Session UO5: Fast Ignition II

Status of the OMEGA EP Laser System

The performance and experimental capabilities of the OMEGA EP Laser System continue to improve. The system, with four NIF-like beamlines, was completed in April 2008. The beams can be operated at 351~nm, with each ultimately producing 6.5 kJ in a 10-ns pulse into the OMEGA EP target chamber. Two of the beams can be operated as high-energy petawatt (HEPW) lasers, each ultimately producing up to 2.6 kJ in a 1053-nm, 10-ps pulse. The HEPW beams can be directed into the OMEGA EP target chamber or into the 60-beam OMEGA target chamber for experiments that combine target compression with HEPW capability. The current and projected status of the laser system performance, laser and target diagnostics, and experimental capabilities will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Implosion of Cone-in-shell Target on Shenguang Laser Facilities

Experiments of indirectly driven implosion of cone-in-shell target for fast ignition have been carried out on Shenguang II and Sheguang III prototype laser facilities. Empty CD shells are imploded using 8 beams on both laser facilities. The neutron yield was measured by the scintillator detectors. 106$\sim $107 neutrons were obtained in the experiments. On Shenguang II, the 9th beam was used to generate a Ti backlighter. The backlit image was measured via a X-ray framing camera with a time-gated resolution of 60 ps. On Shenguang III prototype, only self-emission of X-ray from the imploded shell was measured.   

Deuteron Beam Driven Fast Ignition of a Pre-Compressed Inertial Confinement Fusion (ICF) Target

Fast Ignition (FI) is recognized as the most promising approach to achieving the high energy gain target performance needed for commercial inertial confinement fusion (ICF). Deuteron beams not only provide heating via linear energy transfer when slowing down, but can also provide extra ?bonus? fusion energy through reactions in the target. In view of the recent observation of ultra-high-density deuterium clusters, in addition to the extra energy gain expected from the deuterons, we estimate the impact of the added deposition energy from the deuteron beam to the target fuel based on calculations using a modified energy multiplication factor Fc. The deuteron beam energy deposition range and time are also calculated in order to estimate optimized deuteron initial energy. The aim of this study is to explore the potential advantages of a deuteron beam driven FI scheme for ICF. However, a much more comprehensive calculation is needed to realize a full 3D experimental design for maximum fusion gain.   

Fast Ignition with Laser-Driven Ion Beams

We report on the US research program on fusion fast ignition (FI) using laser-driven ion beams. Compressed DT fuel can ignite with beams of quasi-monoenergetic ions, such as carbon at $\sim $ 500 MeV. The FI simulations that set the requirements and guide our work are shown. The mechanisms that enable a short-pulse laser to drive the ignitor ion beam are summarized, especially how the $\sim $ GeV ion energies required are attained. We also aim to reduce the C-ion energy spread to $\sim $ 10{\%} over the energy range of $\sim $ 0.1 -- 1 GeV. The latest 1, 2 and 3 dimensional simulations on ion acceleration are shown. Those simulations guide our ion experimental work at the Los Alamos Trident laser facility. Our experimental results are summarized, followed by a discussion of the salient issues and the way our project is addressing them.   

Proton Beam Focusing Using Novel Structured Targets

For proton cone-guided FI, the proton source foil is placed within a cone, which can affect the proton beam properties (i.e. focusing, conversion efficiency and maximum energy). The effect of a surrounding structure on the proton beam properties has been investigated using the Trident laser at LANL delivering 80 J in 0.5 ps. High-density carbon hemispherical shells (600 $\mu m$ diameter, 10 $\mu m$ thick) were placed at the end of an aluminum structure, where the segment capped a cylindrical or conical hole and were compared to ``freestanding'' shells. A stack of RCF and a Cu mesh placed rear target normal was used to diagnose the beam properties. Results show that surrounding the hemisphere with a structure not only changed the maximum proton energy but also reduced the conversion efficiency. Comparison of experimental results with LSP modeling will be discussed.   

Short-Pulse Laser-Produced Electron Temperature, Divergence and Conversion Efficiency Measurements Using Multiple X-Ray Spectrometers

The bremsstrahlung radiation emitted from a target irradiated by high-intensity-short-pulse lasers carries information about the electron population. Electron conversion efficiency, temperature, beam divergence and pointing can all be calculated if multiple spectrometers are used. In the experiment described here, five X-ray spectrometers recorded bremsstrahlung radiation from 500 $\mu$m thick silver foils. Modeling of the target to these electrons was performed using Integrated Tiger Series 3.0 (ITS 3.0) code, allowing inference of the electron distribution from the bremsstrahlung emission. These experimental results are compared to the predictions of particle-in-cell (PIC) codes.   

Stochastic Ultra-Intense Laser-Plasma Interactions

Characterization of ultra-intense, short-pulse laser plasma experiments has evidenced fluctuations of up to 2x in electron number and spot size from K-edge emission [1]. Stochastic interaction of the laser with preplasma is possible, but there have not been sufficient repetitive measurements to investigate the cause. We report on an experiment designed for this issue on the Gemini-Astra laser at RAL. Its capability for 1~ppm, 10~J, 0.25~ps shots allowed 10 repeated, nominally identical shots during scans of prepulse conditions. Laser-plasma coupling was measured with x-ray spectrometers recording the fluorescence intensity from buried Cu and Ni layers. Both refluxing (35$\,\mu$m thick) and non-refluxing ($>$1000$\,\mu$m thick) foils were used; the former measured deposited energy, the latter the number of electrons. Care was used to minimize variance in target dimensions and fluorescence measurements so that measured variance could be related to the Laser Plasma interaction. Results will be discussed.\par \vskip6pt \noindent [1] RB Stephens, et al., Phys. Rev. E {\bf 69}, 066414 (2004).   

