Tetsuya Kawachi

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.

Kinetics modeling of Ni-like multiple charged ions

Abstract Atomic processes affecting the population kinetics of Ni-like Ag are investigated using a collisional radiative model that includes the Cu-, Ni-, and Co-like ions. The atomic processes, which determine the steady-state ion abundance and detailed level populations, are investigated to determine the population inversion for the X-ray laser application, with several different sets of configurations. The dielectronic processes are considered by including doubly excited and inner-shell-excited states explicitly which allows one to identify the dominant channel of the dielectronic recombination from Co-like to Ni-like, and Ni-like to Cu-like ion. Further, the resonant excitation from ground state to low excited states of Ni-like ion in high-density plasmas ( n e >10 20 /cm 3 ) is also studied.

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) .

Coherent X-ray beam metrology using 2D high-resolution Fresnel-diffraction analysis

Direct metrology of coherent short-wavelength beamlines is important for obtaining operational beam characteristics at the experimental site. However, since beam-time limitation imposes fast metrology procedures, a multi-parametric metrology from as low as a single shot is desirable. Here a two-dimensional (2D) procedure based on high-resolution Fresnel diffraction analysis is discussed and applied, which allowed an efficient and detailed beamline characterization at the SACLA XFEL. So far, the potential of Fresnel diffraction for beamline metrology has not been fully exploited because its high-frequency fringes could be only partly resolved with ordinary pixel-limited detectors. Using the high-spatial-frequency imaging capability of an irradiated LiF crystal, 2D information of the coherence degree, beam divergence and beam quality factor M2 were retrieved from simple diffraction patterns. The developed beam metrology was validated with a laboratory reference laser, and then successfully applied at a beamline facility, in agreement with the source specifications.

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.

Burst intensification by singularity emitting radiation in multi-stream flows

Burst Intensification by Singularity Emitting Radiation (BISER) is proposed. Singularities in multi-stream flows of emitting media cause constructive interference of emitted travelling waves, forming extremely localized sources of bright coherent emission. Here we for the first time demonstrate this extreme localization of BISER by direct observation of nano-scale coherent x-ray sources in a laser plasma. The energy emitted into the spectral range from 60 to 100 eV is up to ~100 nJ, corresponding to ~1010 photons. Simulations reveal that these sources emit trains of attosecond x-ray pulses. Our findings establish a new class of bright laboratory sources of electromagnetic radiation. Furthermore, being applicable to travelling waves of any nature (e.g. electromagnetic, gravitational or acoustic), BISER provides a novel framework for creating new emitters and for interpreting observations in many fields of science.

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.

X‐ray Lasers Driven by Optical Lasers

Recent topics in the optical‐laser‐driven x‐ray lasers are reviewed. With the collisional excitation x‐ray lasers, pumping laser energy has been reduced over 100 times by reducing the pumping pulse width from ns to ps. A high gain of 30–40 cm−1 has been achieved with the transient collisional excitation using ps pumping. A new scheme for a charge exchange x‐ray laser using clusters such as C60 irradiated with ultra short laser pulses has been introduced. In the inner shell ionization x‐ray lasers, creation of a population inversion utilizing the difference in the Coster‐Kronig decay rates between the upper and lower laser levels has been proposed as a scheme to enable pumping with short bursts of electrons. High gain over long duration is expected with hollow atoms created under intense broad‐band x‐ray irradiation emitted from oscillating electrons under relativistic intensities (Larmor radiation). The x‐ray laser program at the Advanced Photon Research Center at the new site in Kyoto is presented.