Session BO3: Fast Ignition

Isochoric Implosions for Fast Ignition

Various gain models have shown the potentially great advantages of Fast Ignition (FI) Inertial Confinement Fusion (ICF) over its conventional hotspot ignition counterpart. These gain models, however, all assume nearly uniform-density fuel assemblies. By contrast, typical ICF implosions yield hollowed fuel assemblies with a high-density shell of fuel surrounding a low-density, high-pressure hotspot. To realize fully the advantages of FI, then, an alternative implosion design must be found which yields nearly isochoric fuel assemblies without substantial hotspots. Here, it is shown that a self-similar spherical implosion of the type originally studied by Guderley [Luftfahrtforschung \textbf{19}, 302 (1942)] may be employed to yield precisely such quasi-isochoric imploded states. The difficulty remains, however, of accessing these self-similarly imploding configurations from initial conditions representing an actual ICF target, namely a uniform, solid-density shell at rest. Furthermore, these specialized implosions must be realized for practicable drive parameters, $i.e.$, accessible peak pressures, shell aspect ratios, etc. An implosion scheme is presented which meets all of these requirements, suggesting the possibility of genuinely isochoric implosions for FI.   

Gain Curves for Fast-Ignition Inertial Confinement Fusion

Hydrodynamic simulations of realistic high-gain fast-ignition targets,\footnote{ R. Betti and C. Zhou, Phys. Plasmas \textbf{12}, 110702 (2005).} including one-dimensional simulations of the implosion and two-dimensional simulations of ignition by electron beams and burn propagation, have been performed. Based on these simulations, a maximum gain curve has been generated that can be used to assess the benefits of fast ignition with respect to conventional hot-spot ignition. Two-dimensional simulations of pseudo cone targets (assuming that the cone walls are rigid and truncated at a given distance from the center) have also been carried out, to determine the gain deterioration due to the cone. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC02-04ER54789 and DE-FC52-92SF19460.   

Fast-Ignition Fuel Assembly: Theory and Experiments

Scaling relations for fast-ignition fuel assembly are derived both analytically and numerically. The stagnation properties of fuel assembly exclusively depend on the in-flight adiabat, implosion velocity, and driver intensity and energy. Since both density and areal density increase with decreasing adiabat, a high-density and areal-density fuel assembly can be generated by a low-adiabat, low-velocity implosion. This requires massive capulses with an initial aspect ratio of about 2. The implosion of such thick shells leads to high energy gains. Implosions driven by 25 kJ to 2 MJ of UV laser energy are simulated in one and two dimensions. Based on the scaling laws, an optimized laser pulse is designed to perform low-speed, low-adiabat implosions on OMEGA. CH shells of 40-\textit{$\mu $}m thickness and 0.86-mm diameter filled with various gas pressures are imploded to reach a peak areal density of 0.3 g/cm$^{2}$. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. ER54789 and DE-FC52-92SF19460.   

High-Areal-Density, Fuel-Assembly Experiments for the Fast-Ignitor Concept

Fast-ignition targets must be imploded to high-areal densities, $\sim $0.5 g/cm$^{2}$, to stop either $\sim $1-MeV electrons or $\sim $18-MeV protons, generated by an intense ultrashort laser pulse. Simulations have shown that high-density and high-areal-density fuel assembly can be achieved by imploding thick cryogenic shells with low velocity on a low adiabat.\footnote{ R. Betti and C. Zhou, Phys. Plasmas \textbf{12}, 110702 (2005).} A scaled noncryogenic version of the proposed design$^{1}$ was tested experimentally. Fuel-assembly experiments using 40-\textit{$\mu $}m-thick, 0.9-mm-diam plastic shells filled with various gas pressures were performed on the OMEGA Laser Facility, using an optimized low-speed spherical implosion. High-areal densities with temporally and spatially averaged values of $\sim $130 mg/cm$^{2}$ were measured with proton wedged range filters\footnote{ F. H. S\'{e}guin \textit{et al}., Rev. Sci. Instrum. \textbf{74}, 975 (2003).} for D$_{2}$ and D$^{3}$He fills of various pressures in the range from 10 to 33 bar. The areal densities compare favorably to one-dimensional, hydrodynamic-simulation predictions if the measured temporal-neutron-production history is taken into account. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Present Status of FIREX Project for Ignition and Burn

