Akihiko Nishimura

Ytterbium-doped glass regenerative chirped-pulse amplifier

Abstract Chirped-pulse amplification in a Yb3+-doped phosphate glass regenerative amplifier pumped by a free-running Ti:sapphire laser is demonstrated. Amplified chirped pulse energies up to 20 mJ were obtained. Amplified bandwidths of 20 nm at reduced energies were obtained using regenerative spectral shaping. Spectroscopic measurements are presented which are necessary to model amplification in a quasi-three-level laser.

Velocity distributions in high density gadolinium atomic beam produced with axial electron beam gun

The parallel and perpendicular velocity distributions of a gadolinium atomic beam produced with an axial electron beam gun have been measured by a Doppler-shift technique and a Doppler limited absorption spectroscopy, respectively. The atomic density of 1018~1019atoms/m3 at a laser irradiated zone was sufficient for the process, such as an atomic vapor laser isotope separation. The atomic beam velocity of 800 m/s was obtained by a free expansion near an evaporation surface. This velocity, however, was 100 m/s slower than that we had measured with a magnetic transverse electron beam gun in a previous experiment. The reasons for the slower velocity in this experiment than that in the previous one are discussed. In keeping with the deposition rate, the parallel translational temperature rapidly decreased to 200 K, while the perpendicular translational temperature gradually increased and approached to the parallel translational temperature. This result suggested that the collisional region where both translat...

Temporal Change of Thermal Lens Effects on Highly Pumped Ytterbium Glass by Wavefront Measurement

The temporal change of thermal lens effects has been measured for the development of high efficient laser system. Ytterbium phosphate glass was used due to its high saturation fluence property. A free running flashlamp pumped Ti:sapphire laser was developed for the high energy pump source. Pumping intensity on the glass surface exceeded 800 kW/cm2 which is enough high for laser diode simulator. Shack-Hartmann wavefront sensor system was applied to measure the thermal lens effects on a highly pumped ytterbium glass oscillator. The temporal change of wavefront was successfully measured at high repetition rate of 100 Hz. The measured wavefront were expanded to Zernike coefficients. Cooling process for the focus coefficient agreed well with the prediction of unsteady one dimensional heat conduction.

Removal of the plasma contained in an atomic beam produced by electron beam heating

Removal of the plasma contained in a gadolinium atomic beam produced by electron beam heating was investigated. A positive or negative electric potential was applied to the plasma removal electrodes which were a pair of parallel electrodes put along the atomic beam. When a positive potential was applied to the plasma removal electrodes, the plasma could not be removed at high evaporation rates. On the other hand, the plasma could be removed by applying a high negative potential to both removal electrodes, even at high evaporation rates. The potentials applied to the electrodes required to remove the plasma were estimated using the model that a plasma at ground potential flows with the atomic beam; ions are extracted from the plasma by negatively biased removal electrodes. The estimated potentials required to remove the plasma agreed well with experimental values.

Metastable states’ population densities in Gd atomic beam at high‐rate evaporation by electron beam heating

Metastable states’ population densities in gadolinium atomic beams, which were produced by an axial electron beam gun, were measured by laser absorption spectroscopy at the atomic densities on the order of 1012 atoms/cm3. The following results were obtained. (1) The atomic excitation temperature derived from the metastable states distributions is considerably lower than the evaporation surface temperature. With increasing evaporation rates, the atomic excitation temperature decreased to about 500 K. (2) The densities of both ground and first metastable states increased linearly with the deposition rates as measured by a quartz crystal sensor on the beam axis, which indicates that the atomic beam is almost free from the cluster formation caused by rapid expansion into a high vacuum.

Demonstration of material processing using JAERI-FEL

Abstract Ablation experiments by the superconducting RF linac FEL in JAERI are planed. The FEL can produce ultrashort laser pulses of 255 fs micro-pulse with the pulse energy of 0.23 ml The high operational frequency over 10 MHz is useful for the high repetitive metal processing which is free from thermal damages. PACS codes: 42.62 .

Effect of Electron Beam on Velocities of Uranium Atomic Beams Produced by Electron Beam Heating

Velocities of uranium atomic beams produced by two types of electron beams, a bent beam and a straight beam, were measured by a laser Doppler-shift technique. The atomic beams were accelerated to 1100 m/s and 900 m/s by free expansion near the evaporation surface with the bent beam and the straight beam, respectively. The higher velocity for the bent-beam evaporator was explained as follows. The evaporated atoms are excited or ionized more efficiently by the electron beam than for the straight-beam evaporator. The excitation and ionization energies contribute to the acceleration of the atomic beam. The attained velocities for the two types of electron beams were roughly estimated using the excitation and ionization energies derived from the electronic stopping power of uranium.

High-energy flash-lamp-pumped Ti-sapphire laser for Yb:glass CPA

A high energy flashlamp pumped Ti:sapphire laser has been developed for the pumping source of Yb:glass chirped pulse amplification. The free running oscillator generates 12 Joule/pulse at 793 nm at 1 Hz repetition. The output energy of 6 Joule/pulse at 920 nm has been obtained.

Ablation of work hardening layers against stress corrosion cracking of stainless steel by repetitive femtosecond laser pulses

Femtosecond laser pulses successfully ablated the work hardening layers on SUS316L used in boiling water reactors. The measurement of hardness inside the material clarified this new technique to reduce the risk of Stress Corrosion Cracking.

Feasibility study of a direct bonding technique for laser crystals

Direct bonding without the use of adhesives was demonstrated on Ti:sapphire laser crystals with a bonding surface of 12 mm x 6 mm and the bonded region was evaluated from the macroscopic to the atomic level by three different methods. Wavefront distortion caused by the bonded region of 10 mm x 5 mm was estimated at 0.031 wavelengths (λ) at 633 nm. Micro defect measurements by a laser tomography method showed that the number of micro defects on the bonded region were much smaller than that of the intrinsic ones inside the crystal. From a magnified inspection, atoms in the bonded region were well arranged with the same regularity as inside the crystal. In addition, micro defects 1 nm in size appeared slightly along the bonded interface where the titanium ion concentration was four times higher than other parts of the crystal.