Daisuke Wakai

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.

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.

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.

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.

Radioactivity produced by a repetitive laser-driven proton beam using a shot-to-shot proton spectral measurement and a direct activation method

A proton beam driven by a repetitive high-intensity-laser is utilized to induce a 7Li(p,n) 7Be nuclear reaction. The total activity of 7Be are evaluated by two different methods. The activity obtained measuring the decay gamma-rays after 1912 shots at 1 Hz is 1.7 ± 0.2 Bq. This is in good agreement with 1.6 ± 0.2 Bq evaluated from the proton energy distribution measured using a time-offlight detector and the nuclear reaction cross-sections. We conclude that the production of activity can be monitored in real time using the time-of-flight-detector placed inside a diverging proton beam coupled with a high-speed signal processing system.

Demonstration of high peak power, high contrast OPCPA/Ti:sapphire hybrid laser system

We have demonstrated over 30 J broadband output energy based on optical parametric chirped-pulse amplification (OPCPA) and Ti:sapphire chirped-pulse amplification (CPA), indicating potential for peak power of 500 TW with extremely high temporal contrast.

Review on Recent High Intensity Physics Experiments Relevant to X-Ray and Quantum Beam Generation at JAEA

The authors describe firstly the lasers for high intensity physics experiments at JAEA including J-KAREN and JLITE-X lasers which can deliver 00TW and 0TW laser power, respectively. Secondly the authors describe demonstration of flying mirror technique which will become a new technique to make a coherent tunable x-ray source. Thirdly a femto-second laser driven incoherent soft x-ray source and its imaging applications for nano-structures are described. The th topic includes femto-second laser driven multiple quantum beam generation and its applications using a laser driven thin foil target.

Generation of above 1010 Temporal Contrast, above 1020 W/cm2 Peak Intensity Pulses at a 10 Hz Repetition Rate using an OPCPA Preamplifier in a Double CPA, Ti:sapphire Laser System

We demonstrate the generation of greater than 1010 temporal contrast with over 1020 W/cm2 peak intensity pulses at a 10 Hz repetition rate from a Ti:sapphire chirped‐pulse amplification (CPA) laser system utilizing double CPA, optical parametric chirped‐pulse amplification (OPCPA), and cryogenic‐cooling techniques. We also successfully produce uncompressed output pulse energy exceeding 30 J, indicating potential for peak power at the 500 TW level.