Yasuaki Okano

Dependence on laser intensity and pulse duration in proton acceleration by irradiation of ultrashort laser pulses on a Cu foil target

The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5μm thick copper tape target was measured. The laser intensity was varied from 8.5×1017W∕cm2 to 1.1×1019W∕cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.

Thin tape target driver for laser ion accelerator

A thin tape target driver for laser ion acceleration was developed. The driver can move a copper tape of 5 μm thickness with a positioning reproducibility of less than 30 μm (peak to valley), which is sufficient for a laser irradiation target. Using this tape target and laser pulses of energy 350 mJ and duration 60 fs, protons of energies of over 1 MeV were accelerated in the forward direction.

Production of relativistic electrons by irradiation of 43-fs-laser pulses on copper film

The energy spectra of fast electrons produced by ultrashort, high-intensity laser pulses were directly measured using a magnetic spectrometer with an imaging plate. The typical temperature was 350 keV for irradiation on 30 μm thick copper film by pulses of width 43 fs, intensity 2.7×1018 W/cm2, repetition rate 10 Hz without artificial prepulses and was found to be close to the ponderomotive potential. In addition, the energy spectra of high-energy photons, which are expected to be produced from the electrons, were calculated.

MeV-order proton and carbon ion acceleration by irradiation of 60 fs TW laser pulses on thin copper tape

We obtained high-energy ions by irradiating 60 fs laser pulses onto a copper tape target of 5 μm thickness. The proton was accelerated with energy of 1.2 MeV at laser intensity of 6.8×1018 W/cm2. The equivalent Boltzmann temperature of the proton was 185 keV which was less than the ponderomotive potential (650 keV) of an electromagnetic standing wave in a laser field. Moreover, we observed the acceleration of carbon ions with energy of more than 0.4 MeV at laser intensity of 3.4×1018 W/cm2.

Observation of femtosecond-laser-induced ablation plumes of aluminum using space- and time-resolved soft x-ray absorption spectroscopy

The dynamics of the laser ablation plume expansion of aluminum was investigated by using space- and time-resolved soft x-ray absorption spectroscopy. Blueshifts of the Al L-shell photoabsorption edge indicating the state of aluminum were observed in the plumes, which were generated by irradiating an aluminum target with 120fs near-infrared pulses at an intensity of 1014W∕cm2. The spatiotemporal evolution of the plumes exhibited a multilayer structure consisting of vaporized aluminum and condensed aluminum particles, following the expansion of plasma, with expansion velocities of 104m∕s for the atomic state and 103m∕s for the condensed state.

γ-H2AX and Phosphorylated ATM Focus Formation in Cancer Cells after Laser Plasma X Irradiation

Abstract The usefulness of laser plasma X-ray pulses for medical and radiation biological studies was investigated, and the effects of laser plasma X rays were compared with those of conventional sources such as a linear accelerator. A cell irradiation system was developed that used copper-Kα (8 keV) lines from an ultrashort high-intensity laser to produce plasma. The absorbed dose of the 8 keV laser plasma X-ray pulse was estimated accurately with Gafchromic® EBT film. When the cells were irradiated with approximately 2 Gy of laser plasma X rays, the circular regions on γ-H2AX-positive cells could be clearly identified. Moreover, the numbers of γ-H2AX and phosphorylated ataxia telangiectasia mutated (ATM) foci induced by 8 keV laser plasma X rays were comparable to those induced by 4 MV X rays. These results indicate that the laser plasma X ray source may be useful for radiation biology studies.

Enhanced generation of fast protons from a polymer-coated metal foil by a femtosecond intense laser field

The results of generation of fast protons from 5-μm-thick copper foil targets by 60fs laser irradiation at 1.5×1017W∕cm2 are presented. Both polyvinylmethylether (PVME)-coated and uncoated copper foil targets are examined. Fast protons are measured using a Thomson mass spectrometer and maximum proton energies are 570 and 280keV for the PVME-coated and the uncoated target, respectively. The intensity of fast protons with energy of 160keV from the PVME-coated target is approximately 80-fold higher than that from the uncoated target.

Note: Application of laser produced plasma Kα x-ray probe in radiation biology

A dedicated radiation biology x-ray generation and exposure system has been developed. 8.0 keV in energy x-ray pulses generated with a femtosecond-laser pulse was used to irradiate sample cells through a custom-made culture dish with a silicon nitride membrane. The x-ray irradiation resulted in DNA double-strand breaks in the nucleus of a culture cell that were similar to those obtained with a conventional x-ray source, thus demonstrating the feasibility of radiobiological studies utilizing a single burst of x-rays focused on single cell specimens.

Time-resolved electron shadowgraphy for 300 ps laser ablation of a copper film

Time-resolved electron shadowgraphy measurement was performed in infrared 300 ps laser ablation of a copper film in order to investigate expanding plasma into vacuum with space-charge separation. The probe-electron pulse was generated by intense femtosecond laser irradiation on a tungsten bulk target. Time-resolved electron shadowgraphs showed evolving “bright” and “shadow” plumes with expanding speeds of 970 and 110 km/s, when the laser intensity was 980 G W/cm2. These are attributed to be space-charge separation field and ion plume, respectively.

Transient observation of extended x-ray absorption fine structure in laser-melted Si by using femtosecond laser-produced-plasma soft x ray

We have demonstrated the time-resolved measurement of the extended x-ray absorption fine structure (EXAFS) in laser-melted Si foil by using a pump-probe absorption spectroscopy system that utilizes a femtosecond laser-produced-plasma soft x-ray source. By using 100-fs laser irradiation, we observed the transient change in the Si L-edge EXAFS, that is, a slight shortening of its oscillation period and a decrease in its oscillation amplitude. This result suggests that the Si-Si atomic distance expanded and structural disordering occurred as a result of the production of liquid Si.