Laser-driven neutron source and nuclear resonance absorption imaging at ILE, Osaka University: review.

Here, we present an overview on the recent progress in the development of the laser-driven neutron source (LDNS) and nuclear resonance absorption (NRA) imaging at the Institute of Laser Engineering (ILE), Osaka University. The LDNS is unique because the number of neutrons per micro pulse is very large, and the source size and the pulse width are small. Consequently, extensive research and development of LDNSs is going on around the world. In this paper, a typical neutron generation process by the laser-driven ion beam, called the pitcher-catcher scheme, is described. The characteristics of the LDNS are compared with those of the accelerator-driven neutron source (ADNS), and unique application of the LDNS, such as NRA imaging, is presented. In the LDNS, NRA imaging is possible with a relatively short beam line in comparison with that of the ADNS since the neutron pulse width and the source size of the LDNS are small. Future prospects in research and development of NRA imaging with the LDNS at ILE Osaka University are also described.

Petapascal Pressure Driven by Fast Isochoric Heating with a Multipicosecond Intense Laser Pulse.

Fast isochoric laser heating is a scheme to heat matter with a relativistic intensity (>10^{18} W/cm^{2}) laser pulse for producing an ultrahigh-energy-density (UHED) state. We have demonstrated an efficient fast isochoric heating of a compressed dense plasma core with a multipicosecond kilojoule-class petawatt laser and an assistance of externally applied kilotesla magnetic fields for guiding fast electrons to the dense plasma. A UHED state of 2.2 PPa is achieved experimentally with 4.6 kJ of total laser energy that is one order of magnitude lower than the energy used in the conventional implosion scheme. A two-dimensional particle-in-cell simulation confirmed that diffusive heating from a laser-plasma interaction zone to the dense plasma plays an essential role to the efficient creation of the UHED state.

Hard-Target Reflection Laser Spectroscopy of Carbon Monoxide Gas Concentration for the Early Detection of Spontaneous Combustion of Coal.

We report on the hard-target reflection spectroscopy of carbon monoxide (CO) gas based on the technique of infrared tunable diode laser absorption spectroscopy aiming at developing a low-cost yet sensitive sensor for the early detection of spontaneous coal combustion. A narrow-band distributed feedback laser emitting around 2333.7 nm is used to monitor CO gas molecules contained in a 5 cm gas cell. The light diffusely backscattered from the surface of a lump of coal placed at the end of a 50 cm light path is detected with a photodiode in the coaxial transmitter/receiver setup. From the variation of the detected signal profile with the CO partial pressure in the cell, the detection limit of the current system is estimated to be about 30 parts per million per meter (ppm·m), which meets the sensitivity required for monitoring the self-heating of coal in mines, silos, or stockpiles.

Demonstration of shape analysis of neutron resonance transmission spectrum measured with a laser-driven neutron source.

Laser-driven neutron sources (LDNSs) can generate strong short-pulse neutron beams, which are valuable for scientific studies and engineering applications. Neutron resonance transmission analysis (NRTA) is a nondestructive technique used for determining the areal density of each nuclide in a material sample using pulsed thermal and epithermal neutrons. Herein, we report the first successful NRTA performed using an LDNS driven by the Laser for Fast Ignition Experiment at the Institute of Laser Engineering, Osaka University. The key challenge was achieving a well-resolved resonance transmission spectrum for material analysis using an LDNS with a limited number of laser shots in the presence of strong background noise. We addressed this by employing a time-gated 6 Li -glass scintillation neutron detector to measure the transmission spectra, reducing the impact of electromagnetic noise and neutron and gamma-ray flashes. Output waveforms were recorded for each laser shot and analyzed offline using a counting method. This approach yielded a spectrum with distinct resonances, which were attributed to 115 In and 109 Ag , as confirmed through neutron transmission simulation. The spectrum was analyzed using the least-square nuclear-resonance fitting program, REFIT, demonstrating the possibility of using an LDNS for nondestructive areal-density material characterization.

Single-shot laser-driven neutron resonance spectroscopy for temperature profiling.

The temperature measurement of material inside of an object is one of the key technologies for control of dynamical processes. For this purpose, various techniques such as laser-based thermography and phase-contrast imaging thermography have been studied. However, it is, in principle, impossible to measure the temperature of an element inside of an object using these techniques. One of the possible solutions is measurements of Doppler brooding effect in neutron resonance absorption (NRA). Here we present a method to measure the temperature of an element or an isotope inside of an object using NRA with a single neutron pulse of approximately 100 ns width provided from a high-power laser. We demonstrate temperature measurements of a tantalum (Ta) metallic foil heated from the room temperature up to 617 K. Although the neutron energy resolution is fluctuated from shot to shot, we obtain the temperature dependence of resonance Doppler broadening using a reference of a silver (Ag) foil kept to the room temperature. A free gas model well reproduces the results. This method enables element(isotope)-sensitive thermometry to detect the instantaneous temperature rise in dynamical processes.

