Demonstration of a spherical plasma mirror for the counter-propagating kilojoule-class petawatt LFEX laser system.

A counter-propagating laser-beam platform using a spherical plasma mirror was developed for the kilojoule-class petawatt LFEX laser. The temporal and spatial overlaps of the incoming and redirected beams were measured with an optical interferometer and an x-ray pinhole camera. The plasma mirror performance was evaluated by measuring fast electrons, ions, and neutrons generated in the counter-propagating laser interaction with a Cu-doped deuterated film on both sides. The reflectivity and peak intensity were estimated as ∼50% and ∼5 × 1018 W/cm2, respectively. The platform could enable studies of counter-streaming charged particles in high-energy-density plasmas for fundamental and inertial confinement fusion research.

Simulation of optical radiation force distribution in interference patterns and necessary conditions for chiral structure formation on dielectrics.

A chiral structure is formed by the optical radiation force induced by a circularly polarized light that has spin angular momentum; chiral structures are expected to be used for light control devices and molecular chirality discrimination devices. In this paper, we clarify the relationship between the differences in the distributions of the optical radiation force and the possibility of formation of chiral structures. We first simulate the optical radiation force distribution in the case of a Gaussian beam that successfully forms a chiral structure. Given a vector [Formula: see text] with a centre of the light spot [Formula: see text] and polar coordinates [Formula: see text], and an optical radiation force vector [Formula: see text] at [Formula: see text], the angle [Formula: see text] and [Formula: see text] must be constant with respect to the declination angle [Formula: see text] for a chiral structure to form. These conditions are fulfilled in the case of a 6-beam interference pattern, but not in the case of a 4-beam interference pattern, which is consistent with the result that no chiral structure is formed in the latter case. The equations derived for simulation of optical radiation force distribution can be used for any optical intensity distribution, and will be of great help in the research of any dielectrics deformation.

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.

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.

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.

Development of Compton X-ray spectrometer for high energy resolution single-shot high-flux hard X-ray spectroscopy.

Hard X-ray spectroscopy is an essential diagnostics used to understand physical processes that take place in high energy density plasmas produced by intense laser-plasma interactions. A bundle of hard X-ray detectors, of which the responses have different energy thresholds, is used as a conventional single-shot spectrometer for high-flux (>10(13) photons/shot) hard X-rays. However, high energy resolution (Δhv/hv < 0.1) is not achievable with a differential energy threshold (DET) X-ray spectrometer because its energy resolution is limited by energy differences between the response thresholds. Experimental demonstration of a Compton X-ray spectrometer has already been performed for obtaining higher energy resolution than that of DET spectrometers. In this paper, we describe design details of the Compton X-ray spectrometer, especially dependence of energy resolution and absolute response on photon-electron converter design and its background reduction scheme, and also its application to the laser-plasma interaction experiment. The developed spectrometer was used for spectroscopy of bremsstrahlung X-rays generated by intense laser-plasma interactions using a 200 µm thickness SiO2 converter. The X-ray spectrum obtained with the Compton X-ray spectrometer is consistent with that obtained with a DET X-ray spectrometer, furthermore higher certainly of a spectral intensity is obtained with the Compton X-ray spectrometer than that with the DET X-ray spectrometer in the photon energy range above 5 MeV.

Heating efficiency evaluation with mimicking plasma conditions of integrated fast-ignition experiment.

A series of experiments were carried out to evaluate the energy-coupling efficiency from heating laser to a fuel core in the fast-ignition scheme of laser-driven inertial confinement fusion. Although the efficiency is determined by a wide variety of complex physics, from intense laser plasma interactions to the properties of high-energy density plasmas and the transport of relativistic electron beams (REB), here we simplify the physics by breaking down the efficiency into three measurable parameters: (i) energy conversion ratio from laser to REB, (ii) probability of collision between the REB and the fusion fuel core, and (iii) fraction of energy deposited in the fuel core from the REB. These three parameters were measured with the newly developed experimental platform designed for mimicking the plasma conditions of a realistic integrated fast-ignition experiment. The experimental results indicate that the high-energy tail of REB must be suppressed to heat the fuel core efficiently.

Accuracy evaluation of a Compton X-ray spectrometer with bremsstrahlung X-rays generated by a 6 MeV electron bunch.

A Compton-scattering-based X-ray spectrometer is developed to obtain the energy distribution of fast electrons produced by intense laser and matter interactions. Bremsstrahlung X-rays generated by fast electrons in a material are used to measure fast electrons' energy distribution in matter. In the Compton X-ray spectrometer, X-rays are converted into recoil electrons by Compton scattering in a converter made from fused silica glass, and a magnet-based electron energy analyzer is used to measure the energy distribution of the electrons that recoil in the direction of the incident X-rays. The spectrum of the incident X-rays is reconstructed from the energy distribution of the recoil electrons. The accuracy of this spectrometer is evaluated using a quasi-monoenergetic 6 MeV electron bunch that emanates from a linear accelerator. An electron bunch is injected into a 1.5 mm thick tungsten plate to produce bremsstrahlung X-rays. The spectrum of these bremsstrahlung X-rays is obtained in the range from 1 to 9 MeV. The energy of the electrons in the bunch is estimated using a Monte Carlo simulation of particle-matter interactions. The result shows that the spectrometer's energy accuracy is ±0.5 MeV for 6.0 MeV electrons.

