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Flyer acceleration experiments are carried out using a KrF laser system with a pulse duration of 10–15 ns and an intensity of ∼1.0 × 1013 W/cm2. Three-layered targets (aluminum–polyimide–tantalum) are used. First, an average velocity of laser-driven tantalum flyers with a thickness of 4 and 8 μm is estimated. Then, in a collision of a flyer with a copper layer attached to a diamond plate, we measure a transit time of a shock wave in the diamond. The impact velocity is estimated based on the transit time and a numerical simulation. This numerical simulation also shows that the initial peak pressure caused by the impact of a 4-μm-thick flyer is kept at 11 Mbar for 12–13 μm in thickness. Finally, whether this thickness is enough for EOS measurements is discussed.

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.

The experiments of flyer acceleration by the irradiation of a high power laser are carried out using the ASHURA system at the Electrotechnical Laboratory, in which the laser has a short wavelength (∼249 nm) and a long pulse duration (∼30 ns). Three-layered targets (aluminum–polyimide–tantalum) are irradiated. The laser ablates the aluminum and polyimide layers and the rear layer (tantalum) is accelerated as a flyer. It is suggested that the tantalum flyer is in a condensed state for the duration of flight. The flyer velocity estimated from the acceleration profile is at least 8 km/s. One-dimensional numerical simulation indicates that the terminal flyer velocity becomes higher than 15 km/s. The energy conversion from laser energy to flyer kinetic energy is more efficient than that in the previous experiments using the three-layered targets with a longer wavelength and a shorter pulse duration (1–2 ns). Thus, it is found that a facility with a long pulse duration and a short wavelength is suitable for the realization of a fast flyer keeping its state in a condensed phase.

A conceptual design of a high efficiency, high repetition-rated (rep-rated) pulsed power system for an electron-beam (e-beam) pumped KrF laser driver for inertial fusion energy (IFE) is presented. The proposed pulsed power circuit incorporates linear induction voltage adders to charge pulse forming lines for the generation of 900 kV, 260 ns e-beamsat several Hz. This rep-rated power system meets the pulsed power requirements of the future IFE drivers that necessitate high amplifier efficiency as well as high laser energy.

We are working on the generation of the high-power UV pulse by backward Raman pulse amplification. This report deals with the generation of the high-power backward first Stokes light using a simple focusing geometry. The high-power Stokes light, which grows from spontaneous scattered light in the focal region, can be obtained by focusing the high-energy KrF pump pulse into a Raman cell filled with methane gas. The intensity of the pump pulse increases significantly near the focus. This pulse produces a serious effect of the nonlinear refractive index. We mixed xenon gas into the Raman medium to compensate for this effect. As a result, a stable generation of the high-power single backward first Stokes light can be obtained.

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 (

The ` Super-ASHURA ' KrF laser system is currently under development at the Electrotechnical Laboratory. An electron-beam pumped large-aperture KrF laser amplifier named ` Amp4 ' has been completed and is operational as the main amplifier. Amp4 and the currently operating ` ASHURA ' pre-amplifiers (660 J, six beams) have been combined to form the upgraded system. Laser energy extraction experiments from Amp4 have been started (2.3 kJ, 12 beams).

We have developed a useful method for power multiplication of KrF laser light, called forward Raman pulse compression. In this method, a multi-pass forward Raman amplifier is used. Adjustment of incident timing between pump and Stokes light into each path provides efficient Raman energy conversion from long KrF laser to short pulse Stokes light. Even in the case of double path configuration, output Stokes was demonstrated to grow up to 1.3 times the pump intensity. By using an intersecting multi-path of Stokes and pump light, spatial non-uniformity of the output Stokes reduced to less than 5%.

Backward Raman pulse compression of high-power KrF laser using focused geometry was investigated. This simple configuration has various advantages suitable for creating not only high-power but also high-energy Stokes pulses. An energy conversion efficiency of 27% and power gain of 30 have been achieved.

We have carried out the design studies of KOYO-Fast laser fusion power plant, using fast ignition cone targets, DPSSL lasers, and LiPb liquid wall chambers. Using fast ignition targets, we could design a middle sized 300 MWe reactor module, with 200 MJ fusion pulse energy and 4 Hz rep-rates, and 1200MWe modular power plants with 4 reactor modules and a 16 Hz laser driver. The liquid wall chambers with free surface cascade flows are proposed for cooling surface quickly enough to a 4 Hz pulse operation. We examined the potential ofYb-YAG ceramic lasers operated at 150 ∼ 225 K for both implosion and heating laser systems required for a 16-Hz repetition and 8 % total efficiency.

Ionization balance of l0ps KrF laser produced plasmas was investigated under various irradiation conditions. From the laser intensity dependence of relative conversion efficiencies of emissions from F-like and Ne-like Ge ions, it was found that higher ionization state was realized, when a prepulse ASE(amplified spontaneous emission) accompanying a l0ps pulse was suppressed. The absolute conversion efficiency of X-rays from 6A to 9A was also estimated.

A new technique to achieve smooth laser irradiation profiles on laser fusion targets is evaluated numerically and experimentally. In this technique, smoothing is obtained by using a spectral dispersing optic and a random-phase plate placed in front of a focusing lens employing the broad-bandwidth of KrF lasers. Experimental results and numerical calculations agree well, verifying that this simple optical system is effective for smooth irradiation on targets.

