Search

The pulsed-power driven double-ended vacuum hohlraum allows highly symmetric capsule implosions, aided by the separation of the capsule from the z-pinch and its associated instabilities, spatial variations, and non-thermal spectral components. Experiments on Sandia National Laboratories' Z facility in a single-sided power feed configuration have been limited thus far to capsule drive temperatures of 75 eV or less. Nonetheless, these experiments have validated the hohlraum energetics, hohlraum coupling, and capsule coupling properties of this configuration. Implosion capsules and symmetry diagnostic capsules have provided a wealth of capsule energetics data in the form of implosion trajectories for comparison to simulation. These measurements have been compared to detailed two-dimensional Lasnex hohlraum simulations, which model the hohlraum radiation transport including the effects of z-pinch source motion as well as hohlraum absorption/re-emission and wall plasma motion.

The double Z-pinch hohlraum high-yield concept [Hammer et al., Phys. Plasmas 6, 2129 (1999)] utilizes two 63-MA Z pinches to heat separate primary hohlraums at either end of a secondary hohlraum containing the cryogenic fusion capsule. Recent experiments on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories have developed an advanced single-sided power feed, double Z-pinch load to study radiation symmetry and pinch power balance using implosion capsules [Cuneo et al., Phys. Rev. Lett. 88, 215004 (2002)]. Point-projection x-ray imaging with the Z-Beamlet Laser mapped the trajectory and distortion of 2-mm diameter plastic ablator capsules. Using the backlit capsule distortion as a symmetry diagnostic, the ability to predictably tune symmetry at the

The Z-pinch-driven hohlraum (ZPDH) [J. H. Hammer et al., Phys. Plasmas 6, 2129 (1999)] is a promising approach to high yield inertial confinement fusion currently being characterized in experiments on the Sandia Z accelerator [M. E. Cuneo et al., Phys. Plasmas 8, 2257 (2001)]. Simulations show that capsule radiation symmetry, a critical issue in ZPDH design, is governed primarily by hohlraum geometry, dual-pinch power balance, and pinch timing. In initial symmetry studies on Z without the benefit of a laser backlighter, highly-asymmetric pole-hot and equator-hot single Z-pinch hohlraum geometries were diagnosed using solid low density foam burnthrough spheres. These experiments demonstrated effective geometric control and prediction of polar flux symmetry at the level where details of the Z-pinch implosion and other higher order effects are not critical. Radiation flux symmetry achieved in Z double-pinch hohlraum configurations exceeds the measurement sensitivity of this self-backlit foam ball symmetry di...

The first controlled experiments measuring the growth of the magneto-Rayleigh-Taylor instability in fast ($\ensuremath{\sim}100\text{ }\text{ }\mathrm{ns}$) Z-pinch plasmas are reported. Sinusoidal perturbations on the surface of an initially solid Al tube (liner) with wavelengths of $25--400\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ were used to seed the instability. Radiographs with $15\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ resolution captured the evolution of the outer liner surface. Comparisons with numerical radiation magnetohydrodynamic simulations show remarkably good agreement down to $50\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ wavelengths.

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).

Summary form only given. The z-pinch driven hohlraum high-yield concept is in the process of being experimentally validated on the Z accelerator, qualifying its performance with respect to hohlraum energetics, radiation symmetry, pulse shaping, capsule preheat, and z-pinch physics scaling. Key parts of the assessment effort are the measurement and simulation of capsule radiation symmetry on Z, and the theoretical scaling of radiation symmetry from the hohlraum conditions on Z (70-90 eV) to the 200-250 eV hohlraum temperatures characteristic of a device for driving high yield capsules. Recently, we have begun to use the Z-beamlet laser (ZBL) to create high-energy (4.75 - 6.7 keV) X-rays for backlighting capsule implosion experiments on Z in point projection mode. The hohlraum geometry is a single-sided power feed analog to the high-yield concept, in which a central secondary hohlraum containing the capsule is driven by a z-pinch primary hohlraum at each end. Images at 4.75 and 6.7 keV backlighting energies have been obtained, mapping the trajectory and distortion of a 2-mm initial diameter plastic ablator capsule over multiple shots.

Summary from only given. The Z-machine is a Z-pinch device, using pulsed power methods to implode wire arrays and create high density plasmas and consequent x-ray emission. Small cavities (holhraums) are used to symmetrically bathe capsules in this radiation, causing the capsules compress for ICF studies. Radiography performed during an accelerator shot shows implosion symmetry and a factor of 2 compression at 14 ns after peak accelerator radiation The new radiography ability provided by a Z-beamlet laser allows this implosion symmetry to be studied and improved.

