Akira Nagashima

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.

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.

Loss Mechanism of Energetic Ions Produced during the Lower Hybrid Heating of JFT-2 Tokamak

The energetic ion (\({\gtrsim}3\) keV) produced by the lower hybrid waves, decays in 200–400 µsec after RF turn-off. The loss mechanism responsible for this fast decay is indicated as scattering into the loss cone and the toroidal ripple loss cone. The fast ions are considered to be present in \(0.6{\lesssim}r/a{\lesssim}0.9\).

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.

Faraday Rotation and Elliptization of Electron Cyclotron Emission on Reactor-Grade Tokamak Plasma

The effects of the polarization state evolution on two-dimensional electron cyclotron emission (ECE) measurements along horizontal lines of sight for a reactor-grade plasma are studied. In order to investigate the polarization evolution, the propagation of ECE through the plasma is calculated. The final polarization states are very different from the initial states. The exact electron temperature is obtained in a reactor-grade plasma by measuring the polarization states described by the Stokes parameters.

A 2-mm Wave Digital Interferometer for Tokamak Discharges in the Upgraded DIVA

A 2-mm wave digital interferometer without source frequency modulation was fabricated to measure the central electron density of tokamak discharges in an upgraded DIVA device. Two methods of processing the interferometic fringe signals were presented: one by using an up/down counter and the other by a computer to obtain direct graphic displays of the complicated time-evolutions of the average electron density. These methods were successfully put into operation with sufficient resolutions of a 1/2 fringe by the counter and a 1/40 fringe by the computer, and the utility of the digital interferometer was fairly evident in practical measurements. The central average density was found to reach 4.2×1013 cm-3 in the upgraded DIVA plasmas by using the present digital interferometer.

X‐ray Lasers Driven by Optical Lasers

Recent topics in the optical‐laser‐driven x‐ray lasers are reviewed. With the collisional excitation x‐ray lasers, pumping laser energy has been reduced over 100 times by reducing the pumping pulse width from ns to ps. A high gain of 30–40 cm−1 has been achieved with the transient collisional excitation using ps pumping. A new scheme for a charge exchange x‐ray laser using clusters such as C60 irradiated with ultra short laser pulses has been introduced. In the inner shell ionization x‐ray lasers, creation of a population inversion utilizing the difference in the Coster‐Kronig decay rates between the upper and lower laser levels has been proposed as a scheme to enable pumping with short bursts of electrons. High gain over long duration is expected with hollow atoms created under intense broad‐band x‐ray irradiation emitted from oscillating electrons under relativistic intensities (Larmor radiation). The x‐ray laser program at the Advanced Photon Research Center at the new site in Kyoto is presented.

Development of Laser‐Driven Ion Source

We have studied the ion emission from a thin‐foil target irradiated by p, s and circularly polarized laser pulses in order to clarify the mechanism of ion acceleration using a Titanium Sapphire laser with a peak irradiance of 3 × 1018 W/cm2. A tape target driver provided a fresh surface of a tantalum foil with the thickness of 3 micro‐meters during the measurements. The laser polarization was changed from p to s‐ and to circular polarization by using the 1/2 and 1/4 wave plates made of mica. To obtain the ion energy spectra and to resolve the ion species, a Thomson parabola ion analyzer was placed behind the target along the target normal direction. The energy spectra close to 1MeV were obtained. Possible explanation of the experimental result is described. Finally, we describe the phase rotation technique to obtain a monochromatic proton beam.

Laser-driven proton sources and their applications: femtosecond intense laser plasma driven simultaneous proton and x-ray imaging

We have performed simultaneous proton and X-ray imaging with an ultra-short and high-intensity Ti: Sap laser system. More than 1010 protons, whose maximum energy reaches 2.5 MeV, were delivered within a ~ps bunch. At the same time, keV X-ray is generated at almost the same place where protons are emitted. We have performed the simultaneous imaging of the copper mesh by using proton and x-ray beams, in practical use of the characteristics of the laser produced plasma that it can provide those beams simultaneously without any serious problems on synchronization.

Development of laser driven proton sources and their applications

We are developing a proton accelerator using an intense lasers with a focused intensity of >1017 W/cm2. To monitor proton energy spectra as well as plasma parameters at each laser shot, we are using real time detectors. The proton energy of ~MeV is stably obtained for applications.