Over the past 10 years, the laser accelerator research has advanced the electron gain of from 22 MeV to 200 MeV. Recently, it has produced 200 MeV electrons from a 2mm-long plasma. This corresponds to 100 GV/m. In Japan, a glass capillary, last year, succeeded in increasing the accelerating plasma length from 2mm to 10mm. Mono-energetic peaks were also found first in Japan. Together, these advances constitute a breakthrough to the second generation of advanced accelerator development. This review introduces these topics as well as the development of the ion acceleration studies.

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I review progress in the generation of ultrahigh peak power optical pulses in the femtosecond range. A design, performance and characterization of a Ti: sapphire laser system based on chirped-pulse amplification, which has produced a peak power of 0.85-PW with 33-fs pulse durations are described. Extension of laser systems to the multi-petawatt power level is also discussed.

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The amplified optical parametric fluorescence (AOPF) from optical parametric chirped-pulse amplifier (OPCPA) was controlled by injecting a residual fundamental pulse from a seeded Q-switch Nd: YAG pumping laser, as a quenching beam. The output pulse from the OPCPA was used as a seed pulse for Ti: sapphire chirped pulse amplifier (CPA) system to generate over 10TW peak power with a high contrast ratio.

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An ultra-intense laser injected a 15 J of power at 1.053μm in 0.5 ps into a glass capillary of 1-cm long and 60 jam in diameter, which accelerated plasma electrons to 100 MeV. 1- and 2-dimensional particle codes describe that a laser wakefield is resonantly excited with a gradient of 10 GV/m, accelerating electrons. The blue shift of the laser spectrum as well as the Raman side peak, supports the plasma of 10¹⁶ cm^-3 inside the capillary. A bump at the high energy tail suggests the electron trapping in the wakefield.

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Recent results of monoenergetic acceleration obtained at several institutes paved the way for the development of laser-driven particle accelerators, which can be miniaturized to a scale of 1/1000. Experiments covered the plasma density range and laser power range of 1.6×10¹⁸-1.5×10²⁰ cm^-3 and 2-30 TW, respectively. The experimental results revealed empirical scaling on energy gain, which was proportional to laser power and inversely proportional to electron density. Monoenergetic beams were obtained in narrow regions of electron density at given laser powers. These features of acceleration suggest that electrons injected by localized wavebreaking were accelerated by a potential well in plasma formed by an ultra-short laser pulse.

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Laser-plasma accelerators are compared with conventional accelerators. In a laser-irradiated foil, electron sheaths function as electrodes to accelerate ions based on the principle of electrostatic accelerators such as Cockroft Walton and Van de Graaf accelerators. Laser wake-field electron accelerators belong to a travelingwave type, in which plasma waves are substituted for RF waves. Though the laser-plasma accelerators have high acceleration gradients, their structures are temporal and hard to control.

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Recent observations of intense beams of multi-MeV ions generated by intense short-pulse lasers irradiating thin solid foils have opened perspectives for important applications like compact high-brightness ion sources as radioisotope generators, proton radiography, or high-energy density matter. There are two main mechanisms that lead to laser acceleration of high-energy ions in the forward direction. The first one is the ion acceleration by the ponderomotive pressure of the laser pulse at the target front side, and the second mechanism is the acceleration by the sheath field excited at the target rear surface by escaping fast electrons. Both mechanisms and their beam qualities are introduced in this article.

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High quality beam generation project based on High-Tech Research Center Project, which has been approved by Ministry of Education, Culture, Sports, Science and Technology in 1999, has been conducted by advance research institute for science and engineering, Waseda University. In the project, laser photo-cathode RF-gun has been selected for the high quality electron beam source. RF cavities with low dark current, which were made by diamond turning technique, have been successfully manufactured. The low emittance electron beam was realized by choosing the modified laser injection technique. The obtained normalized emittance was about 3 mm mrad at 100 pC of electron charge. The soft X-ray beam generation with the energy of 370 eV, which is in the energy region of so-called “water window” , by inverse Compton scattering has been performed by the collision between IR laser and the low emittance electron beams.

