Effect of target material on fast-electron transport and resistive collimation

Direct observation of imploded core heating via fast electrons with super-penetration scheme

Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. To achieve ignition, the energy of a short-pulse laser is required to be delivered efficiently to the pre-compressed fuel core via a high-energy electron beam. Therefore, understanding the transport and energy deposition of this electron beam inside the pre-compressed core is the key for FI. Here we report on the direct observation of the electron beam transport and deposition in a compressed core through the stimulated Cu Kα emission in the super-penetration scheme. Simulations reproducing the experimental measurements indicate that, at the time of peak compression, about 1% of the short-pulse energy is coupled to a relatively low-density core with a radius of 70 μm. Analysis with the support of 2D particle-in-cell simulations uncovers the key factors improving this coupling efficiency. Our findings are of critical importance for optimizing FI experiments in a super-penetration scheme.

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.

Channel optimization of high-intensity laser beams in millimeter-scale plasmas

Channeling experiments were performed at the OMEGA EP facility using relativistic intensity (>10^{18}W/cm^{2}) kilojoule laser pulses through large density scale length (∼390-570 μm) laser-produced plasmas, demonstrating the effects of the pulse's focal location and intensity as well as the plasma's temperature on the resulting channel formation. The results show deeper channeling when focused into hot plasmas and at lower densities, as expected. However, contrary to previous large-scale particle-in-cell studies, the results also indicate deeper penetration by short (10 ps), intense pulses compared to their longer-duration equivalents. This new observation has many implications for future laser-plasma research in the relativistic regime.

Collimated Propagation of Fast Electron Beams Accelerated by High-Contrast Laser Pulses in Highly Resistive Shocked Carbon

Collimated transport of ultrahigh intensity electron current was observed in cold and in laser-shocked vitreous carbon, in agreement with simulation predictions. The fast electron beams were created by coupling high-intensity and high-contrast laser pulses onto copper-coated cones drilled into the carbon samples. The guiding mechanism-observed only for times before the shock breakout at the inner cone tip-is due to self-generated resistive magnetic fields of ∼0.5-1  kT arising from the intense currents of fast electrons in vitreous carbon, by virtue of its specific high resistivity over the range of explored background temperatures. The spatial distribution of the electron beams, injected through the samples at different stages of compression, was characterized by side-on imaging of hard x-ray fluorescence.

Systematic search for spherical crystal X-ray microscopes matching 1-25 keV spectral line sources

Spherical-crystal microscopes are used as high-resolution imaging devices for monochromatic x-ray radiography or for imaging the source itself. Crystals and Miller indices (hkl) have to be matched such that the resulting lattice spacing d is close to half the spectral wavelength used for imaging, to fulfill the Bragg equation with a Bragg angle near 90^∘ which reduces astigmatism. Only a few suitable crystal and spectral-line combinations have been identified for applications in the literature, suggesting that x-ray imaging using spherical crystals is constrained to a few chance matches. In this article, after performing a systematic, automated search over more than 9 × 10⁶ possible combinations for x-ray energies between 1 and 25 keV, for six crystals with arbitrary Miller-index combinations hkl between 0 and 20, we show that a matching, efficient crystal and spectral-line pair can be found for almost every He_α or K_α x-ray source for the elements Ne to Sn. Using the data presented here it should be possible to find a suitable imaging combination using an x-ray source that is specifically selected for a particular purpose, instead of relying on the limited number of existing crystal imaging systems that have been identified to date.

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.

Blind deconvolution for spatial distribution of Kα emission from ultraintense laser-plasma interaction

The spatial distributions of the Kα emission from foil targets irradiated with ultra-intensity laser pulses have been studied using the x-ray coded imaging technique. Due to the effect of hard x-ray background contamination, noise as well as imperfection of imaging system, it is hard to determine the PSF analytically or measure it experimentally. Therefore, we propose a blind deconvolution method to restore both the spatial distributions of the Kα emission and the system's PSF from the coded images based on the maximum-likelihood scheme. Experimental restoration results from penumbral imaging and ring coded imaging demonstrated that both the structure integrity and the rich detail information can be well preserved.

Scaling of resistive guiding of laser-driven fast-electron currents in solid targets

The resistive magnetic field plays a crucial role in determining the laser produced fast-electron transport in solid targets. The scaling of the resistive guiding is derived and benchmarked against two-dimensional collisional particle-in-cell simulations. We study the impact of the initial state of the material (Z dependence, conductor, or insulator) on global electron-transport patterns, and conclude that the initial state of a conductor or insulator is not important. Instead, global transport patterns depend on the material Z. The fast-electron transport seen in the simulations is consistent with the derived scaling rule. Previous experimental observations [e.g., R. B. Stephens et al., Phys. Rev. E 69, 066414 (2004) and Y. Sentoku et al., Phys. Rev. Lett. 107, 135005 (2011)] that show confinement or divergence in various regimes are also explained by our scaling. The presented scaling then becomes a useful tool to design compact radiation sources or fast ignitor experiments.

Deflection of MeV electrons by self-generated magnetic fields in intense laser-solid interactions

We show that the interaction of relativistic-intensity, picosecond laser pulses with solid targets is affected by the reflected light through the strong currents and 10(4)  T magnetic fields it produces. Three-dimensional particle-in-cell simulations, with the axisymmetry broken by a small angle of incidence, show that these magnetic fields deflect the laser-accelerated electrons away from the incident laser axis. This directly impacts the interpretation of electron divergence and directionality in applications such as laser-driven ion acceleration or fast-ignition inertial fusion.