Summary form only given, as follows. Capsule radiation symmetry is a crucial issue in the design of the z-pinch driven hohlraum approach to high-yield inertial confinement fusion. Capsule symmetry may be influenced by the power imbalance of the two z-pinch X-ray sources, and by hohlraum effects (geometry, time-dependent albedo, wall motion). 2-D and 3-D radiosity ("viewfactor") models have been used to design hohlraums that optimize the capsule radiation symmetry for experiments on Z as well as for high-yield targets. The goals of capsule symmetry experiments on Z include commissioning the Z-beamlet backlighter laser and requisite initial imaging detectors, measuring capsule flux asymmetry at the several percent level, and demonstrating understanding of the factors influencing capsule asymmetry in z-pinch hohlraums. Design and simulation efforts have focused on: (a) hohlraum radiation transport models to determine the sensitivity of capsule flux asymmetry to hohlraum geometry, top-bottom pinch power imbalance, and pinch mistiming, (a) 2-mm diameter, 60-/spl mu/m thick plastic shells designed to implode within 10 ns after the secondary hohlraum reaches its peak drive temperature of 75-85 eV, providing a suitable target for initial experiments at lower backlighting energies (2.5-3 keV), (b) 3- to 5-mm diameter thin (20-40 /spl mu/m) Ge-doped plastic shells designed for backlighting with Ti K-shell X-rays to assess symmetry early in the capsule implosion, and (c) uniform density foam spheres (/spl sim/50-100 mg/cc) also designed for Ti backlighting to image the trajectory of the shock/ablation front feature throughout the drive pulse. We present the results of postprocessed 1- and 2-D radiation-hydrodynamics calculations for each type of symmetry target, describing their advantages and disadvantages, and show comparisons with preliminary experimental data.

The radiation and shock generated by impact of an annular tungsten Z-pinch plasma on a 10-mm diam 5-mg/cc CH(2) foam are diagnosed with x-ray imaging and power measurements. The radiative shock was virtually unaffected by Z-pinch plasma instabilities. The 5-ns-duration approximately 135-eV radiation field imploded a 2.1-mm-diam CH capsule. The measured radiation temperature, shock radius, and capsule radius agreed well with computer simulations, indicating understanding of the main features of a Z-pinch dynamic-hohlraum-driven capsule implosion.

Monochromatic 6.151-keV radiographs of a highly unstable inertial confinement fusion capsule are shown. The capsule was driven by a 70-eV peak radiation temperature and exhibits numerous small-scale features down to the resolution of the backlighting diagnostic (about 15 μm ). The capsule experiment was done using the double-ended Z -pinch-driven hohlraum on the Sandia Z facility.

A recent publication [D. B. Sinars et al., Phys. Rev. Lett. 105, 185001 (2010)] describes the first controlled experiments measuring the growth of the magneto-Rayleigh–Taylor instability in fast (∼100 ns) Z-pinch plasmas formed from initially solid aluminum tubes (liners). Sinusoidal perturbations on the surface of these liners with wavelengths of 25–400 μm were used to seed single-mode instabilities. The evolution of the outer liner surface was captured using multiframe 6.151 keV radiography. The initial paper shows that there is good agreement between the data and 2-D radiation magneto-hydrodynamic simulations down to 50 μm wavelengths. This paper extends the previous one by providing more detailed radiography images, detailed target characterization data, a more accurate comparison to analytic models for the amplitude growth, the first data from a beryllium liner, and comparisons between the data and 3D simulations.

Wire array z-pinches have been used successfully for many years as a powerful x-ray source, as a dynamic hohlraum, and as an intense K-shell radiation source. Significant progress has been made in the effective modeling of these three-dimensional (3D) resistive plasmas. However, successful modeling also requires an accurate representation of the power delivered to these loads from the generator, which is an uncertainty potentially as large as the magnetohydrodynamic (MHD) implosion dynamics. We present 3D resistive MHD simulations of wire arrays that are coupled to transmission line equivalent models of the Z generator, driven by voltage sources derived directly from electrical measurements. Significant (multi-mega-ampere) current losses are shown to occur in both the convolute and the final feed. This limits the array performance and must be correctly accounted for to accurately represent the generator response to the load. Our simulations are validated against data for compact: 20 mm diameter, 10 mm long wire arrays that have produced the highest x-ray power densities on Z. This is one of the most comprehensive experimental data sets for single and nested wire arrays and includes voltage, current, x-ray power and energy, and multiple mass distribution measurements. These data tightly constrain our simulation results and allow us to describe in detail both the implosion and stagnation, and how energy is delivered to, and radiated from z-pinch loads. We show that the radiated power is consistent with the kinetic energy delivered to a distributed 3D mass profile over its implosion and stagnation. We also demonstrate how the local inductance of the transmission line connecting to the wire array is responsible for delivering more than 50% of the total radiated power. This makes the power output dependent on the design of specific elements of the generator, and their response to the imploding load, and not just on the peak current that can be delivered.

The implosion phase of a wire-array Z pinch is investigated using three-dimensional (3D) simulations, which model the mass ablation phase and its associated axial instability using a mass injection boundary condition. The physical mechanisms driving the trailing mass network are explored, and it is found that in 3D the current paths though the trailing mass can reduce bubble growth on the imploding plasma sheath, relative to the 2D (r,z) equivalent. Comparison between the simulations and a high quality set of experimental radiographs is presented.

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...

When used for the production of an x-ray imaging backlighter source on Sandia National Laboratories’ 20MA, 100ns rise-time Z accelerator [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], the terawatt-class, multikilojoule, 526.57nm Z-Beamlet laser (ZBL) [P. K. Rambo et al., Appl. Opt. 44, 2421 (2005)], in conjunction with the 6.151keV, Mn–Heα curved-crystal imager [D. B. Sinars et al., Rev. Sci. Instrum. 75, 3672 (2004)], is capable of providing a high quality x radiograph per Z shot for various high-energy-density physics experiments. Enhancements to this imaging system during 2005 have led to the capture of inertial confinement fusion capsule implosion and complex hydrodynamics images of significantly higher quality. The three main improvements, all leading effectively to enhanced image plane brightness, were bringing the source inside the Rowland circle to approximately double the collection solid angle, replacing direct exposure film with Fuji BAS-TR2025 image plate (read with a Fuji BAS-5000 sc...