H. Daido

Review of soft x-ray laser researches and developments

In this paper, the author reviews the plasma-based x-ray lasers which we have already demonstrated saturated amplification whose wavelengths are between 50 and 6?nm. Section?1 describes the motivation of this review paper which includes basic ideas, developments and their applications of x-ray lasers. In section?2, the author describes the early x-ray laser researches on the recombination and the electron collisional excitation schemes including the hydrogen-like and lithium-like ion recombination schemes and the electron collisional excitation scheme. Section?3 describes the first demonstration of significant lasing at Livermore for the electron collisional excitation scheme of neon-like selenium ions at a wavelength of 20.6 and 20.9?nm and Princeton for the recombination scheme of hydrogen-like carbon ions at a wavelength of 18.2?nm. In section?4, the author describes the electron collisional excitation type soft x-ray lasers which are at present the most successful x-ray lasers. The subjects with which the author deals are saturated amplification neon-like soft x-ray lasers, improvement of neon-like soft x-ray laser performance using multi-layer mirrors, atomic physics issues of the neon-like soft x-ray lasers, gain guiding of the x-ray laser beam propagation, discharge-pumped compact repetitive neon-like ion soft x-ray lasers, the collisional excitation nickel-like ion soft x-ray lasers, high gain and saturated amplification nickel-like soft x-ray lasers at wavelengths as short as 7?nm, the short wavelength nickel-like x-ray lasers whose wavelengths are close to the longest wavelength edge the water window of 4.4?nm, transient collisional excitation scheme which is currently the most popular soft x-ray lasers pumped by short-pulse compact lasers with a laser energy of a few J to a few tens of?J. In section?5, the author describes various plasma-based x-ray laser schemes other than the recombination and the collisional schemes, such as the optical field ionization schemes and inner-shell ionization schemes. Section?6 includes soft x-ray laser applications such as soft x-ray holography, soft x-ray interferometers, soft x-ray microscopy and other applications. In section?7, the author summarizes this review paper and he proposes a future direction for x-ray laser researches.

Investigation of soft X-ray emission from a gas puff target irradiated with a Nd:YAG laser

Abstract Experiments investigating soft X-ray emission in the 1–22 nm wavelength range from plasmas produced using a gas puff target irradiated with a Nd:YAG laser are described. Gas puff targets were created by pulsed injection of high-density gas through a nozzle. The use of the gas puff targets eliminates the production of debris associated with solid targets. Laser pulses of either 0.9 ns or 10 ns time duration with energies up to 0.7 J were used to produce plasmas. X-ray emissions from laser-produced gas puff plasmas were characterized for various gases. The spectral measurements of the soft X-ray emission were performed with the use of grating spectrographs equipped with the back-illuminated CCD camera. The source size was measured using the Fresnel zone plate imaging system and the grating spectrograph equipped with a slit placed perpendicularly to the dispersion direction. The obtained results would allow to develop an efficient and debrisless laser-produced X-ray source to be useful for applications in various fields.

Strong extreme ultraviolet emission from a double-stream xenon/helium gas puff target irradiated with a Nd:YAG laser

Abstract Extreme ultraviolet (EUV) emission in the 8–18 nm wavelength range from a new double-stream gas puff target irradiated with a Nd:YAG laser has been studied for the first time. The target was formed by pulsed injection of a xenon gas stream into a hollow gas stream from helium by using a double-nozzle setup. Strong EUV production near 11 nm from the double-stream xenon/helium target exceeding emissions from iron, copper, and tin solid targets was observed. This new result may be useful to design a laser-produced radiation source for EUV lithography.

Detailed space-resolved characterization of a laser-plasma soft-x-ray source at 13.5-nm wavelength with tin and its oxides

Space-resolved soft-x-ray spectra of laser-produced plasmas of pure-Sn metal and its oxides were measured in the spectral range 7–23 nm. We established a comprehensive spectroscopic database of the emission characteristics of the transition array of highly ionized Sn near 13.5-nm wavelength by varying the incident laser energy and the angle between the observation axis and the target normal. We examined the narrow spectral bandwidth of the transition array obtained by use of a gas-mixed fine-particle (SnO2 powder) target proposed by Matsui [Proc. SPIE3886, 610 (2000) ]. We selected pure-Sn metal, SnO and SnO2 powder, and SnO2 thin-film targets with which to clarify the roles of additional constituent ions, such as O and Ar, in plasmas of the gas-mixed fine-particle targets. The space-resolved spectra show that the bandwidth of the transition array broadens dramatically and that the wavelength at peak intensity shifts slightly toward longer wavelengths with increasing distance from the original target surface or with decreasing incident laser energy. The origins of the broadening and the wavelength shift can be explained in terms of an increase in the range of ion stages that contribute to the transition array and in terms of transfer of the dominant ion stages to lower stages. The narrow bandwidth of the gas-mixed fine-particle target is probably due to the presence of a narrow range of moderate ion stages.

