Hideki Dewa

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.

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).

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.

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.

Fast Z-pinch optical guiding for laser wakefield acceleration

Abstract 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 (>10 17 W / cm 2 ) 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.

Recent developments for the 2nd generation LWFA experiments

The laser wakefield acceleration (LWFA) experiments has been conducted by the use of 2 TW. 90 fs laser pulses synchronized with an electron beam injected from a 17 MeV RF lmac. Preliminary experiments demonstrated the energy gam more than 200 MeV attributed to ionization-induced self-modulation and self-guiding of intense ultrashort laser pulses in plasmas. Measurements of the plasma density oscillation and laser-plasma interactions support the enhanced acceleration. We are also developing an RF photo-injector producing low emittance beams, a capillary plasma wave guide generating long stable wakefields and a energy measurement system with emulsion technique.

Femtosecond Quantum Beam Science and New Pump-and-Probe Analysis

A 700 fs(FWHM) electron single pulse from the S-band (2.856 (GHz) linear accelerator (linac) of the Nuclear Engineering Research Laboratory of University of Tokyo was generated in 1994. A femtosecond electron pulse measurement system has also been developed using a femtosecond streak camera and coherent far-infrared transition radiation interferometry. Then it was synchronized with a 100 fs(FWHM) 3 TW laser pulse from a T3 (Table-Top Terawatt) laser with picosecond time resolution. The synchronized laser and electron linac are used for laser wakefield acceleration, femtosecond X-ray generation via Thomson scattering and pulseradiolysis for radiation chemistry. Recently, the laser photocathode RF(radio frequency) electron gun was introduced to the linac and 440 fs elcetron pulse was produced. Further we generated a picosecond X-ray pulse by irradiating a Cu target by picosecond electron pulse and obtained Bragg diffraction from an NaCl ionic monocrystal using a high sensitivity X-ray imaging plate Finally, we propose and discuss a new pump-and-probe analysis to observe dynamics of atoms by using the synchronized laser (pump) and X-ray (probe).