Kenichi Kinoshita

Refraction effects on the cavity formation and interaction of an intense ultra-short laser pulse with a gas jet

Formation of a plasma cavity with a shock wave in gas jets irradiated by tightly focused femtosecond laser pulses causes the wave break of the laser wake field at the front of the shock wave and, as a result, the injection of electrons into the acceleration phase of the wake-field wave. A strong crescentlike deformation of the cavity and a change in electron signal are observed with gas density growth. It is attributed to a mutual effect of the cavity on the laser pulse propagation and break of the plasma wake field due to refraction of the laser pulse.

Effects of density gradient on short-bunch injection by wave breaking in the laser wake field acceleration

Effects of density gradient on the self-injection of plasma electrons in the phase of laser pulse wake for further acceleration, is studied for moderate laser intensities, a0⩽3. It is shown that transverse wave breaking can shorten the length of accelerated electrons, whereas effective longitudinal wave breaking requiring steep plasma density interface increases their total charge. For the considered range of laser intensities, the total charge of electrons injected by wave breaking rises exponentially with a0.

Femtosecond electron beam generation and measurement for laser synchrotron radiation

One of the S-band twin linacs (18L linac) of Nuclear Engineering Research Laboratory of University of Tokyo is modified in order to produce femtosecond electron single bunch for femtosecond X-ray generation via Thomson backward scattering, namely laser synchrotron radiation. Laser photocathode RF gun and chicane-type magnetic pulse compressor are installed at the S-band linac. 10 ps (FWHM) laser pulse generates 5 MeV, 10 ps (FWHM), 1 nC electron single bunch, which is accelerated up to 20 MeV in the S-band accelerating tube and compressed to 200 fs (FWHM) by the chicane. Design study has been performed by using the code of PARMELA and the installation has been finished. For precise and reliable measurement of the compressed pulse length, the comparison of measurement between the femtosecond streak camera and coherent transition radiation interferometry was carried out. Good agreement between them for 1—10 ps (FWHM) pulses was achieved. A new Michelson interferometer for the 200 fs pulse is now under construction. ( 1998 Elsevier Science B.V. All rights reserved.

Experimental verification of laser photocathode RF gun as an injector for a laser plasma accelerator

The feasibility of the laser photocathode RF gun, BNL/GUN-IV, as an injector for a laser plasma accelerator was investigated at the subpicosecond S-band twin linac system of the Nuclear Engineering Research Laboratory, University of Tokyo. Electron beam energy of 16 MeV, emittance of 6/spl pi/ mm mrad, bunch length of 240 fs (FWHM), and charge per bunch of 350 pC were confirmed at 10 Hz. As for diagnosis of the femtosecond electron bunch, the quantitative comparison of performance of the femtosecond streak camera, the coherent transition radiation (CTR) Michelson interferometer, and the far-infrared polychromator was carried out. We concluded that the streak camera is the most reliable up to 200 fs and that the polychromator is the best for the shorter electron bunch. The 3.5-ps (rms) resolved synchronization between the YLF laser driver for the gun and the electron bunch was achieved. Based on the above experiences, we have designed and installed a much better laser-electron synchronization system using the Kerr-lens mode-locked Ti:Sapphire laser with the min harmonics synchrolocker and the stable 15-MW klystron. The timing jitter is expected to be suppressed down to 320 fs (rms).

Femtosecond electron beam generation by S-band laser photocathode RF gun and linac

A laser photocathode RF electron gun was installed in the second linac of the S-hand twin linac system of Nuclear Engineering Research Laboratory (NERL) of University of Tokyo in August in 1997. Since then, the behavior of the new gun has been tested and the characteristic parameters have been evaluated. At the exit of the gun, the energy is 3.5 MeV, the charge per bunch 1∼2 nC, the pulse width is 10 ps(FWHM), respectively, for 6 MW RF power supply from a klystron. The electron bunch is accelerated up to 17 MeV and horizontal and vertical normalized emittances of 3 π mm.mrad are achieved. Then, the bunch is compressed to be 440 fs(FWHM) with 0.35 nC by the chicane-type magnetic pulse compressor. The linac with the gun and a new femto- and picosecond laser system is planned to be installed for femtosecond pulseradiolysis for radiation chemistry in 1999.

Time-resolved X-ray diffraction at NERL

For ultrafast material analyses, we constructed the time-resolved X-ray diffraction system utilizing ultrashort X-rays from laser-produced plasma generated by the 12-TW-50-fs laser at the Nuclear Engineering Research Laboratory. Ultrafast transient changes in laser-irradiated GaAs crystals were observed as X-ray diffraction patterns. Experimental results were compared with numerical analyses.

Propagation of an intense femtosecond laser pulse through a thin foil filter

In order to improve the contrast of a nanosecond prepulse, the interaction of 12 TW, 50 femtosecond laser pulses with thin foil, as a filter, is investigated. Due to the collisional absorption, the prepulse produces rapidly expanding plasma and can lose 90% of its energy. At a properly chosen thickness of the filter, more than 60% of energy of the femtosecond main pulse is shown to be transmitted through the expanded underdense plasma with reduced prepulse contrast ratio.

Compact x-ray sources by intense laser interactions with beams and plasmas

Short pulsed X-rays have been experimentally generated by 90 degree 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. 100 TW laser and microtron as a 150 MeV electron beam source will be prepared, and the laser and the electron beam will be interacted as a hard X-ray source. Soft X-ray may be generated via laser-plasma nonlinear Thomson scatterings as a source of X-ray microscope.

Self-interaction of subpico-second electron bunch traveling through a chicane-based bunch-compressor

Abstract A photo-cathode RF-gun and a chicane-based bunch-compressor are installed on an S-band linac which had been used for a UT-FEL experiment. Electron bunches extracted from the photo-cathode RF-gun are accelerated by an S-band structure up to 20 MeV and compressed by a chicane magnet. Since the bunch has very small longitudinal size and relatively low energy, coherent synchrotron radiation emitted from the bunch in the chicane creates a nonuniform energy loss in the bunch and degrades the performance of the bunch compressor. In the present paper, the performance of the bunch-compressor under the influence of coherent synchrotron radiation is studied. Preliminary experimental results are also presented.

Efficiency of Laser Plasma Kα Emission for Time-Resolved X-ray Imaging

Laser Plasma X-rays (LPX) with a duration of 10 ps or less, emitted from a solid target irradiated by a femtosecond laser pulse, is useful for time-resolved measurements. The intensity of this radiation is 4×109 photons/shot/4πsr at maximum, which corresponds to the conversion efficiency of laser energy into Kα X-rays of 3.4×10-5. By simulations of the interaction of a laser pulse with a solid target, the efficiency is shown to increase more than ten times by suppressing the energy of a laser prepulse.