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We report on the J-KAREN-P laser facility at QST, which can provide PW peak power at 0.1 Hz on target. The system can deliver short pulses with an energy of 30 J and pulse duration of 30 fs after compression with a contrast level of better than 1012. Such performance in high field science will give rise to the birth of new applications and breakthroughs, which include relativistic particle acceleration, bright x-ray source generation, and nuclear activation. The current achieved laser intensity on target is up to > 9x1021 Wcm-2 with an energy of ~9 J on target. The interaction with a 3 to 5- μm stainless steel tape target provides us electrons with a typical temperature of more than 10 MeV and energetic proton beams with typical maximum energies of > 40 MeV with good reproducibility. The protons are accelerated in the Target Normal Sheath Acceleration regime, which is suitable for many applications including as an injector into a beamline for medical use, which is one of our objectives.

Following three different types of high power lasers at Kansai Photon Science Institute are overviewed and controlling the laser damages in these laser systems are described: (1) PW-class Ti:sapphire laser for high field science, (2) zig-zag slab Nd:glass laser for x-ray laser pumping, and (3) high-repetition Yb:YAG thin-slab laser for THz generation. Also reported is the use of plasma mirror for characterization of short-wavelength ultrashort laser pulses. This new method will be useful to study evolution of plasma formation which leads to laser damages.

Experimental demonstration of multi-charged heavy ion acceleration from the interaction between the ultra-intense short pulse laser system and the metal target is presented. The laser pulse of

We demonstrate a new ion diagnosis method for high energy ions by utilizing a combination of a single CR-39 detector and plastic plates, which enables to detect high energy ions beyond the detection threshold limit of the CR-39. This detection method coupled with a magnetic spectrometer is applied to identify high energy ions of 50 MeV per nucleon in laser-driven ion acceleration experiments using cluster-gas targets.

We present experimental results, theory, and simulations demonstrating two novel sources of coherent X-ray radiation generated in the relativistic laser (>10 18 W/cm 2 ) interaction with easily accessible, repetitive, and debris-free gas jet targets. The first source is based on a relativistic mirror reflecting a counter-propagating laser pulse. A strongly nonlinear breaking wake wave driven by an intense laser pulse can act as a semi-transparent relativistic flying mirror. Such a mirror directly converts counter-propagating laser light into a high-frequency (XUV or X-ray) ultrashort pulse due to the double Doppler effect. In the experimental demonstration with the 9 TW J-KAREN laser, the flying mirror generated in a He gas jet partially reflected a 1 TW pulse, providing up to ~10 10 photons, 60 nJ (~10 12 photons/sr) in the XUV range (12.8-22 nm). The second source is demonstrated with the laser power ranging from 9 to 170 TW in experiments with the J-KAREN and Astra Gemini lasers. The odd and even order harmonics generated by linearly as well as circularly polarized pulses are emitted forward out of the gas jet. The 120 TW laser pulses produce harmonics with ~3×10 13 photons/sr (~600 μJ/sr) in the 120±5 eV spectral range. The observed harmonics cannot be explained by previously known mechanisms (atomic harmonics, betatron radiation, nonlinear Thomson scattering, etc.). We introduce a novel mechanism of harmonic generation based on the relativistic laser-plasma phenomena (self-focusing, cavity evacuation, bow wave generation), mathematical catastrophe theory which explains the formation of structurally stable electron density singularities, spikes, and collective radiation of a compact charge driven by a relativistic laser.

This paper presents the experimental investigation of laser-driven proton acceleration using a table top Ti:Sapphire laser system interacting with the thin-foil targets during the course of medical application of the laser-driven proton beam. The proton beam with maximum energy of upto 14~MeV is generated in 60 TW mode. The number of protons at ~10 MeV is estimated to be over 105 proton/sr/MeV/shot with beam having half divergence angle of 5~degree. If 10 Hz operation is assumed 2 Gy dose is possible to irradiate during 10 min onto a ~1 mm tumor just under the skin. In contrast to the previous condition of our apparatus with which we demonstrated the DNA double-strand breaking by irradiating the laser-driven proton beam onto the human cancer cells in-vitro test, the result reported here has significant meaning in the sense that pre-clinical in-vivo test can be started by irradiating the laser-driven proton beam onto the skin of the mouse, which is unavoidable step before the real radiation therapy.

