N. Nakanii

Transport study of intense-laser-produced fast electrons in solid targets with a preplasma created by a long pulse laser

The effect of preplasma on fast electron generation and transport has been studied using an intense-laser pulse (I=2×1018 W/cm2) at the Osaka University. An external long pulse laser beam (E<1.5 J) was used to create various levels of preplasmas in front of a planar target for a systematic study. Kα x-ray emission from a fluorescence layer (copper) was absolutely counted and its spatial distribution was monitored. Experimental data show Kα x-ray signal reduction (up to 60%) with an increase in the preplasma level. In addition, a ring structure of Kα x rays was observed with a large preplasma. The underlying physics of the ring structure production was studied by integrating the modeling using a radiation hydrodynamics code and a hybrid particle-in-cell code. Modeling shows that the ring structure is due to the thermoelectric magnetic field excited by the long pulse laser irradiation and an electrostatic field due to the fast electrons in the preplasma.

Model experiment of cosmic ray acceleration due to an incoherent wakefield induced by an intense laser pulse

The first report on a model experiment of cosmic ray acceleration by using intense laser pulses is presented. Large amplitude light waves are considered to be excited in the upstream regions of relativistic astrophysical shocks and the wakefield acceleration of cosmic rays can take place. By substituting an intense laser pulse for the large amplitude light waves, such shock environments were modeled in a laboratory plasma. A plasma tube, which is created by imploding a hollow polystyrene cylinder, was irradiated by an intense laser pulse. Nonthermal electrons were generated by the wakefield acceleration and the energy distribution functions of the electrons have a power-law component with an index of ∼2. The maximum attainable energy of the electrons in the experiment is discussed by a simple analytic model. In the incoherent wakefield the maximum energy can be much larger than one in the coherent field due to the momentum space diffusion or the energy diffusion of electrons.

Development of multi-channel electron spectrometer

In order to obtain the angular dependent electron energy distributions, we developed a multichannel electron spectrometer (MCESM) with high energy and angular resolutions. The MCESM consists of seven small electron spectrometers set in every 5° on the basement, each of which detection range is up to 25 MeV. In the experiment, we successfully obtained electron spectra from imploded cone-shell target as well as gold plane target irradiated by ultraintense (300 J/5 ps) laser beam.

Absolute calibration of imaging plate for GeV electrons

An imaging plate has been used as a useful detector of energetic electrons in laser electron acceleration and laser fusion studies. The absolute sensitivity of an imaging plate was calibrated at 1 GeV electron energy using the injector Linac of SPring-8. The sensitivity curve obtained up to 100 MeV in a previous study was extended successfully to GeV range.

Approaching the diffraction-limited, bandwidth-limited Petawatt

J-KAREN-P is a high-power laser facility aiming at the highest beam quality and irradiance for performing state-of-the art experiments at the frontier of modern science. Here we approached the physical limits of the beam quality: diffraction limit of the focal spot and bandwidth limit of the pulse shape, removing the chromatic aberration, angular chirp, wavefront and spectral phase distortions. We performed accurate measurements of the spot and peak fluence after an f/1.3 off-axis parabolic mirror under the full amplification at the power of 0.3 PW attenuated with ten high-quality wedges, resulting in the irradiance of ~1022 W/cm2 and the Strehl ratio of ~0.5.

Laboratory investigations on the origins of cosmic rays

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas.

Autoinjection of electrons into a wake field using a capillary with attached cone

By using a cone attached to a capillary, electrons generated through a laser interaction were autoinjected and accelerated in a low-density wake field. The cone attached to the entrance of the capillary serves as an electron supplier. It increases the number of electrons from below the detection limit to 1.1 pC and the energy from 4 to 30 MeV. A two-dimensional particle-in-cell simulation reveals that a significant number of energetic electrons are extracted from the surface of the cone and are subsequently trapped in the wake field and accelerated in the capillary.

Spectrum modulation of relativistic electrons by laser wakefield

Energetic electrons were generated by the interaction of a high-intensity laser pulse with a plasma preformed from a hollow plastic cylinder via laser-driven implosion. The spectra of a comparatively high-density plasma ∼1019 cm−3 had a bump around 10 MeV. Simple numerical calculations explained the spectra obtained in this experiment. This indicates that the plasma tube has sufficient potential to convert a Maxwellian spectrum to a comparatively narrow spectrum.

Effect of halo on the stability of electron bunches in laser wakefield acceleration

The stability of short electron bunches formed and accelerated by laser wakefield in gas jets strongly depends on parameters of pre-plasma due to poor pulse focus-ability. Instabilities provoked by light energy remained out of focus spots (halo), usually 30–50%, are shown to result in broad and spotted transverse distributions of electrons in uniform gas jets radiated by high-contrast laser pulses. Shaped pre-plasma may drastically improve pulse focus-ability with stable, well-pointed, and well-collimated electron bunches.

Absolute calibration of imaging plate for electron spectrometer measuring GeV-class electrons

An electron spectrometer (ESM) using imaging plate (IP) is designed and tested to measure relativistic electrons which are generated from laser-plasma interactions. The measurable energy range extends to 1 GeV or even higher. The absolute sensitivity of IP for 1 GeV electrons was calibrated using electrons from Linac in SPring-8. An IP has enough sensitivity for 1-GeV electrons. The electron spectrometer, which is measurable to ~ 1 GeV, has been developed using IP detector.