Kevin N. Austin

A new time and space resolved transmission spectrometer for research in inertial confinement fusion and radiation source development

We describe the design and function of a new time and space resolved x-ray spectrometer for use in Z-pinch inertial confinement fusion and radiation source development experiments. The spectrometer is designed to measure x-rays in the range of 0.5-1.5 Å (8-25 keV) with a spectral resolution λ/Δλ ∼ 400. The purpose of this spectrometer is to measure the time- and one-dimensional space-dependent electron temperature and density during stagnation. These relatively high photon energies are required to escape the dense plasma created at stagnation and to obtain sensitivity to electron temperatures ≳3 keV. The spectrometer is of the Cauchois type, employing a large 30 × 36 mm2, transmissive quartz optic for which a novel solid beryllium holder was designed. The performance of the crystal was verified using offline tests, and the integrated system was tested using experiments on the Z pulsed power accelerator.

Performance of a radial vacuum insulator stack

Electrostatic breakdown (“flashover”) can limit the maximum delivered power in pulsed power systems. One problematic region is the solid-vacuum interface. The solid/vacuum interface surface is generally the component most likely to suffer undesired breakdown in a well-designed pulsed power system driving a load in vacuum. A relatively small (in total number) and localized amount of neutrals or plasma can be enough to allow a self-sustaining discharge. A discharge along the solid-vacuum interface can readily and quickly desorb ions and neutrals from the insulator material. The spurious discharge can carry almost unlimited current, generally constrained only by the current source or the inductance of the current path. Radial insulator assemblies, where power flows along the axis of the insulator (used in a coaxial power feed with concentric insulators of differing diameters, maintaining radial electric field direction) can be lower insertion inductance and smaller in total size than axial insulators. Radial insulators have additional electric field grading and mechanical stress issues compared to an axial insulator system. With an interest in quantifying and improving performance of a radial insulator system, we have built a prototype 60 cm diameter assembly with four series insulator rings and 0.23 m2 total stressed area. By driving the insulator with a ~2 MV, 15 ns (effective time) negative pulse from a low energy system, we are able to study insulator performance of a multi-level stack with a reasonable data rate in a relevant regime (~180 kV/cm). The prototype insulator assembly is designed to demonstrate key concepts important for a large system, including 1) improved electric field uniformity using equipotential redistribution, 2) mechanical assembly viability with reasonable machining tolerances, and 3) useful electric field hold-off. The system is scalable to larger size insulators with acceptable electric field uniformity and mechanical stress in the insulating plastic. Mated to a (generally optimal) 40Ω oil-insulated transmission line, the electric field uniformity on the plastic-vacuum interface tested here is better than ±1%. Without equipotential redistribution, the electric field non-uniformity would be of order ±46%. To facilitate future fabrication of larger insulator assemblies, the Rexolite® 1422 cross-linked polystyrene plastic rings are cut from sheet or individually cast, then machined; a large monolithic slab of insulator material is not needed. We will discuss basic properties of vacuum interfaces, discuss scaling formulae used to relate performance of different systems, and show results of testing.

Packaging Strategies for Printed Circuit Board Components Volume I: Materials & Thermal Stresses

Decisions on material selections for electronics packaging can be quite complicated by the need to balance the criteria to withstand severe impacts yet survive deep thermal cycles intact. Many times, material choices are based on historical precedence perhaps ignorant of whether those initial choices were carefully investigated or whether the requirements on the new component match those of previous units. The goal of this program focuses on developing both increased intuition for generic packaging guidelines and computational methodologies for optimizing packaging in specific components. Initial efforts centered on characterization of classes of materials common to packaging strategies and computational analyses of stresses generated during thermal cycling to identify strengths and weaknesses of various material choices. Future studies will analyze the same example problems incorporating the effects of curing stresses as needed and analyzing dynamic loadings to compare trends with the quasi-static conclusions.