Showing papers by "Roger Alan Vesey published in 2007"

Target design for high fusion yield with the double Z-pinch-driven hohlraum

Abstract: A key demonstration on the path to inertial fusion energy is the achievement of high fusion yield (hundreds of MJ) and high target gain. Toward this goal, an indirect-drive high-yield inertial confinement fusion (ICF) target involving two Z-pinch x-ray sources heating a central secondary hohlraum is described by Hammer et al. [Phys. Plasmas 6, 2129 (1999)]. In subsequent research at Sandia National Laboratories, theoretical/computational models have been developed and an extensive series of validation experiments have been performed to study hohlraum energetics, capsule coupling, and capsule implosion symmetry for this system. These models have been used to design a high-yield Z-pinch-driven ICF target that incorporates the latest experience in capsule design, hohlraum symmetry control, and x-ray production by Z pinches. An x-ray energy output of 9MJ per pinch, suitably pulse-shaped, is sufficient for this concept to drive 0.3–0.5GJ capsules. For the first time, integrated two-dimensional (2D) hohlraum/ca...

Fill-Tube-Induced Mass Perturbations on X-Ray-Driven, Ignition-Scale, Inertial-Confinement-Fusion Capsule Shells and the Implications for Ignition Experiments

Abstract: On the first inertial-confinement-fusion ignition facility, the target capsule will be DT filled through a long, narrow tube inserted into the shell. microg-scale shell perturbations Delta m' arising from multiple, 10-50 microm-diameter, hollow SiO2 tubes on x-ray-driven, ignition-scale, 1-mg capsules have been measured on a subignition device. Simulations compare well with observation, whence it is corroborated that Delta m' arises from early x-ray shadowing by the tube rather than tube mass coupling to the shell, and inferred that 10-20 microm tubes will negligibly affect fusion yield on a full-ignition facility.

Compensation for Time-Dependent Radiation-Drive Asymmetries in Inertial-Fusion Capsules

Abstract: An approach is presented to design inertial-fusion capsules compensated for time-dependent radiation-drive asymmetries. This approach uses in depth variable doping of the capsule ablator, i.e., the addition of small amounts of material to tailor the opacity. Simulations show that an inertial-fusion capsule, using a beryllium ablator variably doped with gold, can be designed to compensate for a constant ${P}_{2}$ radiation asymmetry as high as 20% and still produce nominal yield (80% of a symmetrically driven capsule). In contrast, without variable doping the ${P}_{2}$ asymmetry must be less than 2% to obtain nominal yield. Similarly encouraging results are obtained for modes ${P}_{1}$, ${P}_{4}$, and ${P}_{6}$. Simulations also demonstrate that variable doping can compensate for nearly arbitrary time-dependent radiation-drive asymmetries by varying the polar dependence of the doping fraction with depth.

Planar wire array performance scaling at multi-MA levels on the Saturn generator.

Abstract: A series of twelve shots were performed on the Saturn generator in order to conduct an initial evaluation of the planar wire array z-pinch concept at multi-MA current levels. Planar wire arrays, in which all wires lie in a single plane, could offer advantages over standard cylindrical wire arrays for driving hohlraums for inertial confinement fusion studies as the surface area of the electrodes in the load region (which serve as hohlraum walls) may be substantially reduced. In these experiments, mass and array width scans were performed using tungsten wires. A maximum total radiated x-ray power of 10 {+-} 2 TW was observed with 20 mm wide arrays imploding in {approx}100 ns at a load current of {approx}3 MA, limited by the high inductance. Decreased power in the 4-6 TW range was observed at the smallest width studied (8 mm). 10 kJ of Al K-shell x-rays were obtained in one Al planar array fielded. This report will discuss the zero-dimensional calculations used to design the loads, the results of the experiments, and potential future research to determine if planar wire arrays will continue to scale favorably at current levels typical of the Z machine. Implosion dynamics will be discussed, includingmore » x-ray self-emission imaging used to infer the velocity of the implosion front and the potential role of trailing mass. Resistive heating has been previously cited as the cause for enhanced yields observed in excess of jxB-coupled energy. The analysis presented in this report suggests that jxB-coupled energy may explain as much as the energy in the first x-ray pulse but not the total yield, which is similar to our present understanding of cylindrical wire array behavior.« less

Optimal X-Ray Pulse Compression with Compact Nested Wire Arrays On Z

Abstract: Summary form only given. Compact nested tungsten wire arrays operating in current transfer mode produce peak soft x-ray powers of 200 to 250 TW on the Z accelerator. Such arrays use outer (inner) array diameters of 20 (12) mm and also employ low density, 4 to 6 mm diameter CH2 foam pulse shaping targets on the axis. These are among the highest powers produced on Z, at any array diameter. Optimized single tungsten wire arrays produce peak powers of 160 TW at 20 mm diameter. These nested arrays also produce the narrowest x-ray bursts of any arrays on Z (1.9 to 2.5 ns) compared to 4 to 7 ns with single arrays. X-ray radiography at 6.151 keV is used to determine the mass density profile as a function of space and time. The density profiles are obtained from the radiographs via Abel inversion. These data indicate that the narrowing of the x-ray pulse of nested arrays is correlated to a narrowing of the measured spatial width of the mass distribution at the axis near stagnation. The nested mass distribution narrows as a result of three factors: (1) Current transfer mode operation allows the Magneto-Rayleigh-Taylor (MRT) perturbation and wavelength to be reset to smaller values at the time of current transfer to the inner array, consistent with an ablation phase of the inner wire array. The wavelength and amplitude of the outer array is not impressed on the inner at switching. The MRT wavelength and perturbation is reduced by a factor of ~3.4 compared to a single wire array when compared at the same implosion radius. (2) Current transfer mode allows the outer array mass to fill the interior of the inner wire array. Thus the MRT growth on the inner wire array is stabilized by mass accretion. (3) Finally, low-density axial foam targets also change the mass profile near the axis and further narrow the pulse through mass accretion. Such optimized nested arrays may permit a peak power of 100 TW in 5 ns width from a 10-mm diameter array. This more compact array would be compatible with driving a scale-0.6 double-ended hohlraum ICF concept, which would permit pulsed-power ignition with fusion yields of 50 MJ at half of the required facility energy storage of the scale-1.0 double-ended hohlraum design (400 MJ yields).