Toshihiro Taguchi

Particle simulation on x-ray emissions from ultra-intense laser produced plasmas

The interaction of ultra-short intense laser with a thin foil has been studied by one- and two-dimensional particle simulations, which include the collisional absorption and the hard x-ray bremsstrahlung processes. The bremsstrahlung processes are modeled in a fully relativistic way in the frame work of quantum electrodynamics and integrated by the Monte Carlo method in particle-in-cell code. For carbon thin foils in which electron density is 200 times critical density, a laser pulse with an intensity greater than 1018 W/cm2 irradiates the foil normally to accelerate electrons on the surface along the laser propagation direction by the oscillating ponderomotive force. As a result, hard x rays over the range of 100 keV are emitted strongly toward the laser direction. The spectrum of the photon energy and scaling of the hard x-ray emission with respect to the various laser intensity are investigated by the newly developed simulation code.

Plasma physics and laser development for the Fast-Ignition Realization Experiment (FIREX) Project

Since the approval of the first phase of the Fast-Ignition Realization Experiment (FIREX-I), we have devoted our efforts to designing advanced targets and constructing a petawatt laser, which will be the most energetic petawatt laser in the world. Scientific and technological improvements are required to efficiently heat the core plasma. There are two methods that can be used to enhance the coupling efficiency of the heating laser to the thermal energy of the compressed core plasma: adding a low-Z foam layer to the inner surface of the cone and employing a double cone. The implosion performance can be improved in three ways: adding a low-Z plastic layer to the outer surface of the cone, using a Br-doped plastic ablator and evacuating the target centre. An advanced target for FIREX-I was introduced to suit these requirements. A new heating laser (LFEX) has been constructed that is capable of delivering an energy of 10 kJ in 10 ps with a 1 ps rise time. A fully integrated fast-ignition experiment is scheduled for 2009.

Resonant heating of a cluster plasma by intense laser light

Cluster heating by a strong laser field is studied using a particle-in-cell code (PIC) for a range of intensities and cluster sizes. Above a threshold intensity, heating is dominated by a nonlinear resonant absorption process.

Study of hot electron beam transport in high density plasma using 3D hybrid-Darwin code

Abstract Hot electron transport has been analyzed by our newly developed hybrid-Darwin code, which is aimed to apply the detail analysis of the fast ignition scheme. Our code divides electrons into two species, fast beam electrons and cold background electrons and fast electrons are described as particles, while background electrons and ions are described as fluids. The results show that the beam electron injected into a high density plasma forms a ring structure followed by the breakup into filaments because of the two stream instability. Our recently developed open boundary code shows such a ring formation and the impedance of the straight flow by the concentration of the hot electron beam due to the strong self-generated magnetic filed. The self-pinch and the following divergence of the electron beam after a relatively short propagation are crucial for the achievement of the fast ignition scheme.

Gases of exploding laser-heated cluster nanoplasmas as a nonlinear optical medium

The manner in which strongly heated nanoclusters explode in the presence of intense laser fields influences all applications of this interaction. By measuring, with femtosecond time resolution, the ensemble average polarizability in a gas of intense laser-heated clusters, we have inferred the cluster explosion dynamics. The time evolution of the polarizability is characteristic of competition in the optical response between supercritical and subcritical density regions of the exploding cluster. These results are consistent with complementary time-resolved Rayleigh scattering measurements and with the predictions of a near-field plasma hydrodynamic model of the laser–cluster interaction. A significant implication of this cluster evolution appears in its macroscopic effect on a laser beam: a gas of exploding cluster plasmas causes nonlinear beam propagation owing to the space and time dependence of the ensemble polarizability. A strong self-focusing effect is observed experimentally that strongly contrasts ...

Control of an electron beam using strong magnetic field for efficient core heating in fast ignition

For enhancing the core heating efficiency in electron-driven fast ignition, we proposed the fast electron beam guiding using externally applied longitudinal magnetic fields. Based on the PIC simulations for the FIREX- class experiments, we demonstrated the sufficient beam guiding performance in the collisional dense plasma by kT-class external magnetic fields for the case with moderate mirror ratio ( 10  ). Boring of the mirror field was found through the formation of magnetic pipe structure due to the resistive effects, which indicates a possibility of beam guiding in high mirror field for higher laser intensity and/or longer pulse duration.

A new FEL concept driven by a vacuum microfielf emitter

Abstract In order to reduce the scale and construction cost of free election lasers, it is necessary to investigate various new ideas with respect to electron beam generation, acceleration and radiation mechanism. In this paper, a microscale free electron laser is proposed which is driven by a very small diameter (less than a few μm) electron beam. The microscopic electron beam can be generated with a needle cathode, for an example, a Spindt cathode. By injecting the microscopic electron beam into a microdielectric channel, we generate radiations of several μm wavelength by Cherenkov emission, Smith-Purcell emission and so on. In the design of a microfield emitter FEL (MFE FEL). It is shown that a 2 mm long dielectric pipe with an electron beam of diameter 1 μm and with a current of 50 μA gives a one pass gain of 50% for a 10 μm wavelength radiation.

Particle in cell analysis of a laser-cluster interaction including collision and ionization processes

A new particle-in-cell (PIC) code which includes collisional and ionization processes has been developed to study laser-plasma interaction. Using the new code, the dynamics of a cluster plasma in a strong laser field has been analyzed and the threshold intensity of the resonant heating, which was previously predicted is accurately evaluated. The angular dependence of ion energy spectrum has also been simulated. As a result, it is found that the anisotropic energy spectrum depends strongly on the presence or absence of collisional processes.

Pseudo-three-dimensional simulation on stability of thin shell target implosion

Pseudo‐three‐dimensional simulation method has been developed to analyze implosion stability of thin shell target in inertial confinement fusion. Nonlinear motion of a thin spherical surface driven by the pressure simulates the implosion of a shell target filled with fuel gas. The simulation shows that acceleration and deceleration of the shell give rise to the Rayleigh–Taylor instability and the instability prevents the target from being compressed uniformly. The results of the simulation show how the maximum volume compression ratio and breakup time of the target depend upon the initial perturbation amplitude. The new simulation scheme has also been applied to another shape of closed surfaces. The implosion phenomenon of the surface of torus is shown as an example. The toroidal target implosion is found to be more stable against the Rayleigh–Taylor instability than that of spherical targets.

Computer simulation of micro-Cherenkov FEL oscillator

Abstract Compact and short-wavelength Cherenkov free-electron laser (FEL) oscillators using a field emission array (FEA) and microdielectric waveguides have been studied with a particle simulation. The particle simulation code that we use calculates all trajectories of electrons in a device interacting with a spatially and temporally varying radiation field self-consistently. Such a kind of simulation is possible because the total number of electrons in the device is small. Using the simulation code, the spatial profile of the radiation amplitude, all electron trajectories and the temporal evolution of the radiation spectrum were calculated. In addition to that, since it is possible to include a random fluctuation of the electron position, we can estimate the start-up time of the micro-Cherenkov FEL oscillator.