K. Kinoshita

Effects of plasma density on relativistic self-injection for electron laser wake-field acceleration

Density effects on the dynamics of a cavity produced in the wake of an ultraintense (a0=eE/mcω≫1) and short (ωplτ/π<1) laser pulse and on the duration of accelerated electrons are studied via two-dimensional particle-in-cell simulation. Formation of a nonbreaking cavity is a crucial part of relativistic self-injection of plasma electrons from the front of a laser pulse and their further acceleration leading to a beam-quality femtosecond bunch. This self-injection appears in a uniform plasma when the group velocity of the pulse becomes smaller than the maximal electron velocity accelerated in the ponderomotive bias, Φ=mc2a02/2. However with increasing density, this mechanism starts to contend with relativistic wave breaking. Though additional injection due to the relativistic wave breaking increases the total charge of energetic electrons, the duration of the bunch increases to the picosecond range and its energy distribution becomes a Maxwellian.

Development of a laser-driven plasma cathode for medical applications

In this article, the present status of radiation therapy in Japan and updated medical accelerators are reviewed. In addition, the potential of laser plasma acceleration as a future medical accelerator is discussed. The updated results of laser plasma cathode experiment by the University of Tokyo are described.

CTR Bunch Length Measurement of Monoenergetic and Maxwellian Electron Beams from Laser Plasma Cathode

Recently, several plasma cathodes at universities and institutes can generate a monoenergetic electron beam. LBL measured the bunch length by the CTR (Coherent Transition Radiation) interferometer to determine it as about 50 fs (FWHM). We are trying to carry out a single‐shot measurement of the bunch length by a infrared polychromator. As the first step forward it, we measured the CTR spectra emitted at a 300microm‐thick Ti foil by the electron beams from our plasma cathode. The laser parameters are 12 TW, 50fs, 3×1019W/cm2. He gas‐jet is used and the electron density is 6×1019cm−3. By using the liquid He bolometer and several optical low‐pass filter, we measured the spectra of the CTR. We can clearly classify the difference of the spectrum and bunch length for the monoenergetic and Maxwellian beams. The numerical analysis of the bunch elongation due to the energy spread confirms the experimental results. The bunch length of the monoenergetic beam is less than 100 fs at FWHM with about 10 pC per bunch, wh...

Generation of femtosecond electron bunches and hard-X-rays by ultra-intense laser wake field acceleration in a gas jet

Summary form only given. Femtosecond electron beams and hard X-rays with /spl lambda//spl sim/0.1 nm may find various applications in biology, chemistry, and molecular electronics giving a new time-scale probe analysis. Such short electron beams can be produced in the wake field acceleration by short relativistically intense laser pulses and then Thomson scattering of a second laser pulse can serve for efficient generation of very short X-rays with use of such electron beams. We study experimentally with 12 TW, 50 fs Ti-sapphire laser set-up and numerically through a multidimensional particle-in-cell simulation two mechanisms of generation of femtosecond electron bunches in gas jet suitable for efficient Thomson scattering. The first is the LWFA of electrons injected due to wave-breaking on a shock-wave produced by a laser prepulse in a He gas-jet. This mechanism allows us to produce a narrow-coned electron bunch with duration around 40 fs. Results of measurements agree well with two-dimensional hydrodynamics and particle-in-cell simulations. Such a beam can scatter up to 10/sup 9/ photons per pulse in 1/spl deg/ cone. Spectrum of scattered light is discussed. Because of large energy spread of electrons in a bunch, the X-ray spectrum is broad. To overcome this problem another mechanism, self-injection of plasma electrons, is proposed and studied numerically. The self-injection of plasma electrons which have been accelerated to relativistic energies by a laser pulse moving with a group velocity less than the speed of light appears when a/sub 0//spl ges//spl radic/2(/spl omega///spl omega//sub pl/)/sup 2/3/ where a/sub 0/ is normalized laser field. In contrast to the injection due to wave-breaking processes, self-injection allows extraction of a beam-quality bunch of energetic electrons. This injection is also expected to be useful in generation of very short pulse, /spl sim/10 fs, electron beams with the charge /spl sim/100 pC. The diameter of such a beam in a gas jet after acceleration is only 5-10 /spl mu/m that makes possible the production of high-brightness hard-X-rays with few percent energy spread by using contrary propagating laser pulses. The efficiency and spectrum of such X-rays are calculated and discussed. We also study density effects on the dynamics of a cavity produced in the wake of an ultra-intense (a/sub 0//spl Gt/1) femtosecond laser pulse via 2D particle-in-cell simulation. Formation of a non-breaking cavity is a crucial part of relativistic self-injection of plasma electrons and their further acceleration leading to a beam-quality femtosecond bunch. This self-injection appears in a uniform plasma when the group velocity of the pulse becomes smaller than the maximal electron energy mc/sup 2/a/sub 0//sup 2//2 so that a/sub 0/>(2/sup 1/4//spl omega///spl omega//sub pl/)/sup 2/3/. With increasing density, the total charge of the bunch increases, however, the energy distribution becomes Maxwellian-like; the bunch lengthens due to the effect of the wave-breaking injection and mutual effects on the cavity.

