We have developed conceptual designs of two petawatt-class pulsed-power accelerators: Z 300 and Z 800. The designs are based on an accelerator architecture that is founded on two concepts: single-stage electrical-pulse compression and impedance matching [Phys. Rev. ST Accel. Beams 10, 030401 (2007)]. The prime power source of each machine consists of 90 linear-transformer-driver (LTD) modules. Each module comprises LTD cavities connected electrically in series, each of which is powered by 5-GW LTD bricks connected electrically in parallel. (A brick comprises a single switch and two capacitors in series.) Six water-insulated radial-transmission-line impedance transformers transport the power generated by the modules to a six-level vacuum-insulator stack. The stack serves as the accelerator's water-vacuum interface. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs), which are joined in parallel at a 10-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator's physics-package load. Z 300 is 35 m in diameter and stores 48 MJ of electrical energy in its LTD capacitors. The accelerator generates 320 TW of electrical power at the output of the LTD system, and delivers 48 MA in 154 ns to a magnetized-liner inertial-fusion (MagLIF) target [Phys. Plasmas 17, 056303 (2010)]. The peak electrical power at the MagLIF target is 870 TW, which is the highest power throughout the accelerator. Power amplification is accomplished by the centrally located vacuum section, which serves as an intermediate inductive-energy-storage device. The principal goal of Z 300 is to achieve thermonuclear ignition; i.e., a fusion yield that exceeds the energy transmitted by the accelerator to the liner. 2D magnetohydrodynamic (MHD) simulations suggest Z 300 will deliver 4.3 MJ to the liner, and achieve a yield on the order of 18 MJ. Z 800 is 52 m in diameter and stores 130 MJ. This accelerator generates 890 TW at the output of its LTD system, and delivers 65 MA in 113 ns to a MagLIF target. The peak electrical power at the MagLIF liner is 2500 TW. The principal goal of Z 800 is to achieve high-yield Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Recent experiments at the Sandia National Laboratories Z Facility have, for the first time, studied the implosion dynamics of magnetized liner inertial fusion (MagLIF) style liners that were pre-imposed with a uniform axial magnetic field. As reported [T. J. Awe et al., Phys. Rev. Lett. 111, 235005 (2013)] when premagnetized with a 7 or 10 T axial field, these liners developed 3D-helix-like hydrodynamic instabilities; such instabilities starkly contrast with the azimuthally correlated magneto-Rayleigh-Taylor (MRT) instabilities that have been consistently observed in many earlier non-premagnetized experiments. The helical structure persisted throughout the implosion, even though the azimuthal drive field greatly exceeded the expected axial field at the liner's outer wall for all but the earliest stages of the experiment. Whether this modified instability structure has practical importance for magneto-inertial fusion concepts depends primarily on whether the modified instability structure is more stable than standard azimuthally correlated MRT instabilities. In this manuscript, we discuss the evolution of the helix-like instability observed on premagnetized liners. While a first principles explanation of this observation remains elusive, recent 3D simulations suggest that if a small amplitude helical perturbation can be seeded on the liner's outer surface, no further influence from the axial field is required for the instability to grow.

Magnetized Liner Inertial Fusion (MagLIF) [1] is a concept that involves using a pulsed electrical current to implode an initially-solid, cylindrical metal tube (liner) filled with preheated and magnetized fusion fuel. One-and two-dimensional simulations predict that if sufficient liner integrity can be maintained throughout the implosion, then significant fusion yield (>100 kJ) is possible on the 25-MA, 100-ns Z accelerator. The greatest threat to the liner integrity is the Magneto-Rayleigh-Taylor (MRT) instability, which first develops on the outer liner surface, and then works its way inward toward the inner surface throughout the implosion. Two-dimensional simulations predict that a thick liner, with R outer /∆R=6, should be robust enough to keep the MRT instability from overly disrupting the fusion burn at stagnation. This talk will present the first experiments designed to study a thick, MagLIF-relevant liner implosion through to stagnation on Z [2]. The use of beryllium for the liner material enabled us to obtain penetrating monochromatic (6151±0.5 eV) radiographs that reveal information about the entire volume of the imploding liner. This talk will also discuss experiments that investigated Z's pulseshaping capabilities to either shock-or shocklessly-compress the imploding liners [3], as well as our most recent experiments that used 2-micron-thick aluminum sleeves to provide high-contrast tracers for the positions and states of the inner surfaces of the imploding beryllium liners. The radiography data to be presented provide stringent constraints on the simulation tools used by the broader high energy density physics and inertial confinement fusion communities, where quantitative areal density measurements, particularly of convergent fusion targets, are relatively scarce. We will also present power-flow tests of the MagLIF load hardware as well as new micro-B-dot measurements of the azimuthal drive magnetic field that penetrates the initially vacuum filled interior of the liner during the implosion.

The purpose of this note is to present the results of numerical calculations of linear stability limits for free convection in layers of porous media or packed beds with throughflow. Effects of flow direction and of different boundary conditions are shown. Previous results were given by Homsy and Sherwood (1 976) for the special boundary conditions of constant velocity and temperature at upper and lower boundaries. These results were valid for any flow rate and direction. Wooding (1960) calculated linear stability limits for a semiinfinite layer with large upward flow when the upper surface was porous and submerged by a layer of liquid at constant temperature. Sutton (1970) presented results valid for small flow rates. For packed-bed applications, e.g., the reaction zone in a catalytic reactor or an ion exchange column, the porous or, as it is sometimes called, constant-pressure boundary condition is more appropriate. In this work, we show results for all combinations of boundary conditions and flow direction. The theory and results of calculations are presented within the framework of thermal convection. As is well known, complete analogy exists with concentration-driven convection and we only have to replace T with concentration, thermal diffusivity a' with the species diffusivity, and the thermal expansivity fl with the appropriate linear concentration coefficient of density.The linear theory has been given previously, for example by Homsy and Sherwood, and by Wooding. In summary, marginal states were calculated from Darcy's law and the conductionconvection energy equation. Property variations (density and viscosity) were assumed to be small-the Boussinesq approximation-and linear with temperature from some reference temperature To:where, for gases, both /3 and p' are positive. After elimination of the pressure and restatement in dimensionless form with the height H of the layer as the reference length, the resulting eigenvalue problem for the mass flux perturbation r and the temperature perturbation 0 is obtained by the usual normal mode analysis:In these equations, D = d/dz and z is the dimensionless vertical coordinate with origin at the lower extremity of the porous layer. The parameter Pe' = HG/a'p, is a modified thermal Peclet number in which a' = k,/pCp, is the modified thermal diffusivity and G is the steady mass flux, which is positive for upward flow. The parameter X is given by X = gfiHATK/u,a' -(p + (3')Pe'AT. The first term in this expression is the RayleighDarcy number and the second term is a contribution due to the throughflow. a = kH is the dimensionless wave number of the disturbance in the horizontal plane. F ( z ) is the dimensionless steady state temperature gradient.A single fourth-order ordinary differential equation is obtained by eliminating 8 in favor of r:with appropriate boundary conditions at z = 0 and 1. For constant mass flux (impermeable to perturbations) and temperature, r = 0,B = 0, or, in terms of I' only, For constant-temperature boundaries that are porous or permeable to f...