Rederivation of the alpha effect in terms of the magnetic fluctuation spectrum

We demonstrate that the alpha effect can be expressed in terms of the integrated current helicity spectrum of the turbulence. This is a much more convenient form than that obtained using a kinematic velocity field description

Shock compression experimental capabilities of the Atlas facility

Atlas is a high-energy pulsed-power facility under construction at Los Alamos National Laboratory. When completed in late 2000, Atlas will provide a laboratory environment to perform shock compression experiments in regimes presently unattainable by other methods. The high-energy-density environment on Atlas will be produced by the rapid ({approximately}4{micro}s) implosion of a 20--40 gram, {approximately}4cm radius, 4 cm length cylindrical aluminum or aluminum/high-Z composite liner, driven by a fast current pulse of {approximately}32 MA from a 24 MJ capacitor bank. Implosion velocities up to 20 km/s are predicted, allowing Hugoniot experiments to {approximately}20 Mbar and quasi-adiabatic compression to several Mbar. However, many issues face scientist in performing such experiments, including how to diagnose conditions inside the imploding liner, how to correct results for distortions and density gradients created by the cylindrical geometry and magnetic drive, and how to prevent geometric distortions and instabilities from degrading results. In this paper, liner performance is predicted for a shock compression experiment utilizing 1-D MHD simulations, and the effect of gradients in density, pressure, and velocity in the impactor prior to collision are discussed

Constraints on the magnitude of alpha in dynamo theory

We consider the backreaction of the magnetic field on the magnetic dynamo coefficients and the role of boundary conditions in interpreting whether numerical evidence for suppression is dynamical. If a uniform field in a periodic box serves as the initial condition for modeling the backreaction on the turbulent EMF, then the magnitude of the turbulent EMF and thus the dynamo coefficient \a, have a stringent upper limit that depends on the magnetic Reynolds number $R_M$ to a power of order -1. This is not a dynamic suppression but results just because of the imposed boundary conditions. In contrast, when mean field gradients are allowed within the simulation region, or non-periodic boundary are used, the upper limit is independent of $R_M$ and takes its kinematic value. Thus only for simulations of the latter types could a measured suppression be the result of a dynamic backreaction. This is fundamental for understanding a long-standing controversy surrounding $\alpha$ suppression. Numerical simulations which do not allow any field gradients and invoke periodic boundary conditions appear to show a strong $\alpha$ suppression (e.g. Cattaneo & Hughes 1996). Simulations of accretion discs which allow field gradients and allow free boundary conditions (Brandenburg & Donner 1997) suggest a dynamo $\alpha$ which is not suppressed by a power of $R_M$. Our results are consistent with both types of simulations.Comment: LaTex, version in press, Ap

Predicted Bremsstrahlung generation by energetic electron beams

The CYLTRAN photon/electron Monte Carlo code has been employed to predict Bremsstrahlung generation by monoenergetic electron beams from 10 to 1000 MeV. The forward-directed Bremsstrahlung intensity is investigated as a function of beam energy converter thickness, and material. At high energies, the forward extraction efficiency is maximized by using converters that are about 0.1-electron ranges thick. The largest intensities are attained with low-Z converter materials such as beryllium. Because the Bremsstrahlung radiation is strongly forward-directed, low divergence of the incident electron beam is crucial. Under deal conditions, a 1000-MeV beam can produce intensities up to 10{sup 8} MeV per steradian, per incident electron. 9 refs., 32 figs., 12 tabs

One-and-Two-Dimensional Simulations of Liner Performance at Atlas Parameters

The authors report results of one-and-two-dimensional MHD simulations of an imploding heavy liner in Z-pinch geometry. The driving current has a pulse shape and peak current characteristic of the Atlas pulsed-power facility being constructed at Los Alamos National Laboratory. One-dimensional simulations of heavy composite liners driven by 30 MA currents can achieve velocities on the order of 14 km/sec. Used to impact a tungsten target, the liner produces shock pressures of approximately fourteen megabars. The first 2-D simulations of imploding liners driven at Atlas current parameters are also described. These simulations have focused on the interaction of the liner with the glide planes, and the effect of realistic surface perturbations on the dynamics of the pinch. It is found that the former interaction does not seriously affect the inner liner surface. Results from the second problem indicate that a surface perturbation having amplitude as small as 0.2 {micro}m can have a significant effect on the implosion dynamics

MHD modeling of atlas experiments to study transverse shear interface interactions

