Shu Nakamura

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.

Real-Time Optimization of Proton Production by Intense Short-Pulse Laser with Time-of-Flight Measurement

A scheme of the real-time optimization of proton production by an intense short-pulse laser interacting with a foil target was developed using a time-of-flight measurement with a plastic scintillator. Owing to special treatments, the detection of protons using a scintillation counter has become possible under heavy backgrounds such as laser light itself, laser-generated hard X-ray, self-emission light, and electrons from the laser-produced plasma. With such a real-time measurement of protons, the energy spectrum of protons could be obtained shot by shot, and the experimental conditions for optimal proton production could be determined very efficiently.

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.

Phase rotation scheme of laser-produced ions for reduction of the energy spread

In order to widely spread out particle beams utilized in cancer therapy, laser-produced ions are developed as the injection beam for an ion synchrotron dedicated for cancer therapy. Such a laser ion source is expected to contribute largely to the realization of compactness of the size and low cost of the cancer therapy accelerator. The energy spectrum of the laser-produced ions, however, has no peak, but their intensities decrease exponentially according to the increase of the energy. For the purpose of modifying such a situation, we have proposed a scheme to rotate the beam in the longitudinal phase space with the use of the RF electric field, which is phase-adjusted with the pulse laser. We aim for a reduction of the energy spread of ± 5% selected by an energy analyzer and slits to ±1% by such phase rotation. For this purpose, a quarter wavelength resonator with two gaps with the same resonant frequency as the source laser has already been fabricated, together with its RF power source. The above phase rotation system and its recent experimental realization are presented.

High-Quality Laser-Produced Proton Beam Realized by the Application of a Synchronous RF Electric Field

A short-pulse (~210 fs) high-power (~1 TW) laser was focused on a tape target 3 and 5 µm in thickness to a size of 11×15 µm2 with an intensity of 3×1017 W/cm2. Protons produced by this laser with an energy spread of 100% were found to be improved to create peaks in the energy distribution with a spread of ~7% by the application of the RF electric field with an amplitude of ±40 kV synchronous to the pulsed laser. This scheme combines the conventional RF acceleration technique with laser-produced protons for the first time. It is possible to be operated up to 10 Hz, and is found to have good reproducibility for every laser shot with the capability of adjusting the peak positions by control of the relative phase between the pulsed laser and the RF electric field.

Simultaneous Proton and X-ray Imaging with Femtosecond Intense Laser Driven Plasma Source

A laser-driven proton beam with a maximum energy of a few MeV is stably obtained using an ultra-short and high-intensity Ti:sapphire laser. At the same time, keV X-ray is also generated at almost the same place where protons are emitted. Here, we show the successful demonstration of simultaneous proton and X-ray projection images of a test sample placed close to the source with a resolution of ~10 µm, which is determined from the source sizes. Although the experimental configuration is very simple, the simultaneity is better than a few hundreds of ps. A CR-39 track detector and imaging plate, which are placed as close as possible to the CR-39, are used as detectors of protons and X-ray. The technique is applicable to the precise observation of microstructures.

Electron Energy Spectrometer for Laser-Driven Energetic Electron Generation

An electron energy spectrometer for studying the energy spectrum of electrons emitted from solid foils irradiated by a femtosecond intense laser pulse and its calibration with a β source is described. The intensity distribution of the magnetic field induced by a dipole magnet and the time decay of the photostimulated luminescence of an imaging plate were determined. Both the energy scale and the electron intensity conversion ratio of the electron energy spectrometer were calibrated with a 90Sr–90Y β source. The energy spectrum of hot electrons emitted from a 3-µm-thick Tantalum foil target irradiated by laser pulses with a pulse duration of 50 fs and a peak intensity of 2×1018 W/cm2 was determined with the calibrated electron energy spectrometer.

Observation of strongly collimated proton beam from Tantalum targets irradiated with circular polarized laser pulses

High-energy protons are generated by focusing an ultrashort pulsed high intensity laser at the Advanced Photon Research Center, JAERI-Kansai onto thin (thickness <10 μm) Tantalum targets. The laser intensities are about 4 × 10 18 W/cm 2 . The prepulse level of the laser pulse is measured with combination of a PIN photo diode and a cross correlator and is less than 10 −6 . A quarter-wave plate is installed into the laser beam line to create circularly polarized pulses. Collimated high energy protons are observed with CH coated Tantalum targets irradiated with the circularly polarized laser pulses. The beam divergence of the generated proton beam is measured with a CR-39 track detector and is about 6 mrad.

New Method to Measure the Rise Time of a Fast Pulse Slicer for Laser Ion Acceleration Research

A dependence of cutoff proton kinetic energy on laser prepulse duration has been observed. Amplified spontaneous emission pedestal duration is controlled by a fast electrooptic pulse slicer where the rise time is estimated to be 130 ps. We demonstrate a new correlated spectral technique for determining this rise time using a stretched frequency-chirped pulse.

HIGH QUALITY LASER-PRODUCED PROTON BEAM GENERATION BY PHASE ROTATION

Laser ion production has been studied for downsizing of the accelerator dedicated for cancer therapy. For optimization of various parameters such as pre-pulse condition, target position, laser spot size on the target, laser pulse width and so on, time of flight (TOF) measurement utilizing the detected signal by a plastic scintillation counter played an essential role for real time measurement. Protons up to ~900 keV and ~600 keV are produced from the thin foil targets of Ti 3 μm and 5 μm in thickness, respectively. Modification of the energy distribution of the laser-produced ions with Maxwell distribution by utilizing an RF electric field synchronized to the pulse laser, which is the rotation of the ion beam in the longitudinal phase space (Phase Rotation), has been demonstrated for the first time. By using the Ti Sapphire laser of the wave length and pulse duration of 800 nm and a few hundreds fs, respectively, intensity increase of factor 3 in a certain energy bins is attained creating energy peaks with the energy spread about 7 %, which is found to be reproduced with good probability.