Yasukazu Izawa

Prepulse-free petawatt laser for a fast ignitor

We have developed a prepulse-free short-pulse Nd:glass laser system of 0.9-PW peak power to heat a pre-imploded high-density plasma. An optical parametric chirped amplification system is introduced to reduce the prepulses to an amplitude (1.5/spl times/10/sup -8/) of that of the main pulse. The compressor is a double-path grating pair system 94 cm in diameter compressing the 50-cm-diameter laser beam to 470 fs. An off-axis parabolic mirror has focused the 420-J energy to an intensity of 2.5/spl times/10/sup 19/ W/spl middot/cm/sup -2/. Part of the front end of the chirped pulse is seeded into the preamplifier of the GEKKO XII laser, used to implode a pellet target, to enable the petawatt laser to irradiate the pre-imploded pellet during stagnation of a few tens of picoseconds.

Scalings of implosion experiments for high neutron yield

A series of experiments focused on high neutron yield has been performed with the Gekko‐XII green laser system [Nucl. Fusion 27, 19 (1987)]. Deuterium–tritium (DT) neutron yield of 1013 and pellet gain of 0.2% have been achieved. Based on the experimental data from more than 70 irradiations, the scaling laws of the neutron yield and the related physical quantities have been studied. Comparison of the experimental neutron yield with that obtained by using a one‐dimensional fluid code has led to the conclusion that most of the neutrons produced in the stagnation phase of the computation are not observed in the experiment because of fuel–pusher mixing, possibly induced by the Rayleigh–Taylor instability. The coupling efficiency and ablation pressure have been calculated using the ion temperature measured experimentally. A coupling efficiency of 5.5% and an ablation pressure of 50 Mbar have been obtained.

Plasma physics and radiation hydrodynamics in developing an extreme ultraviolet light source for lithography

Extreme ultraviolet(EUV)radiation from laser-produced plasma (LPP) has been thoroughly studied for application in mass production of next-generation semiconductor devices. One critical issue for the realization of an LPP-EUV light source for lithography is the conversion efficiency (CE) from incident laser power to EUVradiation of 13.5-nm wavelength (within 2% bandwidth). Another issue is solving the problem of damage caused when debris reaches an EUV collecting mirror. Here, we present an improved power balance model, which can be used for the optimization of laser and target conditions to obtain high CE. An integrated numerical simulation code has been developed for the target design. The code agrees well with experimental results not only for CE but also for detailed EUV spectral structure. We propose a two-pulse irradiation scheme for high CE, and reduced ion debris using a carbon dioxide laser and a droplet or a punch-out target. Using our benchmarked numerical simulation code, we find a possibility to obtain CE up to 6–7%, which is more than twice that achieved to date. We discuss the reduction of ion energy within the two-pulse irradiation scheme. The mitigation of energetic ions by a magnetic field is also discussed, and we conclude that no serious instability occurs due to large ion gyroradius.

Nd-doped phosphate glass laser systems for laser-fusion research

Nd:phosphate laser glasses are evaluated in this paper as the active medium for high-power and high-energy laser systems for laser-fusion research. Small-signal gains and parasitic limits of rod amplifiers and disk amplifiers are first shown. Then system performances at a short-pulse, high-power regime and at a long-pulse high-energy regime are investigated. At a short-pulse regime, very high output power of 3.4 terawatts (TW) has been obtained because of the large stimulated emission cross section and the small nonlinear refractive index. At a long-pulse regime, we found that the effective saturation fluence of the phosphate glass LHG-8 is 4.0 J/cm2and we can efficiently extract the energy stored in the laser glass. This lead to the conclusion that Nd: phosphate glasses are very suitable for short-pulse as well as for long-pulse amplifications. In this paper, system descriptions and hardware specifications of the glass laser systems at the Institute of Laser Engineering (ILE), Osaka, University, are also briefly given.

