T. Ohkubo

Effects of density gradient on short-bunch injection by wave breaking in the laser wake field acceleration

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.

Wave-breaking injection of electrons to a laser wake field in plasma channels at the strong focusing regime

Efficient and fast self-injection of plasma electrons into the wake-field acceleration phase can be procured during the transverse wake-wave breaking when the wake-wave is generated by the high-intensity laser pulse propagating in a narrow plasma channel. In the strong focusing regime, when the laser pulse power exceeds critical for the self-focusing power threshold, the injected electron bunch length becomes comparable with the plasma wavelength and the bunch has the femtosecond duration. The total charge of self-injected electrons depends strongly on the laser pulse amplitude.

Time-resolved X-ray diffraction at NERL

For ultrafast material analyses, we constructed the time-resolved X-ray diffraction system utilizing ultrashort X-rays from laser-produced plasma generated by the 12-TW-50-fs laser at the Nuclear Engineering Research Laboratory. Ultrafast transient changes in laser-irradiated GaAs crystals were observed as X-ray diffraction patterns. Experimental results were compared with numerical analyses.

Propagation of an intense femtosecond laser pulse through a thin foil filter

In order to improve the contrast of a nanosecond prepulse, the interaction of 12 TW, 50 femtosecond laser pulses with thin foil, as a filter, is investigated. Due to the collisional absorption, the prepulse produces rapidly expanding plasma and can lose 90% of its energy. At a properly chosen thickness of the filter, more than 60% of energy of the femtosecond main pulse is shown to be transmitted through the expanded underdense plasma with reduced prepulse contrast ratio.

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.

Numerical Study of Injection Mechanisms for Generation of Mono-Energetic Femtosecond Electron Bunch From the Plasma Cathode

Acceleration gradients of up to the order of 100GV/m and mono-energetic electron bunch up to 200MeV have recently been observed in several plasma cathode experiments. However, mechanisms of self-injection in plasma are not sufficiently clarified, presently. In this study, we carried out 2D PIC simulation to reveal the mechanisms of Laser Wake Field Acceleration (LWFA). Electron density gradient at vacuum-plasma interface is important condition for electron’s self-injection owing to plasma wave breaking. Steep electron density gradient (∼ plasma wave length) causes rapid injection and produces an electron bunch with rather high charge and less than 100fs duration. By this calculation, we obtained the electron bunch width of about 40fs and the maximum energy achieved more than 20MeV. The charge amount of > 3MeV electrons in one bunch was estimated as 50pC with the steep density gradient, while it was 10pC with the gentle one.

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.