The critical electric field of quantum electrodynamics, called also the Schwinger field, is so strong that it produces electron-positron pairs from vacuum, converting the energy of light into matter. Since the dawn of quantum electrodynamics, there has been a dream on how to reach it on Earth. With the rise of laser technology this field has become feasible through the construction of extremely high power lasers or/and with the sophisticated use of nonlinear processes in relativistic plasmas. This is one of the most attractive motivations for extremely high power laser development, i.e. producing matter from vacuum by pure light in fundamental process of quantum electrodynamics in the nonperturbative regime. Recently it has been realized that a laser with intensity well below the Schwinger limit can create an avalanche of electron-positron pairs similar to a discharge before attaining the Schwinger field. It has also been realized that the Schwinger limit can be reached using an appropriate configuration of laser beams. In experiments on the collision of laser light and high intensity electromagnetic pulses generated by relativistic flying mirrors, with electron bunches produced by a conventional accelerator and with laser wake field accelerated electrons the studying of extreme field limits in the nonlinear interaction of electromagnetic waves is proposed. The regimes of dominant radiation reaction, which completely changes the electromagnetic wave-matter interaction, will be revealed. This will result in a new powerful source of high brightness gamma-rays. A possibility of the demonstration of the electronpositron pair creation in vacuum via multi-photon processes can be realized. This will allow modeling under terrestrial laboratory conditions neutron star magnetospheres, cosmological gamma ray bursts and the Leptonic Era of the Universe.

Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.

We describe results of experiments on laser-driven proton acceleration and corresponding laser-plasma diagnostics performed with the multi-10-TW J-KAREN laser. The laser consists of a high-pulse-energy oscillator, saturable absorber, stretcher, Optical Parametric Chirped Pulse Amplifier (OPCPA), two 4-pass Ti:Sapphire amplifiers, and compressor. The final amplifier is cryogenically cooled down to 100 K to avoid thermal lensing. The laser provides ~30 fs, ~ 1 J, high-contrast pulses with the nanosecond contrast better than 1010. The peak intensity is 1020 W/cm2 with the 3- 4 μm focal spot. Using few-μm tape and sub-μm ribbon targets we produced protons with the energies up to 4 MeV. The tape target and repetitive laser operation allowed achieving proton acceleration at 1 Hz. We found significant differences in stability and angular distribution of proton beam in high-contrast and normal-contrast modes. The plasma diagnostics included interferometry and measurement of the target reflectivity. The latter provides convenient diagnostics of the laser contrast in the ion acceleration, harmonics generation, and other laser - solid target interaction experiments.

We describe the demonstration of the high pulse energy and high efficiency second-harmonic generation (SHG) at 532-nm using large aperture CsLiB6O10 (CLBO) crystals for the first time to our knowledge. A pulsed green energy of 25 J has been generated with 34 J of input 1064-nm Nd:glass laser radiation using a two-stage crystal architecture. High conversion efficiency of 74% has been attained at fundamental laser intensity of only 370MW/cm2. This result represents the highest green pulse energy ever reported using the CLBO crystals. We discuss in detail the design and performance of SHG using CLBO crystals.

A four-pass quadrature frequency conversion scheme was developed to generate green output with high efficiency for pumping an ultrashort pulse laser system. With this scheme, a efficiency from fundamental energy into total second harmonic energy in excess of 80% was achieved for frequency doubling of 1064-nm in KTP with a low input fundamental laser intensity of 76 MW/cm 2 . A total second-harmonic output of 486 mJ was obtained with 607 mJ of the input 1064- nm fundamental laser at 10 Hz.

