron collisional excited x-ray lasers. We calculate the ion abundance and soft x-ray gain for the 4d-4p transition of Ni-like multiple charged ion, in short pulse laser irradiated plasmas. We combine a detailed model using the atomic data calculated by HULLAC code and the averaged model based on the screened hydrogenic approximation. In order to choose a proper set of the levels to be included in the atomic model, investigation of the model dependence of the gain is carried out. The population in the fine structure levels in 3d94s and 3d94f configuration, autoionizing double excited configurations of Ni-like and Cu-like ions are found to cause approximately factor of 2 difference in the soft x-ray gain. The steady-state gain and its dependence on plasma density and temperature are investigated over elements from z equals 45 to 65. The time dependent calculation for a plasma condition corresponds to a thin Ag foil irradiated by tow short laser pulse shows the transient gain which is 40 times greater than the steady state gain.

Near-field imaging of Ni-like silver transient collisional x-ray laser

We review our recent progress in the development of transient x-ray lasers and of their application to plasma diagnostic. The first observation of C-ray laser emission at the new PHELIX-GSI facility is reported. This TCE X-ray laser will be a promising tool for heavy-ion spectroscopy. We then present the main results obtained at the LULU-CPA facility with a compact high-resolution X-UV imaging device. This device was used to investigate the spatial source structure of the Ni-like silver transient X-ray laser under different pumping conditions. The key-role of the width of the background laser pulse on the shape of the emitting aperture is demonstrated. Finally the imaging device was used as an interference microscope for interferometry probing of a laser-produced plasma. We describe this experiment performed at APRC-JAERI.

Simulations of transient collisional x-ray lasers

We have developed an atomic kinetics model of transient collisionally excited x-ray lasers by integrating a detailed model for the Ni-like ion based on the atomic data calculated by the HULLAC code, and an averaged model using super levels for a wide range of charge states. Calculations of the temporal and spatial evolution of the gain of the Ni-like Ag laser are carried out using this code as a postprocessor of 1-dimensional hydrodynamics code to show qualitative agreement with experiments.

Development of a CPA Nd:glass/Ti sapphire laser system for generation of coherent x-ray laser radiation

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.

Observation of the Ferroelectric Material with Instantaneous X‐ray Laser Speckles

Picosecond x‐ray speckles experiment has been conducted with a simple setup. The source was a compact silver‐plasma‐based x‐ray laser, with a wavelength of 13.9 nm and pulse duration of 7 ps. The sample was a single crystal of BaTiO3, with ferroelectric multi‐domain structure (a/c domains). The matter correlation function, including statistical information of those randomly distributed scatterers (domains here) within the sample, was extracted by deconvolution of the speckle pattern. The instantaneous x‐ray speckle technique has proved to be particularly efficient to be used to observe fast microscopic‐scale phenomena that are hard to access with other methods currently.