The first phase of the FIREX (Fast Ignition Realization Experiment) project was officially approved by Japanese Government on January 2003. The goal of FIREX-I is to demonstrate fast heating of a fusion fuel up to the ignition temperature of 5-10 keV. Although the fuel of FIREX-I is too small to be actually ignited, sufficient heating will provide the scientific viability of ignition-and-burn by increasing the laser energy thereby increasing the fuel size. Based on the result of FIREX-I, the decision of the start of FIREX-II to achieve ignition-and-burn will be made. The FIREX program is being carried out by collaboration of the Institute of Laser Engineering, Osaka University and the National Institute for Fusion Science, including development of cryogenic targets, holistic simulation systems, and diagnostic equipment. We have recently made high energy of 10 kJ from the heating laser. The segmentation of gratings for pulse compressors has been demonstrated. The cryogenic target development and diagnostic development will be extensively reviewed. We also consider hydrodynamic heating (impact fast ignition) as an alternative heating mechanism to those based on fast electrons and/or ions.   

Collimation of laser-induced high energy density electrons in imploded cylinder plasmas

We have studied propagation of high energy density electrons in long dense plasmas interacted with ultra-intense laser light. The experiments have been carried out with implosion of a cone attached hollow cylinder target. Ultra-intense laser light with an energy of 120J and a pulse duration of 1ps has been injected into the cone target to heat the imploded cylindrical imploded dense plasmas with the energetic electrons generated at the cone tip. Heating of the imploded plasma has been proved with measurements of thermal neutrons to be $2\times 10^5$ indicating efficient coupling of 15-20{\%} of the laser energy. The results imply that collimation of the high energy density electrons in the cylindrical imploded plasma with a length of 300$\mu $m. PIC simulations also predict the collimation of the electrons in the long dense plasmas with a magnetic field induced by spatial gradient of resistivity due to the temperature gradient.   

Increased Hot Electron Production with Low-Density Gold Foams for Fast Ignition

Foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition is proposed. A thin low-density gold foam is to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.   

2-D Simulations of OMEGA Fast-Ignition Cone Targets

The fast-ignition (FI) concept requires the assembly of a dense fuel, while allowing access for a high-energy ignitor beam. One approach to FI relies on the insertion of a high-density gold cone into the capsule, with its tip near the center of the capsule, allowing the injection of the ignitor beam close to the compressed high-density fuel mass. The insertion of the gold cone may degrade the uniformity of the assembled fuel, however, possibly degrading the yield. Two-dimensional (2-D) simulations of FI cone implosions are required to characterize nonuniformities introduced by the FI cone, which will allow the optimization of cone implosions on the OMEGA laser system. Current work on 2-D FI cone-in-shell simulations is presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Hydrodynamic Simulations of Integrated Fast-Ignition Experiments Planned for OMEGA/OMEGA EP Laser Systems

Integrated fast-ignition experiments for the combined OMEGA/OMEGA EP laser systems have been simulated with the multidimensional hydrodynamic code \textit{DRACO} using a straight-line electron-transport model. The model has been improved to include the effects of blooming and straggling. Electric fields caused by return currents, a Gaussian spatial-source profile, and beam divergence have been included. Simulations of an OMEGA cryogenic DT target designed to reach a 1-D fuel \textit{$\rho $R} of 500 mg/cm$^{2}$ have been carried out in 2-D (with and without perturbations) to assess the sensitivity to energy, timing, and irradiance of the fast-ignitor beam. A moderate increase in neutron yield caused by the heating beam is observed for nonperturbed targets. This increase is larger for perturbed targets. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Proton Beam Source Suitable for Fast Ignition Targets

A focused proton beam, which has potential to ignite fast ignition targets, is generated from a sharply defined metal-vacuum interface facing the compressed fuel kernel. Since it must be quite close (100s of $\mu$m) to that kernel to limit time spread of the proton beam, this surface must be protected from the shell implosion by a reentrant cone. The walls of that cone modify the proton beam by limiting the accelerating area and, potentially, refocusing the protons, particularly the low energy component. We have examined the effect of such walls using a test setup and will report on the results.   