Achieved performance of and the future plan for the ASHURA, a 700-J KrF laser, are described. Nanosecond pulses (3–20 ns) with a bandwidth of 50 cm-1 have been produced by gain switching of a preamplifier. The pulses have been six-time angularly multiplexed and double-pass amplified by two electron-beam-pumped amplifiers. The output from the main amplifier is strongly saturated, and maximum energy of 660 J has been extracted in 90 ns with a stage gain of 40. Focused by an f = 1m spherical lens, maximum power density of 6 × 1014 W/cm2 can be obtained on target. The energy density ratio of prepulse on target has been suppressed below 1/107. The ASHURA is being upgraded by adding a next-stage amplifier that aims at 10-kJ output energy in 270 ns.

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.

Fundamental parameters are studied for the detailed design of a picosecond high power KrF laser system. The saturation energy density of 2.3 mJ/cm2and gain recovery time of 1.9 ns in ane-beam pumped KrF laser are observed. A 6 ps pulse is amplified to 1.9 J. The system at the Electrotechnical Laboratory is expected to deliver 10 ps pulses with total energy of several tens of joules in several beams.

Summary form only given. It is very difficult to create energetic high-intensity lasers in the UV region on account of the low damage thresholds of optical components and the low upconversion efficiencies for short pulses. We have been exploring an alternative approach to the high intensity UV pulse generation by combining a large KrF laser amplifier and a backward Raman pulse compression technique. The Raman conversion of KrF laser has many advantages suitable for large systems. For example, two photon absorption coefficient in fused silica window, which is important window material for large system, at the Stokes light wavelength (268 nm for methane gas) is more than three times smaller than the coefficient at KrF laser wavelength.

Reported is a search for the x‐ray amplification in a recombining fluorine plasma produced by the irradiation of a 10 ps KrF laser pulse. The pulse energy was 1 J and the maximum line‐focus length was 1 mm with 20 μm width. Fluorine Hα (81 A) intensity at 50 μm showed exponential growth of gl≊2, and gl≊1.5 for 11s2P‐1s3d line (103.8 A) at 100 μm. From the spatial distributions of He‐like and Li‐like ions, we discuss the detail of the plasma produced.

Resonantly photo‐excited Al He α (7.76 A) fluorescence is observed in the configuration where the photo‐excited Al plasma expanded parallel to the blow off direction of the other Al plasma as a flash lamp. Both plasmas are produced by the irradiation of 10 ns KrF laser pulses on an Al slab target with a step‐height of around 500 μm. Doppler de‐coupling between two plasmas caused by the steep velocity gradient in expanding plasmas is reduced in this configuration. From the spatial distribution of the fluorescence, the scale length of velocity gradient is evaluated.

Soft X-ray contact microscopy has been performed using a short-lived laser-produced plasma as the X-ray source. Single-shot exposures have been recorded in X-ray photoresists, for both dried and in vivo conditions using the high-power Nd : glass and KrF excimer laser systems, available at the SERC Central Laser Facility. The short exposure times of 1 ns and 30 ns respectively allow latent images of the specimens to be recorded before the onset of radiation damage.

A l-kj KrF laser system under development is described. The system has an oscillator, three electron-beam (e-beam) amplifiers, and a Raman pulse compression system. The second stage e-beam amplifier has generated 170 J at the overall efficiency of 1.9%. The final stage amplifier is designed to deliver a 1-kJ optical output in 100 ns.

The present status of the development of a high-power KrF laser system, Ashura, is described. The main amplifier has generated 710 J (95 ns) at the pumping density of 1·lMW/cm3 with the wall plug efficiency of 2·0%. Maximum power of 9·0 GW (200 J/22 ns) per beam has been obtained from the beam lines of six-time pulse multiplexing. Power density of 1 × 1014 W/cm2 has been achieved on target with a 10−6 pre-pulse.

The feasibility of using high-power pulsed lasers (several tens of joules per pulse) to produce plasmas as soft X-ray sources for contact microscopy is discussed and demonstrated for a wide range of biological and other material. As in other forms of X-ray microscopy, complicated and probably artefact-inducing specimen preparation is not necessary. The short pulse lengths of the lasers mean that, for single shot exposures, images can be obtained before radiation damage has altered the structure of the specimen, and this higher resolutions are possible, in principle, than for other forms of X-ray microscopy.

The development of plasma sources of soft x‐rays, using pulsed KrF (600mJ) and Nd:YAG (500mJ) lasers, is described. The intensities and spectral properties of the sources have been investigated for various target materials. The use of the sources in soft x‐ray contact microscopy will be described.

An experimental study of a high-power forward Raman amplifier pumped by a multibeam KrF laser is presented. The active medium is either CH4 at 3-bar pressure or H2 at 1-bar pressure. As many as 56 pump beams confined in a light guide were combined into a single low-divergence Stokes beam. In CH4 a gain of up to 300 and a power-conversion efficiency of 70% was obtained. The maximum Stokes output energy was 8.4 J. The dependence of gain on pump bandwidth was investigated in H2. A gain of 500 and a conversion efficiency of 65% were obtained. The maximum output energy was 5.0 J.