Targets meant to achieve ignition on the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] have been redesigned and their performance simulated. Simulations indicate dramatically reduced growth of short wavelength hydrodynamic instabilities, resulting from two changes in the designs. First, better optimization results from systematic mapping of the ignition target performance over the parameter space of ablator and fuel thickness combinations, using techniques developed by one of us (Herrmann). After the space is mapped with one-dimensional simulations, exploration of it with two-dimensional simulations quantifies the dependence of instability growth on target dimensions. Low modes and high modes grow differently for different designs, allowing a trade-off of the two regimes of growth. Significant improvement in high-mode stability can be achieved, relative to previous designs, with only insignificant increase in low-mode growth. This procedure produces capsule designs that, in simulations, tolerate several times the surface roughness that could be tolerated by capsules optimized by older more heuristic techniques. Another significant reduction in instability growth, by another factor of several, is achieved with ablators with radially varying dopant. In this type of capsule the mid-Z dopant, which is needed in the ablator to minimize x-ray preheat at the ablator-ice interface, is optimally positioned within the ablator. A fabrication scenario for graded dopants already exists, using sputter coating to fabricate the ablator shell. We describe the systematics of these advances in capsule design, discuss the basis behind their improved performance, and summarize how this is affecting our plans for NIF ignition.

A copper doped beryllium ablator capsule design is geometrically scaled from 190 kJ to 600 kJ absorbed energy for use as an ignition capsule driven at 250 eV on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)]. The capsule design was previously optimized for 190 kJ fixed capsule absorbed energy. The optimization is confirmed at 377 kJ. Two-dimensional simulations are reported that determine surface roughness requirements and tolerance to radiative drive asymmetry over this absorbed energy range.

The stability of a capsule intended to produce ignition on the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] is examined. Sensitivity to target fabrication defects in spherical harmonics modes L⩽12 is quantified in terms of a mode dependent tolerance to surface perturbations. Simulations of NIF capsule implosions with single mode perturbations on a single surface allow the determination of modal growth factors. Simulations with large initial perturbations were also done to determine how large a final perturbation could be tolerated. Combining the growth factors and tolerances determines specifications on initial perturbations. This allows an estimate of modal tolerance for each capsule surface and thickness variation for a polyimide ablator capsule design with central gas fills of 0.3 or 0.6 mg/cm3.

Target designs are described that are meant to achieve ignition on the National Ignition Facility. Simulations of recent indirect drive cryogenic capsule designs indicate dramatically reduced growth of short wavelength hydrodynamic instabilities, resulting from two changes in the designs. First, better optimization results from systematic mapping of the ignition target performance over the parameter space of ablator and DT-ice thickness combinations, using techniques developed by one of us (Herrmann). After the space is mapped with one-dimensional simulations, exploration of it with two-dimensional simulations quantifies the dependence of instability growth on target dimensions. Low modes and high modes grow differently in different regions of the space, allowing a trade-off of the two regimes of growth. Significant improvement in high-mode stability can be achieved, relative to previous designs, with only insignificant increase in low-mode growth. This procedure produces capsule designs that, in simulations, tolerate several times the surface roughness that could be tolerated by capsules optimized by older more heuristic techniques. Another significant reduction in instability growth, by another factor of several, is achieved with ablators with 'graded dopants.' In this type of capsule the mid-Z dopant, which is needed in the ablator to minimize x-ray preheat at the ablator-ice interface, ismore » optimally positioned within the ablator. A fabrication scenario for graded dopants already exists, using sputter coating to fabricate the ablator shell. We describe the systematics of these advances in capsule design, discuss the basis behind their improved performance, and summarize how this is affecting our plans for NIF ignition.« less