Proton acceleration with high intensity lasers interacting on micro-cone targets

In the last few years, intense research has been conducted on laser-accelerated ion sources and their applications [1,2]. Proton beams accelerated from solid planar targets have exceptional properties that open new opportunities for ion beam generation and control. Experiments conducted at LANL and LULI have shown that high intensity lasers interacting on micro-cone targets can produce proton beams more collimated and more energetic than with planar targets. These micro-cone targets are composed of a curved cone attached to a micro-table. 2D PIC simulations were performed to understand the experiments and separate the effect of the cone from the effect of the micro-table. These new targets could help increase the laser-accelerated protons maximum energy to the 100 MeV range. [1] J. Fuchs et al., Nature Physics 2, 48 (2006). [2] T.Toncian et al., Science Vol. 312, 21 April 2006, p.410-413.   

The LANL Research Program on Ion-Based Fast Ignition

A few LANL research programs are engaged in activities that are being coordinated as a research program to evaluate fusion fast ignition (FI) initiated by laser-driven ion beams heavier than protons. Compared to protons, FI based on heavier ions has the potential advantage of a more localized energy deposition, which could minimize the required total ion-beam energy ($\sim $ 10 kJ). This FI scheme also requires about 100-fold fewer ions to deliver the necessary energy to ignite. Key ingredients necessary to implement this scheme include the generation of a sufficiently monoenergetic beam, at a sufficiently high ion energy, along with a sufficiently high conversion efficiency of laser to beam energy. Moreover, a better understanding of ion stopping power in dense matter is necessary. The research elements addressing these issues, both in the area of theory and experiment, are summarized in this presentation.   

Above scaling short-pulse ion acceleration from flat foil and ``Pizza-top Cone'' targets at the Trident laser facility

Ion-driven Fast Ignition (IFI) has certain advantages over electron-driven FI due to a possible large reduction in the amount of energy required. Recent experiments at the Los Alamos National Laboratory's Trident facility have yielded ion energies and efficiencies many times in excess of recent published scaling laws, leading to even more potential advantages of IFI. Proton energies in excess of 35 MeV have been observed from targets produced by the University of Nevada, Reno - dubbed ``Pizza-top Cone'' targets - at intensities of only 1x10$^{19}$ W/cm$^{2}$ with 20 joules in 600 fs. Energies in excess of 24 MeV were observed from simple flat foil targets as well. The observed energies, above any published scaling laws, are attributed to target production, preparation, and shot to shot monitoring of many laser parameters, especially the laser ASE prepulse level and laser pulse duration. The laser parameters are monitored in real-time to keep the laser in optimal condition throughout the run providing high quality, reproducible shots.   

Laser-Driven Super-High Velocity Targets for Impact Fast Ignition

In Impact Fast Ignition\footnote{M. Murakami et al., Nucl. Instrum. Meth. Phys. Res. A \textbf{544}, 67 (2005).} (IFI), a compressed main fuel is ignited by impact collision of a fragment of separately imploded fuel (impactor). A most critical requirement for the IFI is to achieve a super-high velocity (1000 km/s) of the impactor to form an igniting hot spot by converting the imploding kinetic energy into its own thermal energy. One then needs to substantially suppress Rayleigh-Taylor (RT) instability for a stable acceleration of the target. The super-high velocity is achieved by utilizing such a suppression technique of RT instability as double ablation in high-Z doped targets.\footnote{S. Fujioka et al., Phys. Rev. Lett. \textbf{92}, 195001 (2004).} We will present the experimental results of the laser-driven planar targets for the IFI.   

Neutron Generation in Colliding Foils for Impact Fast Ignition

We have proposed a new ignition scheme of Fast Ignition, called ``Impact Fast Ignition\footnote{M. Murakami et al., Nucl. Instrum. Meth. Phys. Res. A \textbf{544}, 67 (2005).} (IFI)'', in which a compressed fuel is ignited by impact collision of a fragment of separately imploded fuel. We have started the experiment that used CD colliding foils as the fundamental experiment of the IFI. We used the target which consists of two CD foils (thickness of 20 $\mu $m) with a 600 $\mu $m separation. We have irradiated one of the CD foils by a drive-laser (the energy of 2 kJ) and accelerated. The accelerated CD foil collides with another foil, and the neutrons were generated by the nuclear fusion reaction on the heated foil. In this experiment, we measured the neutron yield of 10$^{6}$. We will present the experimental details and results.