H. Kotaki

Optical guidance of terrawatt laser pulses by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary

A new method of optical guidance by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary is proposed. An imploding plasma column has a concave electron-density profile in the radial direction, just before a stagnation phase driven by a converging current sheet and a shock wave. The feasibility of optical guidance of a high-intensity (>1 x 10(17) W/cm(2)) Ti:sapphire laser pulse by use of this method over a distance of 2 cm, corresponding to 12.5 times the Rayleigh length, has been experimentally demonstrated. The guiding-channel formation process was directly probed with a He-Ne laser beam. The electron density in the fully ionized channel was estimated to be 2.0 x 10(17) cm(-3) on the axis and 7.0 x 10(17) cm(-3) on the peaks of the channel edge, with a diameter of 70 mum, as indicated by the experimental results, which were corroborated by a magnetohydrodynamics simulation.

Head-on injection of a high quality electron beam by the interaction of two laser pulses

High quality intense relativistic electron beams are generated by the interaction of two colliding laser pulses to inject plasma electrons into a wakefield excited by one of the laser pulses. The mechanism of the injection is analyzed theoretically and the generation of a high quality electron beam is verified by the numerical simulation. An electron beam has a small energy spread of 1%, ultrashort pulse duration less than 10 fs and normalized transverse emittance less than 1 π mm mrad.

Quasi-monoenergetic electron beam generation during laser pulse interaction with very low density plasmas

The results of experiments are presented for the single laser pulse interaction with a very low density gas target, under the conditions when the generated wake wave is below the wave-breaking threshold and the laser pulse power is lower than the critical power for relativistic self-focusing. A quasi-monoenergetic electron beam is found to be stably generated for various laser pulse intensity values by controlling the acceleration length. The results of two-dimensional particle-in-cell simulations show that for the electron acceleration an additional mechanism of electron injection into the acceleration phase is required. It is demonstrated that the longitudinal inhomogeneity of the plasma density leads to the electron injection.

Fixed blueshift of high intensity short pulse lasers propagating in gas chambers

An explanation is given for the fixed blueshift observed of an intense short pulse laser propagating in a gas chamber filled with helium. The observed shift is found to be independent of the laser power and gas pressure above some critical power. The results are explained by the formation of filaments, which occur due to nonlinear gas polarization effects. The blueshift is in qualitative agreement with a one dimensional particle-in-cell simulation including ionization of the gas.

Suppression of electron scattering by the longitudinal components of tightly focused laser fields

Relativistic electron scattering by a high intensity linearly polarized Gaussian (TEM00 mode) laser beam is studied in detail using three-dimensional numerical simulations. It is observed that the longitudinal components of the electromagnetic field in a tight focus effectively suppress transverse electron scattering in the relativistic laser ponderomotive acceleration scheme. The simulations show that the relativistic ponderomotive acceleration can produce high quality electron bunches characterized by an extremely short bunch length of subfemtosecond, energy spread less than 1%, and normalized transverse emittance less than 10πmmmrad.

Direct measurement of coherent ultrahigh wakefields excited by intense ultrashort laser pulses in a gas-jet 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. 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.

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.

Production and utilization of synchronized femtosecond electron and laser single pulses

A subpicosecond (700 fs at FWHM) electron pulse from the S-band (2.856 GHz) linear accelerator (linac) of the NERL (Nuclear Engineering Research Laboratory) was synchronized with a femtosecond (100 fs at FWHM) laser pulse from a T3 (table-top terawatts) laser with a picosecond time whose standard deviation is 3.7 ps. Then we generated a picosecond characteristic X-ray pulse by irradiating through the electron pulse a Cu target (Kα, 8.1 keV, 1.54A) and obstained the Bragg diffraction from a NaCl ionic monocrystal using a high sensitivity X-ray imaging plate. Further, we discuss its applications to observe lattice vibration of the monocrystal by using the synchronized laser (pump) and X-ray (probe).

Experimental Results of Laser Wakefield Acceleration Using a Femtosecond Terawatt Laser Pulse.

Laser wakefield acceleration (LWA) experiments have been carried out in an underdense plasma driven by a 2 TW, 90 fs laser pulse synchronized with a 17 MeV RF linac electron injector at 10 Hz. Around optimum plasma densities for LWA, we have observed electrons accelerated to 35 MeV. Wakefield excitation has been confirmed by measuring the electron density oscillation with a frequency domain interferometer. At plasma densities higher than the optimum density, we have also observed high energy electrons over 100 MeV up to 200 MeV.

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.