T. Yabuuchi

Calibration of imaging plate for high energy electron spectrometer

A high energy electron spectrometer has been designed and tested using imaging plate (IP). The measurable energy range extends from 1to100MeV or even higher. The IP response in this energy range is calibrated using electrons from L-band and S-band LINAC accelerator at energies 11.5, 30, and 100MeV. The calibration has been extended to 0.2MeV using an existing data and Monte Carlo simulation Electron Gamma Shower code. The calibration results cover the energy from 0.2to100MeV and show almost a constant sensitivity for electrons over 1MeV energy. The temperature fading of the IP shows a 40% reduction after 80min of the data taken at 22.5°C. Since the fading is not significant after this time we set the waiting time to be 80min. The oblique incidence effect has been studied to show that there is a 1∕cosθ relation when the incidence angle is θ.

Plasma devices to guide and collimate a high density of MeV electrons

The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser–matter interactions at petawatt (1015 W) power levels can create pulses of MeV electrons with current densities as large as 1012 A cm-2. However, the divergence of these particle beams usually reduces the current density to a few times 106 A cm-2 at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser–matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.

Basic and integrated studies for fast ignition

Basic and integrated studies are conducted on fast ignition (FI) using various large laser systems. A Peta watt (PW) laser system is used to study the basic elements relevant to FI and can also be injected to a compressed core to test the FI integrated experiment when coupled with a GEKKO twelve laser beam system. Using a spherical target inserted with a Au cone guide for the PW laser pulse, an imploded core is heated up to 1 keV resulting in neutron increase 1000 times more than that without heating pulse. Details of the implosion are examined at the Omega laser system of this type target with indirect implosion scheme and are compared with simulation results. LASNEX simulation indicates that a 400 g/c.c. high density core could be achieved with this scheme at 1.8 MJ laser input.

Laser generated proton beam focusing and high temperature isochoric heating of solid matter

The results of laser-driven proton beam focusing and heating with a high energy (170J) short pulse are reported. Thin hemispherical aluminum shells are illuminated with the Gekko petawatt laser using 1μm light at intensities of ∼3×1018W∕cm2 and measured heating of thin Al slabs. The heating pattern is inferred by imaging visible and extreme-ultraviolet light Planckian emission from the rear surface. When Al slabs 100μm thick were placed at distances spanning the proton focus beam waist, the highest temperatures were produced at 0.94× the hemisphere radius beyond the equatorial plane. Isochoric heating temperatures reached 81eV in 15μm thick foils. The heating with a three-dimensional Monte Carlo model of proton transport with self-consistent heating and proton stopping in hot plasma was modeled.

On the behavior of ultraintense laser produced hot electrons in self-excited fields

A large number of hot electrons exceeding the Alfven current can be produced when an ultraintense laser pulse irradiates a solid target. Self-excited extreme electrostatic and magnetic fields at the target rear could influence the electron trajectory. In order to investigate the influence, we measure the hot electrons when a plasma was created on the target rear surface in advance and observe an increase of the electron number by a factor of 2. This increase may be due to changes in the electrostatic potential formation process with the rear plasma. Using a one-dimensional particle-in-cell simulation, it is shown that the retardation in the electrostatic potential formation lengthens the gate time when electrons can escape from the target. The electron number escaping within the lengthened time window appears to be much smaller than the net produced number and is consistent with our estimation using the Alfven limit.

Measurements of fast electron scaling generated by petawatt laser systems

Fast electron energy spectra have been measured for a range of intensities between 1018 and 1021Wcm−2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5to5ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40° in p-polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions.

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.

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.

Study of ultraintense laser propagation in overdense plasmas for fast ignition

Laser plasma interactions in a relativistic regime relevant to the fast ignition in inertial confinement fusion have been investigated. Ultraintense laser propagation in preformed plasmas and hot electron generation are studied. The experiments are performed using a 100 TW 0.6 ps laser and a 20 TW 0.6 ps laser synchronized by a long pulse laser. In the study, a self-focused ultraintense laser beam propagates along its axis into an overdense plasma with peak density 1022/cm3. Channel formation in the plasma is observed. The laser transmission in the overdense plasma depends on the position of its focus and can take place in plasmas with peak densities as high as 5×1022/cm3. The hot electron beams produced by the laser-plasma interaction have a divergence angle of ∼30°, which is smaller than that from laser-solid interactions. For deeper penetration of the laser light into the plasma, the use of multiple short pulse lasers is proposed. The latter scheme is investigated using particle-in-cell simulation. It ...

Evidence of anomalous resistivity for hot electron propagation through a dense fusion core in fast ignition experiments

Anomalous resistivity for hot electrons passing through a dense core plasma is studied for fast ignition laser fusion. The hot electrons generated via the ultra-intense laser pulse and guiding cone interactions are measured after they pass through a dense plasma with a density of 50?100?g?cm?3 in a radius of 15?25??m. When significant neutron enhancements are achieved by the ultra-intense laser pulse injection, the energy reduction of fast electrons is observed. Also, a reduction in the number of electrons with energy up to 15?MeV can be seen. We offer a new physical mechanism for the stopping of electrons, involving electron magnetohydrodynamic shock formation in the inhomogeneous plasma density region. The dissipation in the shock region can explain electron stopping with energies of the order of 15?MeV.