Ann Kaul

10 New γ Doradus and δ Scuti Stars

We present high-resolution spectroscopy and precision photometry of five new γ Doradus and five new δ Scuti variables. The five new γ Doradus variables substantially increase the number of confirmed stars of this class. All 10 stars fall in the spectral class range F0–F2, but they are cleanly separated into two groups by their luminosity and photometric periods. However, the period gap between the γ Doradus and δ Scuti stars is becoming very narrow since we confirm that HD 155154 is a γ Doradus star with the shortest periods reported to date (the shortest of its four periods is ~0.312 days). We do not find any evidence in our sample for stars exhibiting both δ Scuti– and γ Doradus–type pulsations.

Experimental Series on Behavior of Post-Damage Recollected Material

Spallation damage, a typical method of failure for ductile materials, results from the nucleation, growth and coalescence of voids caused by high tensile stress. Specific areas of research on spallation damage include the damage initiation range in convergent geometry, behavior of material recollected after damage, and effects of convergent geometry (shear stresses, etc.) on the material response. Spallation phenomena models are primarily based on experiments using a planar configuration, where a significant body of data exists from gas gun, laser and high-explosive experiments. Planar experiments allow for one-dimensional analysis of the evolution of failure characteristics.

Pulsed Power Hydrodynamics: An Application of Pulsed Power and High Magnetic Fields to the Exploration of Material Properties and Problems in Experimental Hydrodynamics

Pulsed-power hydrodynamics (PPH) is an evolving application of low-impedance pulsed-power technology. PPH is particularly useful for the study of problems in advanced hydrodynamics, instabilities, turbulence, and material properties. PPH techniques provide a precisely characterized controllable environment at the currently achievable extremes of pressure and material velocity. The Atlas facility, which is designed and built by Los Alamos National Laboratory, is the worlds first, and only, laboratory pulsed-power system designed specifically for this relatively new family of pulsed-power applications. Atlas joins a family of low-impedance high-current drivers around the world, which is advancing the field of PPH. The high-precision cylindrical magnetically imploded liner is the tool most frequently used to convert electromagnetic energy into the hydrodynamic (particle kinetic) energy needed to drive strong shocks, quasi-isentropic compression, or large-volume adiabatic compression for the experiments. At typical parameters, a 30-g 1-mm-thick liner with an initial radius of 5 cm and a moderate current of 20 MA can be accelerated to 7.5 km/s, producing megabar shocks in medium density targets. Velocities of up to 20 km/s and pressures of > 20 Mbar in high-density targets are possible. The first Atlas liner implosion experiments were conducted in Los Alamos in September 2001. Sixteen experiments were conducted in the first year of operation before Atlas was disassembled, moved to the Nevada Test Site (NTS), and recommissioned in 2005. The experimental program resumed at the NTS in July 2005. The first Atlas experiments at the NTS included two implosion dynamics experiments, two experiments exploring damage and material failure, a new advanced hydrodynamics series aimed at studying the behavior of particles of damaged material ejected from a free surface into a gas, and a series exploring friction at sliding interfaces under conditions of high normal pressure and high relative velocities. Longer term applications of PPH and the Atlas system include the study of material interfaces subjected to multimegagauss magnetic fields, material strength at high strain rate, the properties of strongly coupled plasmas, and the equation of state of materials at pressures approaching 10 Mbar.

A consistent approach to modeling MHD-driven cylindrical damage experiments

Well-characterized experimental data is essential for development of models describing complex material processes such as damage and failure. There is currently a dearth of experimental data capturing material behavior for the processes of non-uniaxial loading to failure and void closure after damage. LANL and VNIIEF recently completed a series of ten helical-generator-driven cylindrical damage experiments using high-precision diagnostics to measure the drive conditions and the material response. These experiments produced a well-characterized damage data set ranging from void initiation to complete failure and recollection for a well-studied material, aluminum. Combining magneto-hydrodynamics (MHD) and material modeling capabilities in a Lagrangian hydrocode allows for self-consistent end-to-end simulations of MHD-driven material property experiments. Simulation results for the damage experiments compared to experimental data will be presented.

Ranchero overview and expectations for performance at currents over 50 MA (LA-UR 11-04345)

Ranchero is a coaxial magnetic flux compression generator (FCG) initiated simultaneously on-axis, which is intended to produce currents approaching 100 MA. We continue with both applications and development, striving for both cost effectiveness and high performance. In this paper we discuss on-going work, provide details of the sophisticated detonation system, and show high current predictions from our active modeling effort. We intend further tests that will push the limits of Ranchero performance.

Current pulse effects on cylindrical damage experiments

A series of joint experiments between LANL and VNIIEF use a VNIIEF-designed helical generator to provide currents for driving a LANL-designed cylindrical spallation experimental load. Under proper driving conditions, a cylindrical configuration allows for a natural recollection of the damaged material. In addition, the damaged material is able to come to a complete stop due to its strength, avoiding application of further forces. Thus far, experiments have provided data about failure initiation of a well-characterized material (aluminum) in a cylindrical geometry, behavior of material recollected after damage from pressures in the damage initiation regime, and behavior of material recollected after complete failure. In addition to post-shot collection of the damaged target material for subsequent metallographic analysis, dynamic in-situ experimental diagnostics include velocimetry and transverse radial radiography. This paper will focus on the effects of tailoring the driving current pulse to obtain the desired data.

Spall and Damage in Convergent Geometry Using Pulsed Power

The ability of pulsed-power to magnetically accelerate and drive high-precision liner-target implosions facilitates studies of material properties such as damage in cylindrically convergent geometry. Spallation damage experiments are usually conducted in a planar geometry, allowing for one-dimensional analysis of evolution of failure characteristics. Cylindrical experiments allow for a careful analysis of the effect of convergence and two-dimensional strains and shear stresses on the spallation profile of a material. This paper reports on a series of recent experiments to provide data describing the onset of failure of a well-characterized material (aluminum) in a cylindrically convergent geometry. Experimental data includes post-shot collection of the damaged target for subsequent metallographic analysis, dynamic VISAR velocimetry to infer the target material spallation strength and damage model parameters, and transverse radial radiography to assess drive and impact symmetry. This data is used to develop and validate damage and failure models. The theoretical basis, designs and results are presented for these experiments using explosively-driven pulsed power facilities.

Spallation Damage Experiments in Cylindrical Geometry

Spallation damage is the process of damage in a ductile material caused by void nucleation, growth and coalescence due to states of high tensile stress. Typical experiments are conducted in a planar, uniaxial stress configuration. A convergent geometry provides a unique opportunity to study multi-dimensional stress states. A series of 3 cylindrical spall experiments on aluminum is planned for this summer, using flux compression generators to drive the experiments. The experiments will explore the spallation damage threshold to determine effects of the cylindrical geometry. In addition, the effect of plastic work on the pressure wave profile as it moves through the material will be studied.