Optimized performance of a capsule intended to produce ignition on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] is presented. Performance is optimized, for a 250 eV isotropic drive on a beryllium(copper) ablator, by varying the ablator outside radius, ablator thickness, the concentration of copper dopant in the ablator, and the fuel thickness, while keeping the absorbed energy fixed. Dopant concentration is constrained to be uniform in the ablator. The drive shock timing is adjusted to produce a low entropy implosion for each set of dimensions. The absorbed energy is kept fixed at 190 kJ, which results in the ablator outside radius remaining practically constant, about 0.137 cm. For capsule geometry near that resulting in optimal implosion yield, the absorbed energy depends only slightly on the ablator or fuel thickness. The parameter space of capsule dimensions was searched for central vapor densities of 0.3 and 0.5 mg/cc. Despite the detailed optimization, it is found that the capsule is notably more unstable than comparable capsules with a graded dopant in the ablator, as reported in previous literature.

Studies on simulation and design of ignition targets for the National Ignition Facility (NIF) are described. Recent effort has emphasized the systematic exploration of the parameter space of possible ignition targets, providing comparisons as specific as possible between the various targets. This study aims at providing guidance for target fabrication R&D, and for other elements of the ignition program. Targets are being considered that span 250350 eV drive temperatures, capsule energies from 150 to 600 U, cocktail and gold hohlraum spectra, and three ablator materials (Be[Cu], CH[Ge] and polyimide). Capsules with graded doped beryllium ablators are found to be very stable with respect to short-wavelength Rayleigh-Taylor growth. Sensitivity to ablator roughness, ice roughness and asymmetry is being explored, as it depends on ablator material, drive temperature and absorbed energy. Three-dimensional simulations are being used to ensure adequate radiation symmetry in three dimensions (3D), and to ensure that coupling of 3D asymmetry and 3D Rayleigh-Taylor does not adversely affect planned performance. Integrated 3D hohlraum simulations indicate that 3D features in the laser illumination pattern affect the hohlraums' performance, and the hohlraum has been redesigned to accommodate these effects.

The J = (3/2) Δ, J = 1/2 Nucleon mass difference shows the quark energies can be spin dependent. It is natural to expect that the quark wave functions also depend on spin. A spin-dependent quark force is fitted to the proton and neutron magnetic moments, axial charge, and spin content using a (1/2 + ) 3 configuration for the quarks and assuming only zero mass u and d quarks are in the nucleon. In the octet, such spin-dependent forces lead to different wave functions for quarks with spin parallel or antiparallel to the nucleon spin. The eigen-energy of this potential is 0.15 GeV higher for quark spin parallel than for the quark spin antiparallel to the proton spin. This potential predicts a single quark energy of 0.37 GeV for mass-less quarks in the Delta. Assuming the quark forces are flavor independent, this potential predicts magnetic moments of a bound strange quark to be very close to those determined empirically from the octet magnetic moments.

The J=3/2 Δ, J=1/2 nucleon mass difference shows that quark energies can be spin dependent. It is natural to expect that quark wave functions also depend on spin. In the octet, such spin dependent forces lead to different wave functions for quarks with spin parallel or antiparallel to the nucleon spin. A two component Dirac equation wave function is used for the quarks assuming small current quark masses for the u and d quarks. Then, the neutron/proton magnetic moment ratio, the nucleon axial charge, and the spin content of the nucleon can all be simultaneously fit assuming isospin invariance between the u and d quarks, but allowing for spin dependent forces. The breakdown of the Coleman–Glashow sum rule for octet magnetic moments follows naturally in this Dirac approach as the bound quark energy also effects the magnetic moment. Empirically the bound quark energy increases with the number of strange quarks in the system. Allowing the strange quark wave function similar spin dependence predicts the magnetic moments of the octet, in close agreement with experiment. Differences between the octet and decuplet magnetic moments are also explained immediately with spin dependent wave functions.

