Hajime Sasao

High temporal and spatial quality petawatt-class Ti:sapphire chirped-pulse amplification laser system

Optical parametric chirped-pulse amplification (OPCPA) operation with low gain by seeding with high-energy, clean pulses is shown to significantly improve the contrast to better than 10(-10) to 10(-11) in a high-intensity Ti:sapphire laser system that is based on chirped-pulse amplification. In addition to the high-contrast broadband, high-energy output from the final amplifier is achieved with a flat-topped spatial profile of filling factor near 77%. This is the result of pump beam spatial profile homogenization with diffractive optical elements. Final pulse energies exceed 30 J, indicating capability for reaching peak powers in excess of 500 TW.

Application of laser-accelerated protons to the demonstration of DNA double-strand breaks in human cancer cells

We report the demonstrated irradiation effect of laser-accelerated protons on human cancer cells. In vitro (living) A549 cells are irradiated with quasimonoenergetic proton bunches of 0.8–2.4 MeV with a single bunch duration of 15 ns. Irradiation with the proton dose of 20 Gy results in a distinct formation of γ-H2AX foci as an indicator of DNA double-strand breaks generated in the cancer cells. This is a pioneering result that points to future investigations of the radiobiological effects of laser-driven ion beams. Unique high-current and short-bunch features make laser-driven proton bunches an excitation source for time-resolved determination of radical yields.

Proton acceleration to 40 MeV using a high intensity, high contrast optical parametric chirped-pulse amplification/Ti:sapphire hybrid laser system.

Using a high-contrast (10(10):1) and high-intensity (10(21) W/cm(2)) laser pulse with the duration of 40 fs from an optical parametric chirped-pulse amplification/Ti:sapphire laser, a 40 MeV proton bunch is obtained, which is a record for laser pulse with energy less than 10 J. The efficiency for generation of protons with kinetic energy above 15 MeV is 0.1%.

Focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II). The challenge of observation over length scales from an ångström to a micrometre

SANS-J (a pinhole small-angle neutron scattering spectrometer at research reactor JRR3, Tokai, Japan) was reconstructed as a focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II). By employing focusing lenses of a biconcave MgF2 crystal or of a sextupole permanent magnet and a high-resolution photomultiplier, the minimum accessible magnitude of the scattering vector qmin was improved from 3 × 10−3 A−1 to an ultra-small-angle scattering (USAS) of 3 × 10−4 A−1. Compared with a Bonse–Hart double-crystal method, the advantages of focusing USAS are the efficient detection of anisotropic USAS with an area detector, an improvement in q resolution Δq/q at conventional magnitudes of the scattering vector q ~ 10−3 A−1 and a gain in neutron flux in the conventional q region of q ~ 10−3 A−1.

Focusing and spectral enhancement of a repetition-rated, laser-driven, divergent multi-MeV proton beam using permanent quadrupole magnets

A pair of conventional permanent magnet quadrupoles is used to focus a 2.4 MeV laser-driven proton beam at a 1 Hz repetition rate. The magnetic field strengths are 55 and 60 T/m for the first and second quadrupoles, respectively. The proton beam is focused to a spot with a size of less than ∼3×8 mm2 at a distance of 650 mm from the source. This result is in good agreement with the Monte Carlo particle trajectory simulation.

Temporal contrast enhancement of petawatt-class laser pulses

We demonstrate the temporal contrast enhancement in a petawatt-class Ti:sapphire chirped-pulse amplification (CPA) laser system. An extra saturable absorber, introduced downstream after a low-gain optical parametric chirped-pulse amplification (OPCPA) preamplifier, has improved the temporal contrast in the system to 1.4×10(12) on the subnanosecond time scale at 70 TW power level. We have achieved 28 J of uncompressed broadband output energy with this system, indicating the potential for reaching peak powers near 600 TW.

High-spatiotemporal-quality petawatt-class laser system

We have developed a femtosecond high-intensity laser system that combines both Ti:sapphire chirped-pulse amplification (CPA) and optical parametric CPA (OPCPA) techniques and produces more than 30 J broadband output energy, indicating the potential for achieving peak powers in excess of 500 TW. With a cleaned high-energy seeded OPCPA preamplifier as a front end in the system, for the compressed pulse without pumping the final amplifier, we found that the temporal contrast in this system exceeds 10(10) on the subnanosecond time scales, and is near 10(12) on the nanosecond time scale prior to the peak of the main femtosecond pulse. Using diffractive optical elements for beam homogenization of a 100 J level high-energy Nd:glass green pump laser in a Ti:sapphire final amplifier, we have successfully generated broadband high-energy output with a near-perfect top-hat-like intensity distribution.

Wide-q observation from 10−4 to 2.0 Å−1 using a focusing and polarized neutron small-angle scattering spectrometer, SANS-J-II

In order to extend an upper q-limit [q is the magnitude of the scattering vector q, defined by q = (4π/λ)sinθ, where λ and 2θ are the wavelength and the scattering angle, respectively], high-angle 3He sub-detectors were installed on a focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II) at JRR-3, Tokai, Japan. Consequently, the upper q-limit was improved from 0.2 to 2.0 A−1. To quantitatively discriminate spin incoherent scattering from hydrogen or to perform nuclear spin polarization contrast variation, a remanent supermirror analyser is also available on the high-angle sub-detector. By combining a focusing ultra-small-angle scattering, realised by compound and/or magnetic lens and high-resolution area detector, SANS-J-II is able to cover from 3 × 10−4 to 2.0 A−1 (four orders of magnitude of q), which benefits investigation of hierarchically ordered systems, found widely in hard, soft and bio-materials.

Complementary Characterization of Radioactivity Produced by Repetitive Laser-Driven Proton Beam Using Shot-to-Shot Proton Spectral Measurement and Direct Activation Measurement

The precision of the time-of-flight method for measuring proton spectra is confirmed by the activation method using a 7Li(p,n)7Be nuclear reaction. The proton beam driven by a repetitive high-intensity laser is utilized to induce the nuclear reaction. The activity measured by direct gamma-ray measurement after 1912 proton beam shots at 1 Hz is 1.7±0.2 Bq. This is in accordance with the activity of 1.6±0.2 Bq obtained from the shot-to-shot proton energy distribution measured using the time-of-flight detector.

Toward laser driven proton medical accelerator

Towards our final goal, such as to establish the laser-driven proton accelerator for the medical application, one of the most important things to establish is to develop the proton transport system. In this continuous work, we demonstrate the focusing system of the laser-driven proton beam with permanent magnet qadrupoles (PMQs), with which a 2.4 MeV laser-driven proton beam, having a divergence angle of ~10 degrees at birth, is focused to a spot whose size is 3×8mm2 at 640mm downstream from the target.