Significant improvement in the efficiency and brightness of the J = 0–1 19.6-nm line of the germanium laser by use of double-pulse pumping

We describe significant improvement in the efficiency and brightness of the J = 0-1 19.6-nm line of the neonlike germanium laser. A curved slab target was irradiated by two laser pulses of 100-ps duration separated by 300 ps with a total energy of 200-300 J on target. Compared with that obtained by 1-ns single-pulse pumping with 1.1 kJ of energy, the J = 0-1 line has a 12 times higher peak intensity, with a smaller beam divergence of 2 mrad, corresponding to a factor-of-25 improvement in brightness.

Soft x‐ray spectra of highly ionized elements with atomic numbers ranging from 57 to 82 produced by compact lasers

X‐ray emission spectra in the spectral range of 2–13 nm from 19 kinds of material with high atomic numbers (lanthanum through lead) were recorded with a grazing incidence spectrometer equipped with a microchannel plate detector. There is an intense, narrow spectral band in these spectra which shifts toward shorter wavelength and becomes weak in intensity with increasing atomic number. The materials were irradiated either by a 4 J/35 ns slab Nd:glass laser or by a 0.5 J/8 ns Nd:YAG laser. The absolute photon intensities of the spectra were determined with an absolutely calibrated charge coupled device camera. The peak spectral brightness of the emission at the peak intensity of the spectral band for lanthanum plasma was estimated to be 2.1×1016 photons/s/mm2/mrad2 in 0.1% bandwidth. The origin of the narrow, intense spectral bands in the recorded spectra and their dependence on target materials and laser wavelength are interpreted.

Line x‐ray emissions from highly ionized plasmas of various species irradiated by compact solid‐state lasers

X‐ray emissions in the spectral 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 the spectra from these materials were determined. The variation of the molybdenum spectrum with laser irradiance was also investigated. Finally, the spectra ranging from 2 to 13 nm produced by the Nd:YAG laser of both the fundamental (1.06 μm) and its second‐harmonic (0.53 μm) wavelengths are compared.

Magnetically insulated and inertially confined fusion — MICF

By combining the benefits of magnetic and inertial fusion, a new fusion scheme is proposed. A plasma with a density of ?1021 cm?3 is confined by the inertia of a heavy, cannonball-type metallic shell; its heat is insulated by a self-generated magnetic field of ?100 T. The plasma and the magnetic field are produced by ablation due to direct impact of a laser (or particle) beam on solid fuel which constitutes the coating of the inner surface of the spherical metallic shell. Preliminary experimental and simulation results, using a 100 J CO2 laser on a target of a few millimetre parylene shell, gave n? ? 5 ? 1012 cm?3?S, with T ? 500 eV. A 1-D spherical hydrodynamic code, HISHO, with the radial heat conductivity reduced by an assumed magnetic field of 103 T, leads to ignition at an absorbed energy of the order of 20 MJ deposited during a confinement time of approximately 100 ns. These results provide supporting evidence for the feasibility of the scheme as a realistic reactor.

Development of beam-pointing stabilizer on a 10-TW Ti:Al2O3 laser system JLITE-X for laser-excited ion accelerator research

We developed a beam-pointing stabilizer for a 10-TW Ti:sapphire laser system to study laser-induced ion acceleration. We confirmed the improvement in the laser beam pointing stability by using a 10-TW laser system (we call it JLITE-X) at JAERI-APRC. The original long-term pointing stability on a 10-TW laser system is ∼70 μrad without beam stabilizing. By using a beam-pointing stabilizer, the laser beam stability was improved within 10 μrad. If f/1 focusing optics and a target which is tilted 45° to the target normal is used, the fluctuation of the laser intensity at the focus will be improved from 80% without stabilizing to <0.4% with it.

Laser polarization dependence of proton emission from a thin foil target irradiated by a 70fs, intense laser pulse

A study of proton emission from a 3‐μm-thick Ta foil target irradiated by p-, s-, and circularly polarized laser pulses with respect to the target plane has been carried out. Protons with energies up to 880keV were observed in the target normal direction under the irradiation by the p-polarized laser pulse, which yielded the highest efficiency for proton emission. In contrast, s- and circularly polarized laser pulses gave the maximum energies of 610 and 680keV, respectively. The difference in the maximum energy between the p- and s-polarized cases was associated with the difference between the sheath fields estimated from electron spectra.