A novel frequency converter architecture of four-pass quadrature frequency conversion scheme was developed aiming to achiev high conversion efficiency. A high conversion efficiency of more than 80 % has been achieved in this scheme forfrequency doubling of 1064-nm in KTP with a low input fundamental laser intensity of 76 MW/cm2. A second-harmonicoutput of 486 mJ was obtained with 607 mJ of the input 1064-nm fundamental laser at 10 Hz.Keywords : quadrature frequency conversion, second-harmonic generation (SHG), KTP, Nd:YAG laser 1. INTRODUCTIONS Frequency conversion through nonlinear processes is widely used for extending the utility of existing laser. Inparticular, generation of pulsed green output is important for application for pumping Ti:sapphire amplifier. The mosteffective way to accomplish this has been through second-harmonic generation (SHG) of 1064-nm Nd:YAG lasers innonlinear optical crystals1'2. For SHG at 1064-nm for Nd:YAG lasers, a commonly employed laser source, conversionefficiency normally exceeds 50 %36• Efficient SHG from the fundamental laser wavelength is also essentially important forthe frequency up-conversion because the performance of higher-order harmonic generation is very much dependent on this.Additionally, in many applications and experiments, a high second-harmonic conversion efficiency is desirable for thepurpose of enhancing the signal-to-noise ratio or reducing the size of the required laser source.In order to achieve high conversion efficiency, a quadrature frequency conversion scheme has been proposed7.Figure 1 shows the quadrature doubling scheme used for SHG. In this scheme, the planes formed by the input laser beampropagation direction and optic axes of two type II crystals are arranged to be orthogonal. The input laser beam leftunconverted from the first crystal due to nonuniformities of the input beam intensity can be converted efficiently in thesecond crystal. Thus, the sensitivity to input laser beam profile, angle and thermal misadjustment is lower than that ofsingle crystal scheme. The second-harmonic output produced in the first crystal is not at the correct polarization forinteraction in the second. Thus, the second-harmonic output from the first crystal passes through the second without back-conversion.In this paper, we describe a four-pass quadrature frequency conversion scheme by using polarization rotation forpumping Ti:sapphire amplifier and report a frequency conversion efficiency of above 80 % for SHG with a low inputfundamental laser beam intensity of 76 MW/cm2. The incorporation of quadrature and multi-pass features in this schemeprovides a frequency conversion performance superior to that of comparable lasers in its class. This scheme can be easilyscaled up by increasing the size of the nonlinear optical crystal to accommodate larger input fundamental laser beam cross-section.

A novel laser architecture of laser-diode pumped eight pass 1064-nm Nd:YAG zig-zag slab laser amplifier with thermal birefringence compensation by use of a 90 degree quartz rotator has been developed aiming to achieve a high average power laser with high efficiency and good beam quality. With a 100 W (1 kHz) class module, the basic performance of the novel concept was examined by demonstrating an average power of 68 W with a high energy extraction efficiency of 61% for the laser mode volume at an initial small signal gain of 3.03 on single pass, and an excellent beam quality, undernegligible thermal lensing with the compensation for thermal birefringence.

The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.

A novel laser architecture of laser-diode pumped eight pass 1064-nm Nd:YAG zig-zag slab laser amplifier was developed aiming to achieve high energy extraction efficiency and good beam quality. A high energy extraction efficiency of up to 73% for the laser mode volume has been achieved in this amplifier operating at an initial small signal gain of 3.36. The excellent beam quality factor M2 of 1.2 has been obtained. The compensation of thermal birefringence in this amplifier was successfully performed.

We report the high-sensitivity measurements of 2D thermal birefringence distribution in LD pumped Nd:glass (HAP4) laser under lasing at 1054 nm and non lasing conditions. The thermal birefringence was reduced under the lasing condition compared to the non lasing condition. By inserting a 90 degree(s) quartz rotator between two identically LD pumped Nd:glass rods, almost perfect birefringence compensation was achieved with very little residual birefringence. A considerable reduction of birefringence loss from over 20% before compensation to less than 1% after compensation was observed.

The design and construction of an efficient near-infrared holosteric optical parametric oscillator amplifier based on KTiOPO4 (KTP) crystal are reported. The system is tunable from 840 to 1451 nm and generates up to 0.45 mJ at 920 nm with 27% efficiency in 4 - 6 ns pulses. The 532 nm pump source is a frequency doubled laser-diode pumped Nd:YAG laser producing up to 2 mJ in 8 ns, 50 Hz pulses. We have used the system to study the fluorescence lifetimes of Yb:YAG and Yb:glass as a function of wavelength. The data reveal interesting variations in the reabsorption enhanced Yb-ion lifetime. Some Yb:YAG samples exhibiting a sharp increase in their lifetime near the peak absorption wavelengths. The longest lifetime measured in a few mm thick sample of Yb:YAG was 3.3 ms, which would be highly beneficial for energy storage applications using laser diodes.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.