Andrew D. Greenwood

A field picture of wave propagation in inhomogeneous dielectric lenses

This article presents the radiation and scattering patterns, and the interior field distributions, of three inhomogeneous dielectric lenses: the Luneburg (1944), Maxwell fish-eye (1860), and Eaton-Lippmann (1952) lenses. The purpose is to provide a picture of wave propagation through these lenses. A better understanding of the field picture, especially the nature of the focal points, is helpful to the practical design of the lenses.

Finite-element analysis of complex axisymmetric radiating structures

The finite element method using mixed edge nodal basis functions and cylindrical perfectly matched layer (PML) is an efficient and accurate method for the analysis of radiation problems. The use of edge elements to expand the transverse field components avoids the problem of spurious modes in the FEM formulation. The use of cylindrical PML for mesh truncation allows an efficient computational domain for almost any problem geometry, and the PML can be made very accurate, allowing the PML to be place near the problem geometry.

Conformal Electromagnetic Particle in Cell: A Review

Conformal (or body-fitted) electromagnetic particle-in-cell (EM-PIC) numerical solution schemes are reviewed. Included is a chronological history of relevant particle physics algorithms often employed in these conformal simulations. Brief mathematical descriptions of particle-tracking algorithms and current weighting schemes are provided, along with a brief summary of major time-dependent electromagnetic solution methods. Several research areas are also highlighted for recommended future development of new conformal EM-PIC methods.

Modeling of Plasma Formation During High-Power Microwave Breakdown in Air Using the Discontinuous Galerkin Time-Domain Method

Rapid plasma formation and evolution during high-power microwave (HPM) air breakdown in an HPM device produce a macroscopic plasma shield to the microwave transmission, which can severely limit the performance of the device. In this paper, the electromagnetic (EM)–plasma interaction and the HPM breakdown in air are modeled by a nonlinearly coupled full-wave Maxwell and plasma fluid system under conditions of high pressure and high collision frequency. The resulting multiphysics and multiscale system is solved by a nodal discontinuous Galerkin time-domain (DGTD) method, which is uniformly high order in both space and time. To demonstrate the capability of the DGTD method in the modeling of the HPM breakdown problems, the air breakdown and plasma formation in a metallic aperture are simulated, from which the underlining physical process can be interpreted and better understood.

All Cavity-Magnetron Axial Extraction Technique

A compact axial π-mode extraction scheme, which is based on a patent by Greenwood, is demonstrated in conjunction with the UM/L-3 relativistic magnetron using the particle-in-cell code ICEPIC. Cases utilizing Greenwoods extraction technique were compared with power extraction using traditional radial waveguides. Average extracted power values in all simulated axial cases were found to be within +/-6.5% of the radial cases. Cases utilizing 85 ° and 90° sector waveguides were found to have efficiencies up to ten percentage points higher than the radial case. The best performing case was found to use a set of three axially oriented 90 ° sector waveguides, shorted on the upstream side, with the short located 15 cm from the center of the magnetron apertures.

Performance Improvements for Efficient Electromagnetic Particle-In-Cell Computation on 1000s of CPUs

The finite-difference time-domain technique for simulation of electromagnetic and low-density plasma phenomena is computationally expensive and can require tens of thousands of computer hours to produce one solution. Substantial gains can be made through memory streamlining (factors of 2.3times faster), efficient cache usage (factors of 3times improvement), and through better parallel design (improving scalability to four times the number of CPUs). These improvements are documented and tested across five different supercomputing hardware platforms for idealized problems designed to highlight the effect of the changes. Then, the cumulative effect of these changes are tested across the five different systems for a typical problem of interest, a relativistic magnetron, on 48 CPUs which shows a factor of two to seven reduction in run-time, or best case, from 21 h to only 3 h.

Virtual prototyping of directed energy weapons

This paper gives an overview of how RF systems, from pulsed power to antennas, may be virtually prototyped with the improved concurrent electromagnetic particle-in-cell (ICEPIC) code. ICEPIC simulates from first principles (Maxwells equations and Lorenzs force law) the electrodynamics and charged particle dynamics of the RF-producing part of the system. Our simulations focus on gigawatt-class sources; the relativistic magnetron is shown as an example. Such simulations require enormous computational resources. These simulations successfully expose undesirable features of these sources and help us to suggest improvements

Virtual Prototyping of Directed Energy Weapons on Thousands of Processors

This paper documents the changes required to permit ICEPIC to scale efficiently to the thousand CPU range. Substantial changes were made to the communication paradigm within the code, so that only one synchronization point is now required. This led to increase of a factor four in the number of processors ICEPIC can productively use on real world problems

Comparison of time and frequency domain antenna measurements

Summary form only given. Currently High-Power Microwave (HPM) systems make use of antennas to radiate the generated microwave pulse. Typically, HPM sources generate a single or dominant frequency with a pulse duration of several hundred microwave cycles. Typically, these antenna systems are calibrated or verified with a frequency domain technique using a Continuous Wave (CW) signal. The radiating system vacuum window or radome may be designed for the dominant frequency of the HPM source. However, the antenna/radome system may not properly accommodate the finite frequency bandwidth due to the finite pulse length or frequency variation. These finite microwave packet size issues can now be accommodated both computationally and experimentally. We will be presenting data comparing frequency domain and time domain simulations, along with experiments. The frequency domain simulations are done with the High Frequency Structure Simulator (HFSS), while the time domain simulations will be done with a Finite-Difference-Time-Domain (FDTD) code such as ICEPIC or VOLMAX. The experiments will compare the standard frequency domain measurements done with a Vector Network Analyzer, while the time domain measurements will be done with packets generated with a CW oscillator and an Arbitrary Waveform Generator (AWG). An example antenna/radome system is our existing Vlasov Antenna and fiberglass belljars. We will also be making use of a TEM to TM/sub 01/ launcher, which has a measured return loss of -20 dB over the frequency range of 1.1 to 1.6 GHz.

Parallel performance characteristics of ICEPIC

Fast, efficient results from the ICEPIC (improved concurrent electromagnetic particle in cell) code are key to the Air Force Research Laboratorys efforts to design high power microwave sources for electronic warfare and nonlethal weaponry. Parallelization of ICEPIC allows the use of DoD supercomputer assets to perform device simulations which would previously have been impossible, and also to obtain these results quickly, with little waste of expensive CPU resources. We explain the parallel design and implementation of ICEPIC. Its parallel PIC loop exploits the possibility of computing and communicating between CPUs concurrently, reducing the waste and overhead inherent to parallel computing. ICEPICs static and dynamic load balancing features ensure that computation is divided evenly among the CPUs, a necessary condition for efficient operation. We show scalability results from both test simulations and actual device simulations, illustrating the effectiveness of ICEPICs parallel design and implementation.