Simulations of spatially resolved K-alpha emission as a tool to measure the divergence of fast electrons relevant to Fast Ignition

The Fast Ignition (FI) concept of Inertial Confinement Fusion relies on fast electrons created from the laser-matter interaction to deposit their energy into the compressed target core and start the fusion burn. Divergence of those electrons is one of the most crucial parameters in FI. Spatially resolved, time-integrated K-alpha x-ray imagers have been used in the past as a way to measure the divergence of fast electrons. Since any electron with energy above some threshold could produce a K-alpha photon, the K-alpha emission distribution can be quite different from that of the fast electrons. We present the results of 2D simulations using the hybrid PIC code LSP on the study of spatially resolved K-alpha emission as a tool to measure fast electron divergence. Among our results, we find that the K-alpha images evolve with time due to refluxing electrons, well after the laser pulse has stopped, giving rise to K-alpha images that suggest a larger electron beam size than is actually present for the FI relevant electrons.   

Modeling the Modification of Escaping Electrons

Ultraintense laser interactions with solid density plasma involve significant transfer of energy to electrons, and the energy and angular distribution of these electrons play a vital role in the Fast Igniter approach to Inertial Fusion Energy. Electrons are typically measured through the generation of secondary radiation or by direct measurement of the electrons which escape the plasma. LSP simulations were performed to determine the extent of modification of the electron spectrum by transport through a dense plasma and the electromagnetic fields produced when electrons leave the plasma. The laser plasma interaction was modeled with an electron distribution based upon ponderomotive scaling and measured laser-to-electron energy efficiency. The simulations include time and space varying sheath fields, Ohmic fields, \textit{and ion acceleration}. The simulated energy spectrum as recorded by an electron spectrometer is found to differ significantly from the spectrum computed within the target. New results including the role of ion expansion will be presented.   

Angular moment model for the charge particle transport in radiation hydrodynamic codes

An accurate and rapid kinetic model, derived from the Fokker-Planck equation, describing the collisional transport of charged particles will be presented. It involves an angular closure in the phase space leading to a set of hyperbolic equations for the moments of the distribution function evolving in time, space and energy. This method provides an alternative to the prohibitive cost of a direct solution to the full kinetic equation. Moreover, it is exact for the limitings cases of collimated beams and quasi-isotropic distributions. The numerical solution can be approximated with the usual schemes of the non-linear hyperbolic analysis. The transport kinetic module is coupled with the radiation hydrodynamic code CHIC and comparisons with several published results related to the fast ignition with inserted cone will be presented. The model will be further improved in order to take into account self consistent fields that are important for the fast electron transport. \\[4pt] [1] Breil et al, J.Co.Ph. 224, 785 (2007)\\[0pt] [2] Dubroca et al, to be published in Eu.J.Ph.D   

Collisional and collisionless beam plasma instabilities

Collisions are a key issue regarding the instabilities involved in the Fast Ignition Scenario for Inertial Confinement Fusion. Because of the plasma density gradient through which the relativistic electron beam travels, unstable modes are collisionless at the beginning of the path, and collisional near the target core. While some works have been done on both regimes, the transition from the former to the later remains unclear. By implementing a hot fluid model accounting for a collisional return current, a theory is presented which bridges between the two regimes. The transition from one regime to the other is detailed in terms of the beam-to-plasma density ratio and the collision frequency. Purely collisional modes are found to arise at very low $k$, compared to the collisionless ones, and generate beam skin-depth size structures in accordance to previous works on resistive filamentation.   

Nuclear Physics in High Energy ICF Fast Ignition

In contradistinction to main stream fast ignition scenario (FIS) based on collisional stopping in compressed DT-fuel of relativistic electron beams(REB)in the 1-2 MeV energy range [1], we discuss the potentialities of ultra-relativistic REB stopped by a possible combination of Langmuir [2] and EMHD collective modes [3] altogether with numerous strongly inelastic electromagnetic reactions such as electron-positron Trident process as well as D and T electro disintegration in single nucleons and negative pion above 140 MeV. We focus on the trapping of this negative pion within a number of stable and Borromeanlike molecules built on a Debyelike interaction,able to initiate an ultrafast pion-catalysed cycle in the very dense compressed fuel at high temperature, thus preventing the occurence of the detriemental alpha particle sticking so common in cold medium. The Au cone used in FIS experiments may also help converting REB in very hard Gamma rays adding their efficient photo disintegration to DT electro disintegration.\\[4pt] [1] C.Deutsch et al PRL 77,2483(1996)\\[0pt] [2] V.M.Malkin and N.J.Fisch PRL 89,125004 [2002)\\[0pt] [3] T.Yabuuchi et al New J Phys 11,093031 (2009)