Sho Amano

The stability of a continuously growing solid-Xe target (thickness 500 Em) on a rotating drum (diameter 17 cm, rotation speed 1000 rpm and up-down length 3 cm) chilled by liquid-N₂ is investigated. The thermal load and the propagation process of a laser-induced shockwave in a solid-Xe target irradiated at a high-repetition rate and a high laser power are calculated theoretically. The thermal load is calculated to be approximately 0.7 W/cm² for a pulse energy of 1 J, a repetition rate of 30 kHz and an average power of 30 kW. Under these conditions the solid-Xe target should be chilled to ≤ 84 K for it to attain thermal equilibrium. The target system has the ability to supply solid-Xe target sufficiently rapidly that foliation of the target due to an accumulation of shockwaves does not occur under these irradiation conditions, since the lifetimes of the shockwaves were estimated to be about 1 μs.

A new-type of laser-excited RF-gun using a needle photocathode is proposed. Design studies of the gun performances are executed with numerical calculations for electric field of the tip of the needle photocathode, laser heating and Joule heating of the tip, and beam parameters of a photoelectron beam in the RF-gun cavity. This calculation assumes a 2856 MHz RF-gun system that has the maximum field of 40 MV/m and a mode-locked pulsed-laser at a wavelength of 351 nm with a micropulse duration of 10 ps. Calculation results show that, for the optimized tip radius of 8.8 μm, a peak photocurrent reaches up to a space-charge limited current of ${\sim}90$ A with a pulse width of ${\sim}6$ ps and an energy spread of ${\sim}1$% at the cavity exit using an optimized laser with a peak intensity of 330 MW/cm2 and a laser injection phase of 20 deg. A preliminary experiment of a needle-RF-gun is also performed.

A femtosecond synchrotron radiation pulse train can be extracted from an electron storage ring by interaction between an ultrashort laser pulse and an electron beam in an undulator. Generation system of a femtosecond soft x-ray pulse by the slicing technique was studied with numerical calculations for its performance, as applicable for the NewSUBARU synchrotron radiation facility at LASTI. The femtosecond electron pulse, that is energy-modulated with a Ti:sapphire laser at a pulse energy of 100 μJ, a pulse width of 150 fs, and repetition frequency of 20 kHz, can be sufficiently separated in a bending magnet. A femtosecond soft x-ray pulse (the critical photon energy of 0.69 keV and a pulse width of 250 fs) is obtained with a collimator (diameter of 800 μmφ), and it has an average brightness 3 × 10⁶ photons/s/mm²/mrad²/0.1 %BW and an average photon flux 10⁵ photons/s/0.1 %BW.

The dependence of quantum efficiency on a high electric field was investigated using a tungsten needle photocathode irradiated by a 3rd-harmonics Nd:YAG laser whose photon energies were lower than the work function of tungsten. The tungsten needles were fabricated by an electrolytic etching technique. The tip radii could be controlled between 0.1 and 1 μm by changing the etching conditions. The obtained quantum efficiency of the needle tip is found to be proportional to the >10th power of the electric over 500 MV/m, and it reached up to 3% at about 800 MV/m. This observed field-enhancement of quantum efficiency is qualitatively explained with a field-emission process including the Schottky effect and photo-excitation.

Image relaying in a high-average-power slab laser system, especially one with thermal lenses, was studied.Ray-trace analysis using an ABCD matrix was conducted for a multi-cascade relaying system. The analysisclarified quantitatively the beam expansion, the focal shift and the power loss by thermal lenses that should betaken into account in the system design. An Nd: YAG slab laser system was developed, and its beam wasrelayed successfully by careful design, taking the above considerations into account. The laser system demonstratedan average power of 235 W with a high beam quality.