J.W. Bates

Shock ignition target design for inertial fusion energy

Continuing work in the design of shock ignition targets is described. Because of reduced implosion velocity requirements, low target adiabats, and efficient drive by short wavelength lasers, these targets produce high gain (>100) at laser energies well below 1 MJ. Effects of hydrodynamic instabilities such as Rayleigh–Taylor or Richtmyer–Meshkov are greatly reduced in these low-aspect ratio targets. Of particular interest is the optimum ratio of ignitor to compression pulse energy. A simple pellet model and simulation-derived coupling coefficients are used to analyze optimal fuel assembly, and determine that shock ignition allows enough control to create theoretically optimum assemblies. The effects on target design due to constraints on the compression and ignitor pulse intensities are also considered and addressed. Significant sensitivity is observed from low-mode perturbations because of large convergence ratios, but a more powerful ignitor can mitigate this.

Acceleration to high velocities and heating by impact using Nike KrF laser

The Nike krypton fluoride laser [S. P. Obenschain, S. E. Bodner, D. Colombant, et al., Phys. Plasmas 3, 2098 (1996)] is used to accelerate planar plastic foils to velocities that for the first time reach 1000 km/s. Collision of the highly accelerated deuterated polystyrene foil with a stationary target produces ∼Gbar shock pressures and results in heating of the foil to thermonuclear temperatures. The impact conditions are diagnosed using DD fusion neutron yield, with ∼106 neutrons produced during the collision. Time-of-flight neutron detectors are used to measure the ion temperature upon impact, which reaches 2–3 keV.

Polar-direct-drive experiments on the National Ignition Facilitya)

To support direct-drive inertial confinement fusion experiments at the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)] in its indirect-drive beam configuration, the polar-direct-drive (PDD) concept [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] has been proposed. Ignition in PDD geometry requires direct-drive–specific beam smoothing, phase plates, and repointing the NIF beams toward the equator to ensure symmetric target irradiation. First experiments to study the energetics and preheat in PDD implosions at the NIF have been performed. These experiments utilize the NIF in its current configuration, including beam geometry, phase plates, and beam smoothing. Room-temperature, 2.2-mm-diam plastic shells filled with D2 gas were imploded with total drive energies ranging from ∼500 to 750 kJ with peak powers of 120 to 180 TW and peak on-target irradiances at the initial target radius from 8 × 1014 to 1.2 × 1015 W/cm2. Results from these initial experi...

DIRECT DRIVE FUSION ENERGY SHOCK IGNITION DESIGNS FOR SUB-MJ LASERS

New approaches in target design have increased the possibility that useful fusion power can be generated with sub-MJ lasers. We have performed many 1D and 2D simulations that examine the characteristics of target designs for sub-MJ lasers. These designs use the recently-proposed shock-ignition target scheme, which utilizes a separate high-intensity pulse to induce ignition. A promising feature of these designs is their significantly higher gains at lower energies (one dimensional (1D) gain ˜ 100 at Elaser ˜ 250kJ) than can be expected for the conventional central ignition scheme. The results of these simulations are shown and we discuss the implications for target fabrication and laser design. Of particular interest are the constraints on the target and laser from asymmetries due to target imperfections and laser imprint.

Direct-drive laser target designs for sub-megajoule energies

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength to lower the laser energy required for substantial gain (>30×) to sub-MJ level. These low laser-energy pellets are useful in systems that could form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [S. P. Obenschain et al., Phys. Plasmas 13, 056320 (2006)]. The short wavelength laser should allow higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities to be achieved while keeping the low aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500kJ. A target design is presented with various laser pulse shapes and both 1D and 2D simulation results are shown. The sensitivity of these targets to both low-mode and high-mode perturbations is examined. The analysis and simulations in this paper indicate that significant gain (G=57) can be a...

