[en] We investigate a mechanism of nonlinear phenomena in laser-plasma interaction, a laser wakefield excited by intense laser pulses, and the possibility of generating an intense bright electron source by an intense laser pulse. We need to understand and further employ some of these phenomena for our purposes. We measure self-focusing, filamentation, and the anomalous blueshift of the laser pulse. The ionization of gas with the self-focusing causes a broad continuous spectrum with blueshift. The normal blueshift depends on the laser intensity and the plasma density. We, however, have found different phenomenon. The laser spectrum shifts to fixed wavelength independent of the laser power and gas pressure above some critical power. We call the phenomenon 'anomalous blueshift'. The results are explained by the formation of filaments. An intense laser pulse can excite a laser wakefield in plasma. The coherent wakefield excited by 2 TW, 50 fs laser pulses in a gas-jet plasma around 10¹⁸ cm^-3 is measured with a time-resolved frequency domain interferometer (FDI). The density distribution of the helium gas is measured with a time-resolved Mach-Zehnder interferometer to search for the optimum laser focus position and timing in the gas-jet. The results show an accelerating wakefield excitation of 20 GeV/m with good coherency, which is useful for ultrahigh gradient particle acceleration in a compact system. This is the first time-resolved measurement of laser wakefield excitation in a gas-jet plasma. The experimental results are compared with a Particle-in-Cell (PIC) simulation. The pump-probe interferometer system of FDI and the anomalous blueshift will be modified to the optical injection system as a relativistic electron beam injector. In 1D PIC simulation we obtain the results of high quality intense electron beam acceleration. These results illuminate the possibility of a high energy and a high quality electron beam acceleration. (author)

[en] The laser wakefield electron acceleration up to 300 MeV has been observed in an underdense plasma driven by a 2 TW, 90 fs laser pulse synchronized with a 17 MeV RF linac at 10 Hz. The electron acceleration was enhanced at a density higher than the resonant density due to gas ionization and self-channeling effects. The wakefield excitation has been confirmed by measuring the electron density oscillation of the plasma wave with the frequency domain interferometer. (author)

[en] We have studied optical guiding of high-intensity laser pulses using fast Z-pinch for channel guided laser wakefield acceleration (LWFA). A fast Z-pinch discharge can produce a long stable plasma channel which has a concave electron density profile in its core. We experimentally demonstrated the feasibility of optical guiding of high-intensity laser pulses (>10¹⁷ W/cm²) using Z-pinch discharge channel. A ray-trace calculation of the Ti:Sapphire laser pulse in the channel has been done, which could well explain the experimental results

[en] The anomalous blue shift of high intensity laser which was discovered by the present authors occurs in the process of gas ionization accompanied with the self-focusing. This shift does not depend either on the laser power or on the gas density and all photons are shifted by a certain frequency, while the one which has been known in common depends on both the intensity and density and only some part of the laser photons is shifted. In order to elucidate this phenomenon, the occurrence conditions of the anomalous blue shift were investigated and the results are compared with theory. The shifts were measured by focusing the laser beam in the gas-filled chamber with an off-axis-parabolic mirror and with a convex lens. When the reflective lens was used the amount of the shift depended significantly on the ionization rate of the plasma, while it depended on the pulse width when the transmission lens was used indicating that the shift is determined by the valence due to the ionization at the focusing point. (S. Funahashi)

[en] A RF photoinjector for the high duty operation was designed and constructed by the BNL/KEK/SHI international collaboration and installed at the S-band linear accelerator of the University of Tokyo. Subpicosecond electron bunches compressed by a magnetic compression is planned to be used for the subpicosecond X-ray generation and the laser acceleration. Preliminary experiments of the RF photoinjector were performed. The measured value of the Cu cathode's quantum efficiency was 5x10^-5. (author)

No abstract available

[en] Short pulsed X-rays were experimentally generated by 90deg Thomson scatterings of 2 TW, 90 fs laser pulses by 17 MeV electron beams. Synchronization between the laser pulses and electron beams were achieved within a few ps. A 100 fs X-ray pulse will be generated via backward Thomson scatterings from a 100 fs electron bunch made by a bunch compression chicane. A high peak power, high brightness, high repetitive laser synchrotron radiation source consisting of counter-colliding laser and electron storage rings is proposed. (author)

[en] Recently there has been a tremendous experimental progress in ultrahigh field particle acceleration driven by ultraintense laser pulses in plasmas. A design of the laser wakefield accelerators aiming at GeV energy gains is discussed by presenting our recent progress on the laser wakefield acceleration experiments, the developments of high quality electron beam injectors and the capillary plasma waveguide for optical guiding of ultrashort intense laser pulses. (author)

[en] Short pulsed X-rays have been experimentally generated by 90 deg. Thomson scattering of 2 TW, 90 fs laser pulses by 17 MeV electron beams. A few 100 fs X-ray pulses have been generated via backward Thomson scattering from a few 100 fs electron bunches made by a bunch compression chicane. Soft X-ray may be generated via laser-plasma nonlinear Thomson scatterings as a source of X-ray microscope

[en] When an intense laser pulse is focused in tenuous gas, the pulse blue-shifts by optical-field ionization and relativistically self-focuses forming a plasma channel. Since the blue-shift changes the critical power of the self-focusing, the plasma channel length can be limited. This mechanism results in the fixed amount of blueshift. (author)