Femtosecond hard-x-rays via Thomson-scattering on electrons produced by ultra-intense laser wake field acceleration

Summary form only given, as follows. In the present work, we study experimentally with 12 TW, 50 fs Ti-sapphire laser set-up and numerically through a multidimensional particle-in-cell simulation two mechanisms of generation of femtosecond electron bunches in gas jet suitable for efficient Thomson scattering. The first is the LWFA of electrons injected due to wave-breaking on a shock-wave produced by a laser prepulse in a He gas-jet. This mechanism allows us to produce a narrow-coned electron bunch with duration around 30 fs. Results of measurements agree well with two-dimensional hydrodynamics and particle-in-cell simulations.

Application of Laser Plasma X-rays to Time-resolved Debye-Sherrer Diffraction

We have studied Laser Plasma X‐ray(LPX) in order to apply to time‐resolved protein crystallography. We consider that our works will contribute for application of X‐ray pulse and breed short pulse handling techniques. LPX pulse duration is femto∼pico‐second. So, we expect that the laser plasma X‐ray system has the potential to satisfy the requirement of time‐scale to resolve the early period of protein’s structural change. We need about more than 1012photns/shot X‐ray to get a diffraction image of organics. We have reinforced our LPX system to get a diffraction image. Now, we try laser pre‐pulse effect by experiments and calculations. As the first step of our aim, we will obtain the Debey‐Sherrer diffraction image of a biological sample.

Fundamental Study for Time‐Resolved Imaging by Laser Plasma X‐rays

Laser plasma X‐ray, generated from solid targets irradiated by an intense short laser pulse, is an ultra‐short pulse with the time‐duration of about 10ps and enables time‐resolved measurements. For dynamic imaging using this laboratory‐scale source at Nuclear Engineering Research Laboratory (NERL), University of Tokyo, we have to increase the X‐ray intensity at least ten times more than present 3 ∼104 photons/cm2/shot at a sample. We carried out simulations of the interaction of a laser pulse with a solid target, which show that the number of hot electrons (>8keV for CuKα emission) become larger by decreasing the intensity of laser Amplified Spontaneous Emission. We have a plan to take time‐resolved images of laser ablation of solids with the time resolution of 10ps.

Relativistic self-injection effects on ultra-intense laser wake field acceleration

Summary form only given, as follows. In the present work, we present a new mechanism of electron injection originating from the relativistic character of the laser-plasma interaction. The self-injection of plasma electrons which have been accelerated to relativistic energies by a laser pulse moving with a group velocity less than the speed of light is found via particle-in-cell simulation to be efficient for laser wake field acceleration.

Intense laser pulse driven ion acceleration in expanding foil plasmas

Summary form only given, as follows. In the present work, we investigate numerically various mechanisms of energetic ion emission from slab targets irradiated by a picosecond pulse laser, as well as very intense subpicosecond pulse lasers and effect of contamination on these mechanisms. We use the particle-in-cell simulation employing the average ion model to include transient plasma ionization and Langevin equation to calculate elastic collisions. Results of recent experiments with foil plasmas irradiated by the 12 TW, 60 fs laser facility of NERL are discussed. The new mechanism of ion acceleration in a solitary wave is proved to be the most efficient providing ion energy E -(3-10) MeV/nucleon at laser intensity I/spl sim/10/sup 19/ W/cm/sup 2/ and useful as a high-current ion source for conventional accelerators for radiology.