Yoshihiro Ochi

Numerical study of Kα emission from partially ionized chlorine

Abstract Population kinetics and spectral synthesis codes have been developed to analyze Kα emission from partially ionized chlorine in hydrocarbon plasmas irradiated with high-intensity ultra-short laser pulses. The population kinetics processes are calculated using a bi-Maxwellian electron temperature distribution composed of fast and cold electrons. The fast electrons dominantly contribute to ionize the K-shell bound electrons. i.e., inner-shell ionization, while the cold electrons produce ionization from the outer-shell electrons. The resultant Kα emission provides a distinct spectral signature for each charge state. Also included in the calculation are the opacity effects in the framework of an escape probability method. It is shown that the Kα emission is saturated at a plasma thickness of more than a few microns. Further, we find that unresolved satellite lines overlap significantly with the corresponding parent Kα lines of the next charge state, and make a large contribution to composite spectrum. Finally, time-dependent properties of the Kα emission are also discussed briefly.

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.

Recent progress in X-ray laser research in JAERI

Recent progress in x-ray laser (XRL) research in Japan Atomic Energy Agency (JAEA) is reviewed. The repetition-rate of the x-ray laser has been improved from each 20 minutes to 10 seconds (0.1 Hz) by installing new driver laser, TOPAZ, which allows us to promote the applications of fully spatial coherent 13.9 nm laser in the wide variety of research fields such as material science, single-shot x-ray holography and atomic physics. In order to improve the present performance of the x-ray lasers, we have investigated the possibilities of the enhancement of the peak brilliance using v-groove target and the generation of circularly polarized x-ray laser under a strong magnetic field. Towards shorter wavelength x-ray lasers, we have investigated several schemes. One is the use of reflection of the light by relativistic plasma mirror driven by laser-wake-field, and the other is photo-pumping scheme using Kα emission from a solid target.

Development of a highly coherent x‐ray laser

We have developed a fully coherent x‐ray laser (XRL) at 13.9 nm by a double‐target configuration. In the experiment, a seed XRL beam from the first target is injected into a plasma amplifier at the second target. The amplified XRL beam has full spatial coherence, 0.2 mrad of the nearly diffraction limited divergence, and 25 nJ of the output energy. For various application researches, we improve the output energy and the beam property of the x‐ray laser. The gain‐length product was gL ∼ 8.2, and the maximum output energy of about 1 μJ is achieved. The divergence of the XRL is affected by the refraction influence in the amplifier medium and/or the diffraction by the small size of the gain medium. We made the 2‐D radiation hydrodynamics simulation code in order to investigate the generation of the gain medium plasma and the refraction influence.

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.

Development of a full spatial coherent x-ray laser at 13.9 nm

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.

Measurement of wavelength and linewidth of transient collisional x‐ray lasers

We have conducted a preliminary measurement of the wavelength and the intrinsic linewidh of the nickel‐like silver x‐ray laser at a wavelength of 13.9 nm in the transient collisional excitaion scheme. The measured wavelength was compared with Multi‐Configuration Dirac‐Fock (MCDF) code (GRASP92) and was used as a benchmark of the accuracy of the MCDF code. The intrinsic linewidth of the x‐ray laser, which is convolution of inhomogeneous and homogeneous components, is investigated theoretically by use of a collisional‐radiative model based on the HULLAC code.