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Recent developments of the synchrotron radiation facilities and intense lasers are now guiding us to new research frontiers with a high energy GeV photon beam and an intense and short pulse MeV γ-ray beam. New directions of the science developments with photo-nuclear reactions are discussed. The inverse Compton yrays have good characteristics as 1) good emmitance, 2) high linear and circular polarization. With these advantages, the photon beams in the energy range from MeV to GeV are used for studying hadron structure, nuclear structure, astrophysics, materials science, as well as for applying medical science.

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In this article, the current status of radiation therapies in Japan and updated medical accelerators are reviewed. For medical use, there is a strong demand of a compact and flexible accelerator. At present, however, we have only two choices of the S-band linac with one or two rotation axis combined with the multi leaf collimator, or the X-band linac with a rather flexible robotic arm. In addition, the laser plasma cathode that is the second generation of the laser wake-field accelerator (LWFA) is studied as a high-quality electron source for medical use though it is still at the stage of the basic research. The potential of LWFA as medical accelerator near future is discussed based on updated results of laser plasma cathode experiment in Univ. of Tokyo.

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Energetic proton generation by irradiation of several kinds of tape targets by ultra-short high-power laser pulses was investigated in the intensity region around 10¹⁸W/cm² and sub-pico second pulse duration. The maximum proton energy showed a dependence on the product of target thickness and mass density. The fast electrons should lose energy after passing through solid targets of high mass density, which was also con-firmed by a Monte Carlo simulation code, Geant4. A deformable mirror was successfully controlled to increase the maximum proton energy. Only 30mJ of laser energy was sufficiento generate 1 Me V protons.

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We have constructed the front-end system using a broadband high-gain OPCPA for chirped pulse amplifica-tion. This system can deliver the output energy of 65 mJ with 6-nm spectral width at a 1.053-lim central wavelength. The stability of output energy was substantially improved compared with the Ti: Al₂O₃ regenera-tive amplifier. These output parameters are appropriate as a seed of PW-class Nd: glass laser system.

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An efficient, compact water-cooled heat sink has been developed for high-power edge-pumped microchip lasers. A vertical, water jet impingement cooling system can reduce the heat sink cross-sectional area to Ο10 mm. Experimental results and theoretical analysis show dramatic enhancement of the heat transfer rate by a microchannel structure on the water-impinged, behind the heat sink top where laser material is bonded. Thermal resistance of 0.25°C/W was achieved in the central Ο5mm area of the heat sink's cooling surface. Continuous-wave (CW) laser output over 300W was successfully obtained from the diode edge-pumped, Yb: YAG microchip core (Ο5 mm area 0.3mm thick) bonded on the new heat sink.

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We have developed the generation system of synchronized pulses of high precision using chirped pulse laser. The PW laser is synchronized to Gekko XII beams within 10 picosecond by injecting part of the PW laser intc the Gekko XII laser system. A part of the 3ns/ 6nm (pulse width/ spectral width) output from the front end is stretched to 5.5ns/5nm and is then siliced to 1.1ns/nm width and injected into Gekko XII system. We have obtained 2.5-kJ output energy at a 532-nm wavelength from 12 semi-Gaussian beams. The pulse width is 1.1 ±0.1 ns (FWHM) and the conversion ef Hciency from 1 to 0.5mm was 43%.

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A method of displacement measurement of a moving object with a diffusing surface has been developed already. This paper presents, an automatic method of the displacement mesurement. The automatic method consists of a feedback loop synchronized with the movement of an object, making fringes using speckle pattern interferometry. This is done by processing the input image of the fringes, including extraction of circles and detection of the difference by moving the reference PZT. The method of displacement measurement is as follows. A collimated laser light beam is transformed into two beams by using a concave lens and a beam-splitter cube, in a Twyman-Green-type interferometer. When a diffusing surface is placed at the focal point of one of two beams, a diverging spherical wave front containing large-sized speckle patterns is scattered. If a smooth surface reference mirror with PZT is placed in the vicinity of the focal point of the other beam, the reflected forms of the two beams have essentially a wave front, with identical shape. The two beams can be combined to form concentric interference fringes which are fairly regular. By observing whether the concentric fringes which flow out or sink in, it is shown that the normal component of the displacement of the diffused surface can be measured with an accuracy of a fraction of a half wavelength.

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