Isao Matsushima

10 kHz 40 W Ti:sapphire regenerative ring amplifier.

A Ti:sapphire regenerative ring amplifier is developed. The power of 40 W before compression and 26 W after compression are achieved at 10 kHz repetition rate. Thermal distortion of a compressor grating is reduced by air flow cooling.

KrF laser picosecond pulse source by stimulated scattering processes

A new short-pulse KrF laser source was developed by combining stimulated scattering processes, namely, Raman, Brillouin, and four-wave-mixing (FWM). Short pulses of 1.1 ps were obtained from 4 ns oscillator output. Only two commercial discharge KrF lasers are required for this system. This method is very simple and could be extended to an ArF laser wavelength in principle.

High-intensity short KrF laser-pulse generation by saturated amplification of truncated leading-edge pulse

Abstract A new method for the generation of high-intensity short KrF laser pulses was developed. The method efficiently uses truncated leading-edge seed-pulse generation by stimulated Brillouin scattering and pulse-shape steepening by saturated amplification, which is mainly based on the Frantz–Nodvik solution. Short KrF pulses of 7 ps were obtained from 20 ns seed pulses without using mode-locked lasers. The experimental results agreed well with the numerical calculations.

Use of tin as a plasma source material for high conversion efficiency

Debris-free generation of a tin plasma was demonstrated in the cavity-confined configuration. Narrow band emission at 13.7-nm was observed in an emission spectrum of a cavity confined tin plasma. The spectral efficiency was as high as 12% and we found the conversion efficiency could reach 6%/2π str ultimately while lots of works are required to achieve this value. We also confirmed a magnetic field has some effect of stopping a plasma.

A 37% Efficiency, Kilohertz Repetition Rate Cryogenically Cooled Ti:Sapphire Regenerative Amplifier

We present a high-efficiency Ti:sapphire regenerative amplifier being developed as a pumping laser for a high-brilliance laser-plasma X-ray source operating at a multi-kHz repetition rate. A ring resonator is adopted for stable single transverse mode operation and a laser rod is cryogenically cooled to suppress the strong thermal lens effect. By reducing losses in the resonator and by matching the pumping mode volume with the resonator mode, the conversion efficiency is increased to 37%. The output power is 7.4 W when pumped by a 1 kHz 20 W green laser.

Overview of `Super-ASHURA' KrF Laser Program

Abstract In the Super-ASHURA KrF Laser at ETL, a main amplifier with full pumping duration (270 ns) and 12 beam lines have been completed. At present, total output energy of 2.7 kJ has been extracted from the main amplifier by double-pass amplified 12 beams (20 ns×12) with an aperture filling factor of 74%. Local intrinsic efficiency has reached 10%. In the Raman amplification which is to be used for high peak power pulse generation, multi-pass forward amplification of nanosecond Stokes pulse showed an energy conversion efficiency of 73%. Also, a multi-joule short Stokes pulse (150 ps) has been obtained from 12.5 J, and 20 ns pulse by saturated backward amplification of a self-generated Stokes pulse. To investigate the effect of irradiation smoothness, the two-dimensional profile of the target rear surface displacement has been measured by using short (

Beam smoothing by broadband random-phase irradiation

Abstract A new technique is proposed to achieve smooth laser irradiation profiles on laser fusion targets employing the broad bandwidth of KrF lasers. In this technique, smoothing is obtained by using a spectral dispersing optical device and a random-phase plate placed in front of a focusing lens. Experimental results demonstrate that this simple optical system is effective for smooth irradiation on targets.

Two-dimensional beam smoothing by broadband random-phase irradiation

A new technique for two-dimensional smoothing is proposed to generate smooth laser irradiation profiles on laser fusion targets employing the broad bandwidth of KrF lasers. In this technique, a wedged etalon is used to get angular dispersion to the orthogonal direction to the one-dimensional broadband random-phase smoothing effect. The preliminary experiment has demonstrated its effectiveness.

Particle-cluster tin target for a high conversion efficiency LPP source for EUVL

Particle-cluster tin target is presented as the solution of a 100W EUV source for EUVL. Theory for maximizing conversion efficiency of a laser-produced plasma is derived and the theory is experimentally confirmed by using a dispersed SnO2 particles. The EUV intensity 4 times higher than that from a plasma on a solid Sn plate target is observed at the optimized density. The achieved conversion efficiency for dispersed particles is estimated to be as high as 3%/(2π str 2%BW) or higher from the value for a Sn plate of 0.8% measured by using two multilayer mirrors and a calibrated photodiode. Theoretical consideration reveals that larger diameter plasma enables higher EUV power. The particle-cluster can be delivered at multi kHz rep-rate by using water droplet. Experimental confirmation of delivering particles by droplets is also reported.

High-Repetition-Rate Electron-Beam-Pumped KrF Laser Technology for Inertial-Fusion-Energy Drivers

The development of a high-repetition-rate (rep-rate) electron-beam (e-beam)-pumped KrF laser is described. In this study, we aim to achieve pulsed laser outputs of 20 J/80 ns at a rep rate of 1 Hz. The key issues are heat management in the diode foils and introduction of long-lifetime switches into the pulsed-power system. E-beam generation experiments have been started with initial test results of twenty consecutive shots at 1 Hz. Improvement in the energy-transfer efficiency in the rep-rate pulsed-power systems is also discussed.