A key factor in parent and fragment ion formation on irradiation with an intense femtosecond laser pulse

Abstract Two pairs of organic molecules (2,3-dimethyl-1,3-butadiene, 2,5-dimethyl-2,4-hexadiene; 1,4-cyclohexadiene, 1,3-cyclohexadiene) were irradiated with a high-intensity 120-fs laser pulse in the intensity region of 10 14 W cm −2 . One molecule in each pair had no allowed electronic transition in the cation at the excitation wavelength of 800 nm, resulting in parent ion dominance. In contrast, the counter molecule, which had a similar structure but with absorption in the cation at the excitation wavelength, showed heavy fragmentation with a negligibly small signal of the parent ion. The previously reported observations are explainable on the basis of the proposed mechanism for fragmentation in femtosecond ionization.

213 W average power of 2.4 GW pulsed thermally controlled Nd:glass zigzag slab laser with a stimulated Brillouin scattering mirror

We report a high-average-power and high-pulse-energy diode-pumped Nd:glass laser amplifier system consisting of two thermally-edge-controlled zigzag slab amplifiers and a stimulated Brillouin scattering mirror. This phase-conjugated system produces an average power of 213 W at 10 Hz in a 8.9 ns pulse (2.4 GW peak power) with an optical-to-optical conversion efficiency of 11.7% and a near-diffraction-limited beam. To the best of our knowledge, this is the highest performance from a Nd:glass-based laser amplifier system ever built.

Pure-tin microdroplets irradiated with double laser pulses for efficient and minimum-mass extreme-ultraviolet light source production

Laser-driven expansion of pure-tin microdroplets was demonstrated to produce an efficient and low-debris extreme-ultraviolet (EUV) light source. The pre-expansion is indispensable for resolving the considerable mismatch between the optimal laser spot diameter (∼300μm) and the diameter (∼20μm) of microdroplets containing the minimum-mass Sn fuel for generating the required EUV radiant energy (∼10mJ/pulse). Explosive expansion of microdroplets was attained by using a laser prepulse, whose intensity was at least 3×1011W∕cm2. The expanded microdroplet was irradiated with a CO2 laser pulse to generate EUV light. A combination of low density and long-scale length of the expanded microdroplet leads to a higher EUV energy conversion efficiency (4%) than that (2.5%) obtained from planar Sn targets irradiated by a single CO2 laser pulse. This scheme can be used to produce a practical EUV light source system.

Optimum laser pulse duration for efficient extreme ultraviolet light generation from laser-produced tin plasmas

Conversion efficiencies (CEs) from incident laser energy to 13.5nm light within a 2% bandwidth were measured with changing laser pulse durations for laser-produced tin plasmas. Experimental results indicate that the optimum pulse duration is determined by two parameters: one is the optical depth of tin plasma for 13.5nm light and the other is laser absorption rate in 13.5nm emission-dominant region. The maximum CE of 2.2% is obtained with pulse duration of 2.3ns.

Properties of ion debris emitted from laser-produced mass-limited tin plasmas for extreme ultraviolet light source applications

Properties of ion debris emitted from laser-produced mass-limited tin plasmas have been experimentally investigated for an application to extreme ultraviolet (EUV) lithography. Simple scaling laws to design the mass-limited target, which is a key technique to minimize contamination of the first EUV collection mirror, is discussed. The measured energy spectrum of the tin ions is consistent with a prediction by the isothermal expansion model. The average charge state of the tin ions is evaluated to be +5 at 180mm away from the plasma, and higher-energy ions have higher charge state. It was found that not only EUV emission but also ion energy spectra are sensitively affected by the target mass limitation.

Low-density tin targets for efficient extreme ultraviolet light emission from laser-produced plasmas

Influence of initial density of tin (Sn) targets has been quantitatively investigated for efficient extreme ultraviolet light emission from laser-produced plasmas. With a decrease in the initial density, conversion efficiency (CE) from incident laser energy to output 13.5nm light energy in a 2% bandwidth increases; 2.2% of the peak CE was attained with use of 7% low-density SnO2 targets (0.49g∕cm3) irradiated with a Nd:YAG laser, of which wavelength, pulse duration, and intensity are, respectively, 1.064μm, 10ns, and 5×1010W∕cm2. The peak CE is 1.7 times higher than that obtained with the use of solid density Sn targets. Experimental results may be attributed to the influence of the initial density and/or microstructure of the targets on expansion dynamics of the plasmas.