J.A. Marozas

Polar direct drive on the National Ignition Facility

Three recent developments in direct-drive target design have enhanced the possibility of achieving high target gain on the National Ignition Facility (NIF): (1) Laser absorption was increased by almost 50% using wetted-foam targets. (2) Adiabat shaping significantly increased the hydrodynamic stability of the target during the acceleration phase of the implosion without sacrificing target gain. (3) Techniques to reduce laser imprint using pulse shaping and radiation preheat were developed. These design features can be employed for direct-drive-ignition experiments while the NIF is in the x-ray-drive configuration. This involves repointing some of the beams toward the equator of the target to improve uniformity of target drive. This approach, known as polar direct drive (PDD), will enhance the capability of the NIF to explore ignition conditions. PDD will couple more energy to the fuel than x-ray drive. The compressed fuel core can be more easily accessed for high-ρR diagnostic development and for fast-ign...

Two-dimensional simulations of plastic-shell, direct-drive implosions on OMEGA

Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are investigated using the multidimensional hydrodynamic code, DRACO [D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)]. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength (l<10) modes due to surface roughness and beam imbalance and the intermediate modes (20⩽l⩽50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1D) predictions. The yield degradation in the thin shells is mainly due to shell breakup at short wavelengths (λ∼Δ, where Δ is the in-flight s...

Multidimensional analysis of direct-drive, plastic-shell implosions on OMEGA

Direct-drive, plastic shells imploded on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] with a 1ns square pulse are simulated using the multidimensional hydrodynamic code DRACO in yield degradation in “thin” shells is primarily caused by shell breakup during the acceleration phase due to short-wavelength (l>50, where l is the Legendre mode number) perturbation growth, whereas “thick” shell performance is influenced primarily by long and intermediate modes (l⩽50). Simulation yields, temporal history of neutron production, areal densities, and x-ray images of the core compare well with experimental observations. In particular, the thin-shell neutron production history falls off less steeply than one-dimensional predictions due to shell-breakup-induced undercompression and delayed stagnation. Thicker, more-stable shells show burn truncation due to instability-induced mass flow into the colder bubbles. Estimates of small-scale mix indicate that turbulent mixing does not influence p...

Improving the hot-spot pressure and demonstrating ignition hydrodynamic equivalence in cryogenic deuterium–tritium implosions on OMEGAa)

Reaching ignition in direct-drive (DD) inertial confinement fusion implosions requires achieving central pressures in excess of 100 Gbar. The OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] is used to study the physics of implosions that are hydrodynamically equivalent to the ignition designs on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. It is shown that the highest hot-spot pressures (up to 40 Gbar) are achieved in target designs with a fuel adiabat of α ≃ 4, an implosion velocity of 3.8u2009×u2009107u2009cm/s, and a laser intensity of ∼1015u2009W/cm2. These moderate-adiabat implosions are well understood using two-dimensional hydrocode simulations. The performance of lower-adiabat implosions is significantly degraded relative to code predictions, a common feature between DD implosions on OMEGA and indirect-drive cryogenic implosions on the NIF. Simplified theoretical models are developed to gain physical understanding of the implosion dynamics th...

Performance of direct-drive cryogenic targets on OMEGA

The success of direct-drive-ignition target designs depends on two issues: the ability to maintain the main fuel adiabat at a low level and the control of the nonuniformity growth during the implosion. A series of experiments was performed on the OMEGA Laser System [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to study the physics of low-adiabat, high-compression cryogenic fuel assembly. Modeling these experiments requires an accurate account for all sources of shell heating, including shock heating and suprathermal electron preheat. To increase calculation accuracy, a nonlocal heat-transport model was implemented in the 1D hydrocode. High-areal-density cryogenic fuel assembly with ρR>200mg∕cm2 [T. C. Sangster, V. N. Goncharov, P. B. Radha et al., “High-areal-density fuel assembly in direct-drive cryogenic implosions,” Phys. Rev. Lett. (submitted)] has been achieved on OMEGA in designs where the shock timing was optimized using the nonlocal treatment of the heat conductio...

Polar-direct-drive simulations and experiments

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] will allow direct-drive ignition experiments on the National Ignition Facility (NIF) [J. Paisner et al., Laser Focus World 30, 75 (1994)] as it is configured for x-ray drive. Optimal drive uniformity is obtained via a combination of beam repointing, pulse shapes, spot shapes, and∕or target design. This article describes progress in the development of standard and “Saturn” [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 0952002 (2005)] PDD target designs. Initial evaluation of experiments on the OMEGA Laser System [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)] and simulations were carried out with the two-dimensional hydrodynamics code SAGE [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005)]. This article adds to this body of work by including fusion particle production and transport as well as radiation transport within the two-dimensional DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] hydr...

