Peter Mardahl

Relativistic magnetron driven by a microsecond E-beam accelerator with a ceramic insulator

Relativistic magnetron experiments performed on a six-cavity device have generated over 300 MW total microwave power near 1 GHz. These experiments were driven by the long-pulse electron beam from an accelerator with parameters as follows: voltage of *300 kV, current of 1-10 kA, and typical pulselength of 0.5 ms. This paper reports investigations of high-power microwave generation, mode competition, and pulse shortening for the relativistic magnetron with a ceramic insulator compared to a plastic insulator. The ceramic insulator improves the vacuum by a factor of ten (to 10/sup *7/ torr range) and flattens the voltage of the accelerator. Relativistic magnetron performance with the ceramic insulator shows increased microwave power and pulselength over the plastic insulator. Effects of RF breakdown in the extraction waveguide on peak microwave power and pulselength are also investigated by utilizing SF/sub 6/ in one or both of the extraction waveguides.

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.

Design and Simulation of a Mega-Watt Class Nonrelativistic Magnetron

Numerical simulations of a prototype conventional magnetron capable of an RF output power exceeding 1.0 MW are presented. Magnetron design evaluation is carried out via numerical simulation using the 3-D Improved Concurrent Electromagnetic Particle-in-Cell code. The magnetron was capable of oscillating in the π mode with little mode competition at 655 MHz over a range of magnetic fields extending from B = 0.186 to B = 0.261 T and voltages ranging from 40 to 64 kV. RF Output power ranged from 400 kW to 2.1 MW over these voltages with efficiencies typically at 60%. RF power propagation upstream was identified as a major source of loss in the design.

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

A High-Efficiency Megawatt-Class Nonrelativistic Magnetron

Numerical simulations of a prototype conventional magnetron capable of an RF output power exceeding 1.3 MW at peak efficiency greater than 87% for relatively low diode voltages of ~ 40 kV are presented. Virtual prototyping of the magnetron design is carried out on massively parallel architecture utilizing the 3-D improved concurrent electromagnetic particle-in-cell code. Simulations demonstrate that the magnetron is capable of stable and robust oscillations in the π mode at saturation with negligible mode competition at 912 MHz over a range of magnetic fields extending from B = 0.18 T to B = 0.275 T and voltages ranging from 37-56 kV. RF Output power ranged from 400 kW-1.5 MW over these voltages with efficiencies typically above 85%. Oscillations in the π mode follow the Buneman-Hartree resonance curve for all magnetic fields sampled with a window of π-mode oscillations typically extending over 6 kV. Electron back bombardment of the cathode as well as collisions with the slow wave structure acted as major loss mechanisms.

Three Dimensional PIC Simulations of Novel Cathodes in the Michigan and AFRL Relativistic Magnetrons

Several novel shaped cathodes for a relativistic magnetron were modeled and optimized using a massively parallel electromagnetic particle-in-cell code ICEPIC. The effect of using a shaped cathode to enhance key performance parameters such as output power, power efficiency and impedance, was examined. In simulations we saw a dramatic increase in the range of magnetic field in which the magnetron functions, an increase of output power, an increase in efficiency, an elimination of mode competition, and immediate start up

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

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.

Compact non-invasive bunch length monitor ICEPIC study

A novel non-invasive, near real-time cavity detector used to monitor the bunch length and profile of photo-emitted electron bunches was simulated in ICEPIC [1] for the purpose of providing a meaningful comparison to experimental data [2] and for predicting the cavity response to a variety of bunch lengths and profiles.

Controllable Axial Overmoding in an MDO

During a computational investigation of a magnetron utilizing a diffraction output, it was discovered that axial overmoding allowed more than one π mode to exist. Further investigation led to the identification of at least four distinct π modes, each with its own frequency and associated 2π/3 and 4π/3 modes, each accessible by appropriate choices of voltage and magnetic field. This result indicates that a discretely tunable GW class device is conceivable.