Kilotesla magnetic field due to a capacitor-coil target driven by high power laser.

Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 µm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 µm, and 5 × 10(16) W/cm(2)).

Monochromatic x-ray radiography for areal-density measurement of inertial fusion energy fuel in fast ignition experiment.

Ultrafast, two-dimensional x-ray imaging is an important diagnostics for the inertial fusion energy research, especially in investigating implosion dynamics at the final stage of the fuel compression. Although x-ray radiography was applied to observing the implosion dynamics, intense x-rays emitted from the high temperature and dense fuel core itself are often superimposed on the radiograph. This problem can be solved by coupling the x-ray radiography with monochromatic x-ray imaging technique. In the experiment, 2.8 or 5.2 keV backlight x-rays emitted from laser-irradiated polyvinyl chloride or vanadium foils were selectively imaged by spherically bent quartz crystals with discriminating the out-of-band emission from the fuel core. This x-ray radiography system achieved 24 µm and 100 ps of spatial and temporal resolutions, respectively.

One-dimensional and multichannels multi-imaging x-ray streak camera for imploded core plasma of shell-cone target.

Recently one-dimensional and multichannels multi-imaging x-ray streak camera (1D+McMIXS) has been proposed as an ultrafast diagnostic tool for imploded core plasma of shell-cone target. This diagnostic system can provide much more information of core plasma dynamics, such as the shell trajectory before the maximum compression, two dimensional (2D) x-ray images, and 2D map of electron temperature. The 1D+McMIXS was used in an implosion experiment of a shell target with a cone for fast ignition at Gekko XII laser facility. The interaction between the core plasma and the tip of the cone was observed and revealed some detailed structures. A series of time-resolved 2D x-ray images were reconstructed and a nominal temperature evolution was also mapped.

Monochromatic x-ray sampling streak imager for fast-ignitor plasma observation.

Ultrafast two-dimensional (2D) x-ray imaging is required to investigate the dynamics of fast-heated core plasma in inertial confinement fusion research. A novel x-ray imager, consisting of two toroidally bent Bragg crystals and an ultrafast 2D x-ray imaging camera, has been demonstrated. Sequential and 2D monochromatic x-ray images of laser-imploded core plasma were obtained with a temporal resolution of 20 ps, a spatial resolution of 31 mum, and a spectral resolution of over 200, simultaneously.

Measurement of heating laser injection time to imploded core plasma by using x-ray framing camera.

A simultaneous measurement of imploded core plasma and injection time of heating laser is conducted by using an x-ray framing camera (XFC). The experiments are performed using Gekko XII laser system for implosion of the deuterated polystyrene (CD) plastic shell target and Peta Watt (PW) laser system for heating. The time of PW laser injection is observed as the bright zone in the XFC image. The measured x-ray intensity profiles fit the Gaussian profiles well. The calculations of microchannel plate by using dynode model explain these broadened temporal profiles qualitatively. The peak position of fitted x-ray intensity profile is almost in agreement with the time when the high energy x ray is observed by x-ray streak camera. Moreover, the peak position is delayed corresponding to the delayed setting of PW laser injection time. From these results, it is concluded that we can estimate the heating laser injection time with resolution of the order of 10 ps by using XFC.

Observation of asymmetrically imploded core plasmas with a two-dimensional sampling image x-ray streak camera.

A shell target with a cone for guiding the heating beam has been proposed for the fast ignition scheme. Implosion of such target is no longer symmetric because of the cone. A fast two-dimensional x-ray imaging technique, two-dimensional (2D) sampling image x-ray streak camera was applied for the first time to observation of the dynamics of implosion and core plasma. X-ray emission image of the plasma was sampled with two-dimensionally distributed image sampling points, streaked with the tube, and the recorded signals were reconstructed as sequential 2D frame images. Shape and movement of the core plasma were clearly observed.

Suppression of the Rayleigh-Taylor instability due to self-radiation in a multiablation target.

A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive inertial fusion target. In a high-Z doped-plastic target, two ablation surfaces are formed separately-one driven by thermal radiation and the other driven by electron conduction. The growth of the Rayleigh-Taylor instability is significantly suppressed on the radiation-driven ablation surface inside the target due to the large ablation velocity and long density scale length. A significant reduction of the growth rate was observed in simulations and experiments using a brominated plastic target. A new direct-drive pellet was designed using this scheme.