The spin crisis of the nucleon is that the quark spin contribution is only a small fraction of the nucleon spin. A relativistic Dirac equation approach is followed assuming three low mass current quarks in the nucleon described by a (1/2 + ) 3 configuration. If the lower component contribution to the normalization of the quark wave function is about 0.18, then the axial charge of the nucleon can be reproduced. However including the same lower component to every quark wave function is not enough to resolve the spin crisis. The net u quark spin z component is predicted as 1.0 and the net d quark spin z component is predicted as −0.25, both in disagreement with experiment. These predictions can be brought into agreement with experiment if flavor independent but spin dependent forces are assumed between the quarks. The strength of the spin dependent force found by empirically fitting the nucleon spin data is shown to be comparable to the spin dependence that can explain the Δ-nucleon mass difference. The spin content of the Δ + is then predicted using the interactions that reproduce the spin content of the proton.

This paper is a general update on the target fab specs for ignition targets for the National Ignition Facility. A general overview of the status of all the requirements is presented, with a historical summary plus recent results. Historical work will be related to recent systematization of the characterization as coordinated by Rich Stephens. New results to be presented include analysis of a polystrene-ablator target, analysis of Rayleigh-Taylor growth on beryllium targets driven at 250eV at various scales, simulations of the effect of fill tubes on the implosion, considerations of hohlraum and capsule size variations, and variations in DT gas density (i.e. capsule temperature).

A variant of a squared three-body Dirac equation is used to determine center-of-mass energy effects in independent particle motion approximations for three quarks in the nucleon. A scalar linear flux tube potential is used to confine the quarks. The relativistic nearly massless three-quark system, in the rest frame where the total momentum is zero, has a squared energy that is 3/5 the value compared to when the quarks are assumed to move independently. This is smaller than the 2/3 energy ratio determined using the non-relativistic harmonic oscillator model. This analytic model has one parameter, the flux tube constant. Choosing the flux tube constant to reproduce the proton rest energy, results in the analytic wave function well reproducing the proton axial charge and rms charge radius. The proton magnetic moment predicted is 2.235, lower than experiment.

The Roper resonances of the nucleon are described as transverse vibrations of astretched flux tube between the three quarks. The proton is modeled using currentmass quarks interacting with a confining linear flux tube potential plus thespin-dependent parts of a one-gluon exchange potential. The proton ground state hasno vibrations and the confining flux tube has the minimum length required toconnect the three quarks. The flux tube has a V or Y shape of two or threesegments, depending on the locations of the quarks. The vibrations of the fluxtube have nodes at the quarks and at the apex of the Y-shaped configuration andprovide the vibrational excitation energy to describe the proton excitations. Theamplitude of the transverse vibrations is found from a geometric analysis, anddepends on the string constant of the flux tube potential. The Roper 1. 440-GeVresonance energy is very nearly reproduced by the vibration with mode number1 acting in only one segment of the flux tube. The vibration with mode number2 in one segment of the flux tube closely reproduces the second proton excitationat 1.710 GeV. The δD excited states are also well reproduced by these modes ofa vibrating flux tube.

A three-quark shell model of the nucleon is used to calculate analytically thepolarizability using a scalar linear flux tube potential with no one-gluon exchangepotential included. A value of 12.02 × 10 −4 f 3 is obtained for the proton, ingood agreement with experiment, with the flux tube constant adjusted to reproducethe proton Δ average rest energy. The magnetic polarizability of the proton isthen calculated as 1.51 × 10 −4 f 3 , which is in agreement with the experimentalvalue. The neutron/proton electric polarizability ratio is calculated as 2/3, andthe neutron electric polarizability is predicted to be 8.01 × 10 −4 f 3 .

The dipole fit to the proton form factor is extended impirically in a three quark analysis of the electromagnetic form factors G(E) and G(M). A relativisitic three quark Dirac shell model wave function is used to describe the quarks in the proton rest frame. Fits beyond the dipole to the electromagnetic form factors lead to the upper and lower components of the shell model wave function, and eventually to a model determination of the potentials acting on the quarks. Asymptotically the scalar potential is a confining linear potential. Its slope allows one to estimate the flux tube constant. A best fit of the electromagnetic form factors using the composite three quark wave function yields similiar values for the flux tube constant. The flux tube constant found is about one half the 0.9 GeV/fm found from meson studies.