[Show abstract][Hide abstract] ABSTRACT: Acceleration of plasma electrons by laser wake-filed is the most promising process to produce the next generation of compact accelerators because of its ultrahigh acceleration gradient. To achieve the efficient and stable mono-energetic acceleration of plasma electrons, we investigated the laser-plasma interaction and its correlation to electron acceleration through single shot measurements. We observed a thin laser channel when a mono-energetic spectrum was generated.
International Journal of Modern Physics A 01/2012; 22(23). · 1.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electron beam injection triggered by intense ultrashort laser pulses, which is called as plasma cathode, is presented. We have studied generation of relativistic electrons by interaction between a high intensity ultrashort laser pulse and gas jet. When a static magnetic field of 0.2 T is applied, the modification of the preplasma cavity, and significant enhancement of emittance and an increase of the total charge of electron beams produced by a 12 TW, 40 fs laser pulse tightly focused in a He gas jet, were observed. And very high stability and reproducibility of the characteristics and position of well-collimated electron beams was detected. We performed an experiment with a magnetic circuit that has more intense magnetic field of 1 T.
[Show abstract][Hide abstract] ABSTRACT: We have performed measurement of electron bunch duration from laser plasma acceleration. Spectrum analysis of the transition radiation from the electron bunch was selected for measurements, using an IR (infrared) bolometer with different filters first. The bunch is estimated to be 30 fs in PIC simulation, but measured duration was 2 ps by the result of bunch elongation at 340 mm downstream from the plasma edge due to broadness of energy spectrum. And then measurement by In-Sb polychromator aiming shot-by-shot measurement is also done. But the signal of transition radiation cannot be detected by X-ray background signal even if shielding by Pb. So we have done optimisation of X-ray shielding and position of detector.
[Show abstract][Hide abstract] ABSTRACT: Pulse length of quasimonoenergetic electrons accelerated by the wakefield generated by 12 TW, 40 fs laser pulses in a gas jet is determined via spectral measurements using a bolometer to detect coherent transition radiation. A quasimonoenergetic electron beam with its mean energy E=21 MeV, dispersion ΔE=4 MeV, total charge q∼30 pC, and the geometrical emittance 0.07π mm mrad is generated with high reproducibility. The averaged duration of only the quasimonoenergetic electron bunches peaked around 20 MeV is 130±30 fs (FWHM), while it is 250±70 fs (FWHM) for electron bunches with quasimonoenergetic distributions peaked around 4 MeV, at a distance of 180 mm far from the gas jet because of relatively large electron energy spread. Pulse elongation of the electron bunch with the quasimonoenergetic distribution after 180 mm path is 60–220 fs (FWHM). Therefore, the initial duration of the electron bunch at the gas-jet rear is expected to be less than 100 fs (FWHM).
Physical Review Special Topics - Accelerators and Beams 03/2007; 10(3). · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Significant enhancement of emittance and an increase of the total charge of femtosecond electron beams produced by a 12 TW, 40 fs laser pulse, tightly focused in a He gas jet, are observed after applying a static magnetic field, B> or =0.2 T, directed along the axis of laser pulse propagation. The effect appears when plasma produced by a laser prepulse becomes magnetized in the vicinity of the focus point: the electron Larmor frequency exceeds the collisional frequency, while periphery of the plasma remains unmagnetized. The entailed change in the shape of the plasma suppresses the diffraction of the main laser pulse that results in a much higher charge of electrons self-injected during the longitudinal wave breaking of the laser wake as well as the excellent stability of the beams.
[Show abstract][Hide abstract] ABSTRACT: We use a one-shot measurement technique to study effects of laser prepulses on the electron laser wakefield acceleration driven by relativistically intense laser pulses (lambda=790 nm, 11 TW, 37 fs) in dense helium gas jets. A quasimonoenergetic electron bunch with an energy peak approximately 11.5 MeV[DeltaE/E approximately 10% (FWHM)] and with a narrow-cone angle (0.04pi mm mrad) of ejection is detected at a plasma density of 8 x 10(19) cm(-3). A strong correlation between the generation of monoenergetic electrons and optical guiding of the pulse in a thin channel produced by picosecond laser prepulses is observed. This generation mechanism is well corroborated by two-dimensional particle-in-cell simulations.
[Show abstract][Hide abstract] ABSTRACT: Single-shot measurement of plasma parameters is very important in laser wakefield acceleration research aiming to generate compact high-quality femtosecond electron beams. To achieve the efficient and stable monoenergetic acceleration of plasma electrons, effects of laser prepulses, plasma density gradients, wave-breaking processes, laser pulse instability, and energy spectra of accelerated electrons have to be measured simultaneously, granting reproducible laser pulse parameters. A strong correlation between the laser prepulse forming the plasma cavity and electron acceleration has been experimentally confirmed.