Proton acceleration to 40 MeV using a high intensity, high contrast optical parametric chirped-pulse amplification/Ti:sapphire hybrid laser system.

Using a high-contrast (10(10):1) and high-intensity (10(21) W/cm(2)) laser pulse with the duration of 40 fs from an optical parametric chirped-pulse amplification/Ti:sapphire laser, a 40 MeV proton bunch is obtained, which is a record for laser pulse with energy less than 10 J. The efficiency for generation of protons with kinetic energy above 15 MeV is 0.1%.

The conceptual design of 1-ps time resolution neutron detector for fusion reaction history measurement at OMEGA and the National Ignition Facility.

The nuclear burn history provides critical information about the dynamics of the hot-spot formation and high-density fuel-shell assembly of an Inertial Confinement Fusion (ICF) implosion, as well as information on the impact of alpha heating, and a multitude of implosion failure mechanisms. Having this information is critical for assessing the energy-confinement time τE and performance of an implosion. As the confinement time of an ICF implosion is a few tens of picoseconds, less than 10-ps time resolution is required for an accurate measurement of the nuclear burn history. In this study, we propose a novel 1-ps time-resolution detection scheme based on the Pockels effect. In particular, a conceptual design for the experiment on the National Ignition Facility and OMEGA are elaborated upon herein. A small organic Pockels crystal "DAST" is designed to be positioned ∼5 mm from the ICF implosion, which is scanned by a chirped pulse generated by a femto-second laser transmitted through a polarization-maintained optical fiber. The originally linearly polarized laser is changed to an elliptically polarized laser by the Pockels crystal when exposed to neutrons, and the modulation of the polarization will be analyzed. Our study using 35-MeV electrons showed that the system impulse response is 0.6 ps. The response time is orders of magnitude shorter than current systems. Through measurements of the nuclear burn history with unprecedented time resolution, this system will help for a better understanding of the dynamics of the hot-spot formation, high-density fuel-shell assembly, and the physics of thermonuclear burn wave propagation.

Note: A Laue crystal imager for high energy quasi-monochromatic x-ray.

A newly designed transmission type x-ray Laue imager for tens of keV hard x-rays is reported. Compared with the traditional reflection type x-ray crystal imager, the transmission geometry produces a much better image quality for high energy hard x-rays. This system was assessed via a calibration experiment performed at the SPring8 synchrotron radiation facility. With a Ta x-ray fluorescer, the mono-energetic x-ray at 70 keV from the synchrotron radiation was converted to an isotropically emitted Ta K-shell source at 57.5 keV and 65 keV. A tungsten pinhole array was employed as the test object, and clear images of the pinholes with a magnification of ∼5 were acquired. These images exhibited superior quality in the dispersion plane. As an extension of this work, a slit-free full-spectral Laue imager is proposed for high resolution hard x-ray imaging.

Magnetized fast isochoric laser heating for efficient creation of ultra-high-energy-density states.

Fast isochoric heating of a pre-compressed plasma core with a high-intensity short-pulse laser is an attractive and alternative approach to create ultra-high-energy-density states like those found in inertial confinement fusion (ICF) ignition sparks. Laser-produced relativistic electron beam (REB) deposits a part of kinetic energy in the core, and then the heated region becomes the hot spark to trigger the ignition. However, due to the inherent large angular spread of the produced REB, only a small portion of the REB collides with the core. Here, we demonstrate a factor-of-two enhancement of laser-to-core energy coupling with the magnetized fast isochoric heating. The method employs a magnetic field of hundreds of Tesla that is applied to the transport region from the REB generation zone to the core which results in guiding the REB along the magnetic field lines to the core. This scheme may provide more efficient energy coupling compared to the conventional ICF scheme.

Control of unsteady laser-produced plasma-flow with a multiple-coil magnetic nozzle.

We report an experimental demonstration of controlling plasma flow direction with a magnetic nozzle consisting of multiple coils. Four coils are controlled separately to form an asymmetric magnetic field to change the direction of laser-produced plasma flow. The ablation plasma deforms the topology of the external magnetic field, forming a magnetic cavity inside and compressing the field outside. The compressed magnetic field pushes the plasma via the Lorentz force on a diamagnetic current: j × B in a certain direction, depending on the magnetic field configuration. Plasma and magnetic field structure formations depending on the initial magnetic field were simultaneously measured with a self-emission gated optical imager and B-dot probe, respectively, and the probe measurement clearly shows the difference of plasma expansion direction between symmetric and asymmetric initial magnetic fields. The combination of two-dimensional radiation hydrodynamic and three-dimensional hybrid simulations shows the control of the deflection angle with different number of coils, forming a plasma structure similar to that observed in the experiment.