Instability of isolated planar shock waves

Previously, expressions governing the temporal evolution of linear perturbations to an isolated, planar, two-dimensional shock front in an inviscid fluid medium with an arbitrary equation of state were derived using a methodology based on Riemann invariants and Laplace transforms [J. W. Bates, Phys. Rev. E 69, 056313 (2004)]. An overlooked yet immediate consequence of this theory is that the stability limits of shocks can be readily determined from an inspection of the poles of the transformed ripple amplitude. Here, it is shown that two classes of instabilities exist for isolated planar shock waves: one in which perturbations grow exponentially in time, and the other in which disturbances are stationary. These results agree with those derived by D’yakov and Kontorovich (by more arduous and somewhat ambiguous means), and serve as an important addendum to our earlier analysis.

Numerical simulations of the ablative Rayleigh-Taylor instability in planar inertial-confinement-fusion targets using the FastRad3D code

The ablative Rayleigh-Taylor (RT) instability is a central issue in the performance of laser-accelerated inertial-confinement-fusion targets. Historically, the accurate numerical simulation of this instability has been a challenging task for many radiation hydrodynamics codes, particularly when it comes to capturing the ablatively stabilized region of the linear dispersion spectrum and modeling ab initio perturbations. Here, we present recent results from two-dimensional numerical simulations of the ablative RT instability in planar laser-ablated foils that were performed using the Eulerian code FastRad3D. Our study considers polystyrene, (cryogenic) deuterium-tritium, and beryllium target materials, quarter- and third-micron laser light, and low and high laser intensities. An initial single-mode surface perturbation is modeled in our simulations as a small modulation to the target mass density and the ablative RT growth-rate is calculated from the time history of areal-mass variations once the target rea...

Developments in Direct Drive Laser Fusion

Abstract Recent designs for laser driven, direct drive inertial confinement fusion (ICF) indicate that substantial gains (G>100) might be achieved with lower total laser energy (E~500 kJ) than previously considered possible. A leading contender is the shock ignition approach which compresses low aspect ratio pellets with high intensity laser pulses (1015 W/cm2) before achieving ignition with a final higher intensity spike (1016 W/cm2). Excimer laser systems based on a krypton-fluoride (KrF) medium are particularly well suited to these new ideas as they operate in the ultraviolet (248 nm), provide highly uniform illumination, possess large bandwidth (1-3 THz), and can easily exploit beam zooming to improve laser-target coupling for the final spike pulse. This paper will examine target physics advantages of KrF lasers in relation to the new implosion designs and the balancing of hydrodynamic instability and laser-plasma instabilities. Supporting experimental and theoretical studies of are being conducted by the Nike laser group at the U. S. Naval Research Laboratory. Recent experimental work has also shown that the high ablation pressures and smooth profiles obtained with the Nike laser can be used to accelerate planar targets to velocities consistent with the requirements of impact ignition.

Progress of impact ignition

Recent progress of impact ignition is reported: First, a maximum velocity ∼ 1000 km/s has been achieved under the operation of NIKE KrF laser at Naval Research Laboratory (laser wavelength = 0.25μm) in the use of a planar target made of plastic. Two-dimensional simulation have been performed for burn and ignition to show the feasibility of the impact ignition. Optimized direct illumination scheme is also addressed.

Response to “Comment on ‘Instability of isolated planar shock waves’” [Phys. Fluids 20, 029101 (2008)]

In a recent article [J. W. Bates, “Instability of isolated planar shock waves,” Phys. Fluids 19, 094102 (2007)], we derived linear instability criteria for an isolated, planar, two-dimensional shock wave propagating through an inviscid fluid with an arbitrary equation of state. The basis for this analysis was a novel solution for the time-dependent Fourier amplitude of a single-mode perturbation on the front, which was expressed in the form of a Volterra equation. In the comment by Tumin [“Comment on ‘Instability of isolated planar shock waves’,” Phys. Fluids 20, 029101 (2008)], the author demonstrated the consistency of our results with those of Erpenbeck, whose mathematical approach avoided the derivation of an integral equation in the time domain, but required a complicated, inverse Laplace-transform operation to ascertain the temporal evolution of disturbances at the shock’s surface. Here, we emphasize that such information is obtained more readily from a direct solution of the aforementioned Volterra...