Improving cryogenic deuterium–tritium implosion performance on OMEGA

A flexible direct-drive target platform is used to implode cryogenic deuterium–tritium (DT) capsules on the OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)]. The goal of these experiments is to demonstrate ignition hydrodynamically equivalent performance where the laser drive intensity, the implosion velocity, the fuel adiabat, and the in-flight aspect ratio (IFAR) are the same as those for a 1.5-MJ target [Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010)] designed to ignite on the National Ignition Facility [Hogan et al., Nucl. Fusion 41, 567 (2001)]. The results from a series of 29 cryogenic DT implosions are presented. The implosions were designed to span a broad region of design space to study target performance as a function of shell stability (adiabat) and implosion velocity. Ablation-front perturbation growth appears to limit target performance at high implosion velocities. Target outer-surface defects associated with contaminant gases in the DT fuel are identified as the dominant per...

Polar direct drive : Proof-of-principle experiments on OMEGA and prospects for Ignition on the National Ignition Facility

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11 2763 (2004)] shows promise for achieving direct-drive ignition while the National Ignition Facility (NIF) [E. M. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41 B39 (1999)] is initially configured for indirect drive. Experiments have been carried out using 40 repointed beams of the 60-beam OMEGA laser system [T. R. Boehly et al., Rev. Sci. Instrum. 66 508 (1995)] to approximate the NIF PDD configuration. Backlit x-ray framing-camera images of D2-filled spherical CH capsules show a characteristic nonuniformity pattern that is in close agreement with predictions. Saturn targets (similar capsules surrounded by a plastic ring) increase the drive on the equator, suggesting that highly symmetric PDD implosions may be possible with appropriate tuning. Two-dimensional (2D) simulations reproduced the approximately threefold reduction in yield found for the non-Saturn PDD capsules. Preliminary simulations for a NIF Saturn design predict a high ...

Shock-tuned cryogenic-deuterium-tritium implosion performance on Omega

Cryogenic-deuterium-tritium (DT) target compression experiments with low-adiabat (α), multiple-shock drive pulses have been performed on the Omega Laser Facility [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to demonstrate hydrodynamic-equivalent ignition performance. The multiple-shock drive pulse facilitates experimental shock tuning using an established cone-in-shell target platform [T. R. Boehly, R. Betti, T. R. Boehly et al., Phys. Plasmas 16, 056301 (2009)]. These shock-tuned drive pulses have been used to implode cryogenic-DT targets with peak implosion velocities of 3×107u2002cm/s at peak drive intensities of 8×1014u2002W/cm2. During a recent series of α∼2 implosions, one of the two necessary conditions for initiating a thermonuclear burn wave in a DT plasma was achieved: an areal density of approximately 300u2002mg/cm2 was inferred using the magnetic recoil spectrometer [J. A. Frenje, C. K. Li, F. H. Seguin et al., Phys. Plasmas 16, 042704 (2009)]. The other condition—a burn-averaged ion temperature ⟨Ti⟩n of 8–10 keV—cannot be achieved on Omega because of the limited laser energy; the kinetic energy of the imploding shell is insufficient to heat the plasma to these temperatures. A ⟨Ti⟩n of approximately 3.4 keV would be required to demonstrate ignition hydrodynamic equivalence [Betti et al., Phys. Plasmas17, 058102 (2010)]. The ⟨Ti⟩n reached during the recent series of α∼2 implosions was approximately 2 keV, limited primarily by laser-drive and target nonuniformities. Work is underway to improve drive and target symmetry for future experiments.Cryogenic-deuterium-tritium (DT) target compression experiments with low-adiabat (α), multiple-shock drive pulses have been performed on the Omega Laser Facility [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to demonstrate hydrodynamic-equivalent ignition performance. The multiple-shock drive pulse facilitates experimental shock tuning using an established cone-in-shell target platform [T. R. Boehly, R. Betti, T. R. Boehly et al., Phys. Plasmas 16, 056301 (2009)]. These shock-tuned drive pulses have been used to implode cryogenic-DT targets with peak implosion velocities of 3×107u2002cm/s at peak drive intensities of 8×1014u2002W/cm2. During a recent series of α∼2 implosions, one of the two necessary conditions for initiating a thermonuclear burn wave in a DT plasma was achieved: an areal density of approximately 300u2002mg/cm2 was inferred using the magnetic recoil spectrometer [J. A. Frenje, C. K. Li, F. H. Seguin et al., Phys. Plasmas 16, 042704 (2009)]. The other condition—a burn...

Triple-picket warm plastic-shell implosions on OMEGA

Warm deuterium-gas-filled plastic shells are imploded by direct irradiation from the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The pulse shapes contain three pickets that precede a sharp rise to a constant laser intensity at ∼4.5×1014u2002W/cm2. The in-flight-aspect-ratio (IFAR), a crucial measure of shell instability to nonuniformity growth, is varied in these implosions by changing picket energies and the timing among the pickets. Simulations that include cross-beam energy transfer in addition to inverse bremsstrahlung for the laser-energy deposition models show better agreement with measurements of the neutron bang time and temporally resolved scattered light and therefore more correctly model the shell kinetic energy. It is also shown that target performance improves significantly as IFAR is reduced. Nearly twice the neutron yield is measured for IFAR∼31 compared to IFAR∼60. The ratio of the measured to simulated neutron yield and areal density increases significantly with decreasin...