Yoneyoshi Kitagawa

Focus optimization of relativistic self-focusing for anomalous laser penetration into overdense plasmas (super-penetration)

Relativistic electron motion in a plasma due to an intense laser pulse modifies the refractive index and leads to two effects: relativistic induced transparency and relativistic self-focusing. A combination of the above two effects enables transmission of laser energy deep into plasmas which is useful for fast ignition of inertial fusion. This so-called super-penetration sensitively depends on the focal position of the laser intensity due to the inhomogeneous density profile of the plasma and convergence of the laser pulse by final focusing optics. Experiments were conducted at vacuum focused laser intensities between 3.3 and 4. 4 × 1018 W cm−2 at peak plasma densities between 23 and 75nc, where nc is the critical density of the plasma. We introduced a scenario: the laser beam diameter at nc/4 density must be smaller than the plasma wavelength to achieve whole beam self-focusing. An optimum focus was found experimentally by measuring the plasma channel, laser transmittance and electron spectra. All three data are consistent with one another and numerical calculations based on a paraxial approximation model suggest that this optimum focus corresponds to the scenario described above.

Formation of Stable Pinch Column in Focus Plasma

The plasma boundary fluted due to the Rayleigh-Taylor instability was observed in the early stage of implosion phase of the focus plasma by using tlae shadow-graph technique. Only in the strong focus region, this instability disappeared at the end of implosion phase and the stable pinch column was formed over 20 ns.

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.

1 Hz fast-heating fusion driver HAMA pumped by a 10 J green diode-pumped solid-state laser

A Ti : sapphire laser HAMA pumped by a diode-pumped solid-state laser (DPSSL) is developed to enable a high-repetitive inertial confinement fusion (ICF) experiment to be conducted. To demonstrate a counter-irradiation fast-heating fusion scheme, a 3.8 J, 0.4 ns amplified chirped pulse is divided into four beams: two counter-irradiate a target with intensities of 6 × 1013 W cm−2, and the remaining two are pulse-compressed to 110 fs for heating the imploded target with intensities of 2 × 1017 W cm−2. HAMA contributed to the first demonstration by showing that a 10 J class DPSSL is adaptable to ICF experiments and succeeded in DD neutron generation in the repetition mode. Based on HAMA, we can design and develop an integrated repetitive ICF experiment machine by including target injection and tracking.

Head-On Inverse Compton Scattering X-rays with Energy beyond 10 keV from Laser-Accelerated Quasi-Monoenergetic Electron Bunches

Inverse Compton X-rays from laser-accelerated multiple electron bunches are observed. A Ti:sapphire laser (pulse energy: 500 mJ; pulse width: 150 fs) beam is divided into two beams. The main beam is focused onto an edge of a helium gas jet to accelerate electrons to energies of 14 and 23 MeV, which inversely scattered the head-on colliding secondary laser beam into 6 and 12 keV X-rays; this agrees well with that calculated from the electron spectra obtained. This demonstrates a first on-axis inverse Compton scattering X-ray energy detection beyond 10 keV induced by laser-accelerated electrons.

Preliminary studies of direct energy conversion in a D-3He inertial confinement fusion reactor

AbstractA direct energy conversion method is proposed for a D-3He inertial confinement fusion reactor. The method utilizes inductive energy recovery through pickup coils in the plasma chamber in which mirror magnetic fields are applied. A method to reduce the problems regarding the handling of ultrahigh voltage inherent in energy recovery of this type is proposed that divides a one-turn pickup coil into a number of pickup segments both axially and azimuthally to reduce the output voltage per pickup segment so that it can be managed by near-term technologies.Analytical results predict that the expanding plasma energy is directly converted to electricity through the recovery circuit using capacitors with an efficiency of >80% when the plasma is assumed to expand cylindrically.

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.

Ultrahigh Pulsed Magnetic Field Produced by a CO2 Laser

A method is presented for generating a high magnetic field using a CO2 laser pulse. The magnetic field, current and voltage in a one-turn coil were measured as functions of the gap width, where the high voltage was induced by laser irradiation at a fixed intensity of 1.3 ? 1014 W/cm2. These results could be explained on the basis of lateral transport by hot-electron E ? B motion and an expansion of the critical density plasma in the gap. The maximum magnetic fields are determined and limited by the filling time of the gap with a critical-density plasma. In order to obtain higher magnetic fields, we tested a cylinder-type one-turn coil attached to the optimized gap. The highest magnetic field observed was 400 T.

Multilayered polycrystallization in single-crystal YSZ by laser-shock compression

A single shot of an ultra-intense laser with 0.8 J of energy and a pulse width of 110 fs (peak intensity of W cm−2) is divided into two beams and the two beams counter-irradiated onto a 0.5 mm-thick single crystal yttria-stabilized zirconia (YSZ), changing the YSZ into a multilayered polycrystalline state. The laser-driven shock wave of the intensity 7.6 Pa penetrated the crystal as deep as 96 m, causing formation of a four-layered structure (the first layer from the surface to 12 m, the second from 12 to 28 m, the third from 28 to 96 m, and the fourth from 96 to 130 m, respectively). The grain size of the first layer was 1 m, while that of the second layer was broken into a few tens nanometers. The grain size of the third layer was a few hundred nanometers to a few ten micrometers. The area deeper than 96 m remained as a single crystal. The plasma heat wave might remelt the first layer, resulting in the grain size becoming larger than that of the second layer. The surface polycrystallization seems to maintain the residual stresses frozen in the film thickness direction. Our experimentally observed spatial profile of the grain size can be explained by this shock and heat waves model.

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.