Wesley C. Fan

Shielding considerations for satellite microelectronics

Shielding for space microelectronics needs to provide an acceptable dose rate with minimum shield mass. The analysis presented here shows that the best approach is, in general, to use a graded-Z shield, with a high-Z layer sandwiched between two low-Z materials. A graded-Z shield is shown to reduce the electron dose rate by more than sixty percent over a single-material shield of the same areal density. For protons, the optimal shield would consist of a single, low-Z material layer. However, it is shown that a graded-Z shield is nearly as effective as a single-material shield, as long as a low-Z layer is located adjacent to the microelectronics. A specific shield design depends upon the details of the radiation environment, system model, design margins/levels, compatibility of shield materials, etc. Therefore, we present here general principles for designing effective shields and describe how the computer codes are used for this application.

An Analysis of the Extended-Transport Correction with Application to Electron Beam Transport

Abstract Charged-particle transport is characterized by scattering cross sections that are extremely large and forward-peaked, requiring specialized treatment as compared with neutral-particle transport. The extended-transport correction (ETC) is known to be an effective method to treat elastic scattering of electrons. We apply the ETC to inelastic downscattering of electrons, and evaluate the effectiveness of the method by comparing the scattering moments for the screened Rutherford scattering kernel and for scattering with a deterministic cosine. The ETC approximation results in a δ-function in angle downscatter source term, for energy loss without direction change, which has been incorporated into the CEPTRE discrete ordinates code in a manner that is compatible with general quadrature sets, not requiring a specialized Galerkin quadrature. The ETC approximation also makes it possible to develop a first-collision source technique that is effective for charged-particle transport, by including particles that have downscattered in energy without direction change in the uncollided-flux solution. We demonstrate the effectiveness of these techniques for problems involving electron beam sources incident on infinite and finite water cylinders and compare the energy- and charge-deposition distributions with ITS Monte Carlo results with good agreement.

Discrete Ordinates Approximations to the First- and Second-Order Radiation Transport Equations

The conventional discrete ordinates approximation to the Boltzmann transport equation can be described in a matrix form. Specifically, the within-group scattering integral can be represented by three components: a moment-to-discrete matrix, a scattering cross-section matrix and a discrete-to-moment matrix. Using and extending these entities, we derive and summarize the matrix representations of the second-order transport equations.

Test facilities for evaluating nuclear thermal propulsion systems

Interagency panels evaluating nuclear thermal propulsion (NTP) development options have consistently recognized the need for constructing a major new ground test facility to support fuel element and engine testing. This paper summarizes the requirements, configuration, and baseline performance of some of the major subsystems designed to support a proposed ground test complex for evaluating nuclear thermal propulsion fuel elements and engines being developed for the Space Nuclear Thermal Propulsion (SNTP) program. Some preliminary results of evaluating this facility for use in testing other NTP concepts are also summarized.

A Discontinuous Phase-Space Finite Element Discretization of the Linear Boltzmann-Vlasov Equation for Charged Particle Transport

We examine the modeling of charged-particle transport when both collision processes with background media and electromagnetic effects are important using the Boltzmann-Vlasov equation. We derive and transform the Boltzmann-Vlasov equation into a form very similar to the standard linear Boltzmann equation with additional operators. We apply the discontinuous finite element methods for discretization in the spatial, energy, and angular variables. An implementation of these methods demonstrates correct transport behavior for fixed electric and magnetic fields. We also demonstrate coupling to Maxwells equations with a simple electromagnetic solver to generate self-consistent fields.

SCEPTRE 1.4 Quick Start Guide

This report provides a summary of notes for building and running the Sandia Computational Engine for Particle Transport for Radiation Effects (SCEPTRE) code. SCEPTRE is a general purpose C++ code for solving the Boltzmann transport equation in serial or parallel using unstructured spatial finite elements, multigroup energy treatment, and a variety of angular treatments including discrete ordinates. Either the first-order form of the Boltzmann equation or one of the second-order forms may be solved. SCEPTRE requires a small number of open-source Third Party Libraries (TPL) to be available, and example scripts for building these TPL’s are provided. The TPL’s needed by SCEPTRE are Trilinos, boost, and netcdf. SCEPTRE uses an autoconf build system, and a sample configure script is provided. Running the SCEPTRE code requires that the user provide a spatial finite-elements mesh in Exodus format and a cross section library in a format that will be described. SCEPTRE uses an xml-based input, and several examples will be provided.

Modeling Electron Transport in the Presence of Electric and Magnetic Fields

This report describes the theoretical background on modeling electron transport in the presence of electric and magnetic fields by incorporating the effects of the Lorentz force on electron motion into the Boltzmann transport equation. Electromagnetic fields alter the electron energy and trajectory continuously, and these effects can be characterized mathematically by differential operators in terms of electron energy and direction. Numerical solution techniques, based on the discrete-ordinates and finite-element methods, are developed and implemented in an existing radiation transport code, SCEPTRE.

Nuclear Electric Propulsion Space Environmental Studies

High energy photons emitted from space fission power supplies should not directly interfere with astronomical satellites in an unacceptable way. However, positrons emitted from space fission power and propulsion systems could potentially lead to interference with sensitive astronomical satellites, such as the Gamma Ray Observatory (GRO). Positrons are created when a high energy photon is attenuated by pair production. If this pair production occurs near the surface of a material (in the outer 2 g/cm2) the positron may escape. Positrons emitted from the reactor can then be transported by the Earths magnetic field down to orbits occupied by astronomical satellites. If the satellite and space fission power supply are properly aligned, the positron could interact with the satellite to form two 0.511 MeV photons (via positron/electron annihilation). The proximity of the photon production to detectors and the potential for bursts could lead to interference. The potential for interference is amplified by the fa...