Y. Toyama

Laser light and hot electron micro focusing using a conical target

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil.

Characterization of a gamma-ray source based on a laser-plasma accelerator with applications to radiography

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated.

Characterization of 7Li(p, n) 7Be neutron yields from laser produced ion beams for fast neutron radiography

Investigations of 7Li(p,n)7Be reactions using Cu and CH primary and LiF secondary targets were performed using the VULCAN laser [C.N. Danson et al., J. Mod. Opt. 45, 1653 (1997)] with intensities up to 3×1019 W cm−2. The neutron yield was measured using CR-39 plastic track detector and the yield was up to 3×108 sr−1 for CH primary targets and up to 2×108 sr−1 for Cu primary targets. The angular distribution of neutrons was measured at various angles and revealed a relatively anisotropic neutron distribution over 180° that was greater than the error of measurement. It may be possible to exploit such reactions on high repetition, table-top lasers for neutron radiography.

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.

Fast ignition relevant study of the flux of high intensity laser-generated electrons via a hollow cone into a laser-imploded plasma

An integrated experiment relevant to fast ignition . A Cu-doped deuterated polymer spherical shell target with an inserted hollow Au cone is imploded by a six-beam 900-J, 1-ns laser. A 10-ps, 70-J laser pulse is focused into the cone at the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of CuKα fluorescence by comparison with a Monte Carlo model. The electrons are estimated to carry about 15% of the laser energy. Collisional and Ohmic heating are modeled, and Ohmic effects are shown to be relatively unimportant. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is calculated in the model. Enhanced scattering by instability-induced magnetic fields is suggested. An extension of this fluor-based technique to measurement of coupling efficiency to the ignition hot spot in future larger-scale fast ignition experiments is outlined.

Integrated implosion/heating studies for advanced fast ignition

Integrated experiments to investigate the ultrafast heating of implosions using cone/shell geometries have been performed at the Rutherford Appleton Laboratory. The experiments used the 1054 nm, nanosecond, 0.9 kJ output of the VULCAN Nd:glass laser to drive 486 μm diameter, 6 μm wall thickness Cu-doped deuterated plastic (CD) shells in 6-beam cubic symmetry. Measurements of the opacity of the compressed plasma using two-dimensional spatially resolved Ti-Kα x-ray radiography suggest that densities of 4 g cm−3 and areal densities of 40 mg cm−2 were achieved at stagnation. Upper limits on the heating with both 1 ps and 10 ps pulses were deduced from the fluorescent yield from the Cu dopant. The data suggest that control of the preformed plasma scale-length inside the cone is necessary for efficient coupling to the compressed plasma.

Fast plasma heating in a cone-attached geometry - towards fusion ignition

We have developed a PW (0.5 ps/500 J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to a value nearly equivalent to the ignition condition. It appears that the efficient heating is realized by the guiding of the PW laser pulse energy within the hollow cone and by self-organized relativistic electron transport. Based on the experimental results, we are developing a 10 kJ-PW laser system to study the fast heating physics of high-density plasmas at an ignition-equivalent temperature.

Ti Kα radiography of Cu-doped plastic microshell implosions via spherically bent crystal imaging

We show that short pulse laser generated Ti Kα radiation can be used effectively as a backlighter for radiographic imaging. This method of x-ray radiography features high temporal and spatial resolution, high signal to noise ratio, and monochromatic imaging. We present here the Ti Kα backlit images of six-beam driven spherical implosions of thin-walled 500-μm Cu-doped deuterated plastic (CD) shells and of similar implosions with an included hollow gold cone. These radiographic results were used to define conditions for the diagnosis of fast ignition relevant electron transport within imploded Cu-doped coned CD shells.

Broad-range neutron spectra identification in ultraintense laser interactions with carbon-deuterated plasma

Detailed neutron energy spectra produced from a CD2 target irradiated by a 450fs, 20J, 1053nm laser at an intensity of 3×1018W∕cm2 have been studied. Wide-ranging neutron spectra were observed from two different observation angles 20° and 70° relative to the rear-side target normal. The experiment and numerically calculated spectra, by a three-dimensional Monte Carlo code, indicate that the range of the measured spectra is larger than that produced by the D(d,n)He3 reaction. An interpretation for the measured spectra is introduced by considering the C12(d,n)N13 and D(c12,n)N13 reactions. In addition, the study revealed that the neutron spectra produced by the D–C and C–D reactions can overlap that produced by the D–D reaction, and due to their high cross sections, comparing to the D–D reaction, both of them effectively participate in the neutron yield.

Fast heating of super-solid density plasmas towards laser fusion ignition

We have studied fast heating of highly compressed plasmas using multi 100 TW laser light. Efficient propagation of the ultra-intense laser light and heating of the imploded plasmas were realized with a cone-attached shell target. Energy deposition rate of the ultra-intense laser pulse into high-density plasmas was evaluated from neutron measurements. Generation and propagation property of energetic electrons in the ultra-intense laser interactions were also investigated with solid density targets. About 40% of the laser energy converted to mega electron volts energetic electrons in the interactions with solid targets at intensities of 10 19 W cm -2 . These electrons propagated in the high-density plasmas with a divergence of 20-30° or jet-like collimation. Taking account of these experimental results, heating laser spot size is optimized for laser fusion ignition with a simple estimation.