Japanese Journal of Applied Physics 04/2006; 45:2757-2761. · 1.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the results of electron generation experiments conducted at the Advanced Photon Research Center, Japan Atomic Energy
Research Institute, using 23-fs relativistically intense 20-TW tightly focused laser pulses with underdense plasma. We observed
electron energies up to 40 MeV characterized by a two-temperature Maxwell distribution. With the help of particle-in-cell
simulations, we found that these are due to different plasma wave-breaking processes. A charge of 5 nC/shot was obtained at
a small solid angle, which corresponds to high peak current generation.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
Physics of Plasmas 03/2006; 13(3):033110-033110-6. · 2.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We are developing two spatial filters for our 12TW–50fs laser system in order to improve a spatial profile of the laser. In our scheme of a laser plasma cathode, electrons are injected into further wake fields by relativistic wave breaking. Previous experiments clearly showed production of well collimated ∼40MeV electrons, which is suggested having 40fs pulse duration by a particle in cell (PIC) calculation. It was found from our recent research that the stability of the electron beam strongly depends on the plasma dynamics. With these spatial filters we expect that we can stabilize the femto-second electron bunch generation and plasma dynamics.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 12/2005; 241(1):901-904. · 1.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Femtosecond electron beams are novel tool for pump-probe analysis of condensed matter. Progress in developing femtosecond electron beams with the use of both conventional accelerators and laser-plasma accelerators is discussed. In conventional accelerators, the critical issue is timing jitter and drift of the linac-laser synchronization system. Sophisticated electronic devices are developed to reduce the jitter to 330fs (rms); the precise control of temperature at several parts of the accelerator lessens the drift to 1ps (rms). We also report on a full-optical X-ray and e-beam system based on the laser-plasma cathode by using a 12 TW 50fs laser, which enables 40MeV (at maximum), 40fs (cal.), 100pC and quasi-monochromatic single electron bunches. Since the synchronization is done by a passive optical beam-splitter, this system intrinsically has no jitter and drift. It could achieve tens of femtoseconds time-resolved analysis in the near future.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 12/2005; 241(1):880-884. · 1.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Physics of Plasmas 10/2004; 11(12):5379-5386. · 2.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Physics of Plasmas 09/2004; 11(10):L57-L60. · 2.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 with I lambda(2)>5 x 10(19) W microm(2)/cm(2) is found via particle-in-cell simulation to be efficient for laser wake-field acceleration. When the matching condition a(0)> or =(2(1/4)omega/omega(pl))(2/3) is met, the self-injection, along with wave breaking, dominates monoenergetic electron acceleration yielding up to 100 MeV energies by a 100 TW, 20 fs laser pulse. In contrast to the injection due to wave-breaking processes, self-injection allows suppression of production of a Maxwell distribution of accelerated particles and the extraction of a beam-quality bunch of energetic electrons.
[Show abstract][Hide abstract] ABSTRACT: 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/4pisr at maximum, which corresponds to the conversion efficiency of laser energy into Kalpha 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.
Japanese Journal of Applied Physics 04/2004; 43:1608-1611. · 1.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We started a project to develop a very compact accelerator for cancer
therapy. To reduce the size of the system, we adopted a laser plasma ion
source using a compact ultra-high intensity laser. We have performed ion
generation experiments in which the laser parameters were as follows:
The wave length and the pulse duration were 800 nm and 50 fs,
respectively. Peak power was 4 - 5TW. The laser pulse with normal
incidence angle to the target was focused onto the target with 15 μm
diameter giving power density of 3 - 4x1018W/cm2.
The thin foil metals (Ti, Al) and plastics (polypropylene, polyethylene)
with the thicknesses of 4 - 100 μm were used for targets. We found
that the angular distribution of ions with an energy of ~0.1 MeV had a
significant peak in the backward and forward in respect to the laser
[Show abstract][Hide abstract] ABSTRACT: We have studied generation of relativistic electrons by interaction
between a high intensity ultra-short laser pulse (Ti:Sapphire, 12 TW, 50
fs, λ=790 nm) and gas jet. In the experiment, spatial and energy
distribution of energetic electrons produced by an ultra-short, intense
laser pulse in a He gas jet are measured. They depend strongly on the
contrast ratio and shape of the laser prepulse. In the case of a proper
prepulse the electrons are injected at the shock front produced by the
prepulse and accelerated by consequent plasma wake-field up to tens MeV
forming a narrow-coned ejection angle. In the case of non-monotonic
prepulse, hydrodynamic instability leads to a broader, spotted spatial
distribution. The numerical analysis based on a 2D hydrodynamics (for
the laser prepulse) and 2D particle-in-cell simulation justify the
mechanism of electron injection and acceleration.