The transverse shear established at the interface of two solids moving at differential velocities on the order of the sound speed is being studied in experiments on the ATLAS capacitor bank at Los Alamos. The ATLAS bank has finished certification tests and has demonstrated peak currents of 27.5 MA into an inductive load with a risetime of 5 microseconds. One- and two-dimensional MHD calculations have been performed in support of these 'friction-like' ATLAS experiments. Current flowing along the outer surface of a thick aluminum liner, 10 mm thick at impact with the interaction target, accelerates the liner to velocities of {approx}1.0-1.5 km/s. This cylindrically imploding liner impacts a target assembly composed of alternating disks of high- and low-density materials. Different shock speeds in the two materials leads to a differential velocity along the interface. Shock heating, elastic-plastic flow, and stress transport are included in the calculations. Material strength properties are modeled with a Steinburg-Guinan treatment in these first studies. Various design configurations for the ATLAS experiments are now being considered and will be presented

Stability of Magnetically Implode Liners for High Energy Density Experiments

Magnetically imploded cylindrical metal shells (z-pinch liners) are attractive drivers for a wide variety of hydrodynamics and material properties experiments. The ultimate utility of liners depends on the acceleration of near-solid density shells to velocities exceeding 20 km/sec with good azimuthal symmetry and axial uniformity. Two pulse power systems (Ranchero and Atlas) currently operational or under development at Los Alamos provide electrical energy adequate to accelerate {approximately}50 gr. liners to 1-2 MJ/cm kinetic energy. As in all z-pinches, the outer surface of a magnetically imploded liner is unstable to magneto-Rayleigh-Taylor (RT) modes during acceleration. Large-scale distortion in the liners from RT modes growing from glide plane interactions or initial imperfections could make liners unusable for man experiments. On the other hand, material strength in the liner should, from first principles, reduce the growth rate of RT modes - and can render some combinations of wavelength and amplitude analytically stable. The growth of instabilities in both soft aluminum liners and in high strength aluminum alloy liners has been studied analytically, computationally and experimentally at liner kinetic energies up to 100 KJ/cm on the Pegasus capacitor bank using driving currents up to 12 MA

Pulsed power hydrodynamics : a new application of high magnetic fields.

Pulsed Power Hydrodynamics is a new application of high magnetic fields recently developed to explore advanced hydrodynamics, instabilities, fluid turbulences, and material properties in a highly precise, controllable environment at the extremes of pressure and material velocity. The Atlas facility at Los Alamos is the world's first and only laboratory pulsed power system designed specifically to explore this relatively new family of megagauss magnetic field applications. Constructed in 2000 and commissioned in August 2001, Atlas is a 24-MJ high-performance capacitor bank delivering up to 30 MA with a current risetime of 5-6 {micro}sec. The high-precision, cylindrical, imploding liner is the tool most frequently used to convert electrical energy into the hydrodynamic (particle kinetic) energy needed to drive the experiments. For typical liner parameters including initial radius of 5 cm, the peak current of 30 MA delivered by Atlas results in magnetic fields just over 1 MG outside the liner prior to implosion. During the 5 to 10-{micro}sec implosion, the field outside the liner rises to several MG in typical situations. At these fields the rear surface of the liner is melted and it is subject to a variety of complex behaviors including: diffusion dominated andor melt wave field penetration and heating, magneto Raleigh-Taylor sausage mode behavior at the liner/field interface, and azimuthal asymmetry due to perturbations in current drive. The first Atlas liner implosion experiments were conducted in September 2000 and 10-15 experiments are planned in the: first year of operation. Immediate applications of the new pulsed power hydrodynamics techniques include material property topics including: exploration of material strength at high rates of strain, material failure including fracture and spall, and interfacial dynamics at high relative velocities and high interfacial pressures. A variety of complex hydrodynamic geometries will be explored and experiments will be designed to explore uristable perturbation growth and transition to turbulence. This paper will provide an overview of the range of problems to which pulsed power hydrodynamics can be applied and the issues associated with these techniques. Other papers at this Conference will present specifics of individual experiments and elaborate on the liner physics issues

Spall experiments in convergent geometry using the atlas pulsed power facility.

{sm_bullet}Four spall experiments have been performed using Atlas {sm_bullet} Purpose was to investigate damage in convergent geometry {sm_bullet} Impact pressures ranged between 45 kbars - 110 kbars {sm_bullet} Diagnostics included VISAR and axial and radial radiographs {sm_bullet} Targets were recovered for post-metallugical analysi