Kazuhisa Nakajima

0.56 GeV Laser Electron Acceleration in Ablative-Capillary-Discharge Plasma Channel

A high-quality electron beam with a central energy of 0.56 GeV, an energy spread of 1.2% rms, and a divergence of 0.59 mrad rms was produced by means of a 4 cm ablative-capillary-discharge plasma channel driven by a 3.8 J 27 fs laser pulse. This is the first demonstration of electron acceleration with an ablative capillary discharge wherein the capillary is stably operated in vacuum with a simple system triggered by a laser pulse. This result of the generation of a high-quality beam provides the prospects to realize a practical accelerator based on laser-plasma acceleration.

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.

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.

Controlled electron acceleration in the bubble regime by optimizing plasma density

Improvement of the quality of the monoenergetic electron bunch generated in the laser wakefield is investigated. The electrostatic field is more intense near the back of the bubble than at other locations in the bubble. By optimizing the density gradient of background plasma, the local dephasing problem can theoretically be overcome and the electron bunch can be stably accelerated at the back of the bubble so that the accelerated electrons experience nearly the same electric field. Three-dimensional simulations were performed. Compared with the standard wakefield acceleration schemes, a better-quality electron bunch, with narrower energy spread and higher energy, is obtained with a shorter acceleration distance.

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.

Generation of a large amount of energetic electrons in complex-structure bubble

By means of particle-in-cell (PIC) simulations, we found that when the focus size of a laser pulse is much larger than the plasma wavelength and when the laser power is hundreds of times larger than the critical power required for relativistic self-focusing, a large complex bubble is formed. The transversal size of the bubble depends on the laser spot size. Owing to the large bubble size, a bunch of electrons with the total charge in the range of a few tens of nano- Coulombs is trapped and accelerated in the bubble. When the plasma density is 2◊10 19 cm 3 , the charge of the energetic electron bunch with energy above 5MeV exceeds 45nC with a laser spot size of 60µm. Electrons continuously self-injected into such a complex bubble serve as an effective source of high- charge electron bunches.

Particle acceleration by ultraintense laser interactions with beams and plasmas

Recently, there has been great interest growing in ultrahigh field particle acceleration driven by ultraintense laser interactions with beams and plasmas. Although numerous concepts of particle acceleration by laser fields have been proposed almost since the beginning of the laser evolution, there has been tremendous progress in recent years on their theoretical and experimental aspects owing to advances in the generation of ultraintense short laser pulses. The laser–plasma accelerator concepts are reviewed on the laser wakefield acceleration mechanism. In particular, the electron acceleration by the laser wakefield in plasmas is illustrated by our recent experimental results, including the propagation of the ultrashort intense laser pulses in plasmas.

A table-top X-ray FEL based on the laser wakefield accelerator-undulator system

Laser wakefield acceleration makes it possible to build a compact electron linac. While acceleration is attributed to longitudinal wakefields, transverse wakefields simultaneously generated by a short laser pulse can serve as a plasma undulator with a very short wavelength equal to half of the plasma wavelength. We propose a new FEL concept for X-rays based on a laser pulses delivered from a table-top terawatt laser. The system is composed of the accelerator and undulator stages in a table-top size. A low energy electron beam is accelerated due to laser wakefields in the accelerator stage. A bunched electron beam travelling to the opposite direction of driving laser pulses produces a coherent X-ray radiation in the undulator stage. A practical configuration and its analysis are presented.

Femtosecond single-bunched linac for pulse radiolysis based on laser wakefield acceleration

A conceptual design of a linac for pulse radiolysis is presented based on laser wakefield acceleration. Pulse radiolysis spectroscopically studies the initial stage of chemical reactions induced by electron beams. Single-bunched beams with a bunch-length on the order of a femtosecond are ideal for this purpose. The present design gives pure 20 pC single-bunches with an RMS bunch length of less than 10 fs. It accelerates and compresses only the head part of a high-current beam from a photocathode. Some practical problems concerning the design are also presented.

Energy spread minimization in a cascaded laser wakefield accelerator via velocity bunching

We propose a scheme to minimize the energy spread of an electron beam (e-beam) in a cascaded laser wakefield accelerator to the one-thousandth-level by inserting a stage to compress its longitudinal spatial distribution. In this scheme, three-segment plasma stages are designed for electron injection, e-beam length compression, and e-beam acceleration, respectively. The trapped e-beam in the injection stage is transferred to the zero-phase region at the center of one wakefield period in the compression stage where the length of the e-beam can be greatly shortened owing to the velocity bunching. After being seeded into the third stage for acceleration, the e-beam can be accelerated to a much higher energy before its energy chirp is compensated owing to the shortened e-beam length. A one-dimensional theory and two-dimensional particle-in-cell simulations have demonstrated this scheme and an e-beam with 0.2% rms energy spread and low transverse emittance could be generated without loss of charge.