Takuya Nayuki

Laser prepulse dependency of proton-energy distributions in ultraintense laser-foil interactions with an online time-of-flight technique

Fast protons are observed by a newly developed online time-of-flight spectrometer, which provides shot-to-shot proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of ∼1018W∕cm2 onto a 5-μm-thick copper foil. The maximum proton energy is found to increase when the intensity of a fs prepulse arriving 9ns before the main pulse increases from 1014 to 1015W∕cm2. Interferometric measurement indicates that the preformed-plasma expansion at the front surface is smaller than 15μm, which corresponds to the spatial resolution of the diagnostics. This sharp gradient of the plasma has the beneficial effect of increasing the absorption efficiency of the main-pulse energy, resulting in the increase in the proton energy. This is supported by the result that the x-ray intensity from the laser plasma clearly increases with the prepulse intensity.

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.

Quasi-monoenergetic electron beam generation during laser pulse interaction with very low density plasmas

The results of experiments are presented for the single laser pulse interaction with a very low density gas target, under the conditions when the generated wake wave is below the wave-breaking threshold and the laser pulse power is lower than the critical power for relativistic self-focusing. A quasi-monoenergetic electron beam is found to be stably generated for various laser pulse intensity values by controlling the acceleration length. The results of two-dimensional particle-in-cell simulations show that for the electron acceleration an additional mechanism of electron injection into the acceleration phase is required. It is demonstrated that the longitudinal inhomogeneity of the plasma density leads to the electron injection.

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.

Measurements of energy and angular distribution of hot electrons and protons emitted from a p- and s-polarized intense femtosecond laser pulse driven thin foil target

The energy spectra and angular distributions of hot electrons as well as protons emitted from a 3-μm-thick tantalum foil irradiated by a 70-fs laser pulse with an intensity of ∼1018W∕cm2 are measured. Three hot electron flows are found, in the rear target normal, specular, and target surface directions. The angular distribution of hot electrons is found to depend on the polarization of the incident light. The measured energy spectrum of hot electrons in the rear target normal direction can explain the generated proton beam.

Laser polarization dependence of proton emission from a thin foil target irradiated by a 70fs, intense laser pulse

A study of proton emission from a 3‐μm-thick Ta foil target irradiated by p-, s-, and circularly polarized laser pulses with respect to the target plane has been carried out. Protons with energies up to 880keV were observed in the target normal direction under the irradiation by the p-polarized laser pulse, which yielded the highest efficiency for proton emission. In contrast, s- and circularly polarized laser pulses gave the maximum energies of 610 and 680keV, respectively. The difference in the maximum energy between the p- and s-polarized cases was associated with the difference between the sheath fields estimated from electron spectra.

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.

TRANSFORMATION OF A LASER BEAM INTENSITY PROFILE BY A DEFORMABLE MIRROR

We developed a deformable mirror with nine actuators for transformation of a laser beam intensity profile. A circular-cross-section Gaussian beam was successfully transformed into a rectangular-cross-section beam by the deformable mirror with conservation of spatial coherency.

Lidar measurement of constituents of microparticles in air by laser-induced breakdown spectroscopy using femtosecond terawatt laser pulses

We experimentally demonstrated remote sensing of the constituents of microparticles in air by combining laser-induced breakdown spectroscopy (LIBS) and lidar, using femtosecond terawatt laser pulses. Laser pulses of 70 fs duration and 130 mJ energy generated filaments when focused at a focal length of 20 m and the pulses irradiated artificial saltwater aerosols in air at a 10 Hz pulse repetition rate. Na fluorescence was observed remotely at a distance of 16 m using a 318 mm diameter Newtonian telescope, a spectrometer, and an intensified CCD camera. These results show the possibility of remote measurement of the constituents of atmospheric particles, such as aerosols, clouds, and toxic materials, by LIBS-lidar using femtosecond terawatt laser pulses.