Alex Burke

Design of a wideband high-power W-band serpentine TWT

The design of a W-band serpentine TWT with >200 W of power over a 4 GHz bandwidth (>100 W over 7 GHz) is presented. The amplifier is driven by a 122mA, 20 kV electron beam generated by a slightly modified version of the demonstrated 670 GHz beamstick at a reduced magnetic field. The design was performed by both the established MAGIC-3D and the recently validated NRL code Neptune, with good agreement between the two codes. Predicted RF peak power is 245 W, corresponding to 10% electronic efficiency.

19.2: Modeling emission processes in the finite-element MICHELLE gun & collector simulation code

The MICHELLE code is a Finite-Element Electrostatic Particle in Cell code for application to 2D and 3D particle beam formation, transport, and collection. Although its initial development focus had been for DC electron guns and depressed collectors, other applications such as RF electron guns, ion thrusters, photocathodes, etc. have become a recent focus. The MICHELLE codes ability to manage large mesh sizes and large particle counts in complex geometries requiring the resolution of disparate spatial scales in 2D and 3D on desktop computers has allowed it to be applied to devices that could not have been readily modeled in recent years. This presentation gives an overview of recent developments in the area of emission physics models including photoemission, dark current, and thermal beams with applications to time-dependent examples.

Ku band, 100 kW traveling wave tube based on large quasi-optical spatial power combining array

We report here further development of the novel quasi-optical spatial power combining array for high power millimeter wave (MMW) traveling wave tubes (TWTs) by demonstrating a Ku-band high power TWT which covers 12-15 GHz and with 100 kilowatt (kW) output power. Specifically, a Ku-band high power TWT which consists of a quasi-optical spatial power combining array of fifteen beam-wave interaction circuit slow wave structures and, as a result, beam width/height aspect ratio of close to 85 was developed to achieve a combined output power of over 100 kW at Ku-band. The 15 individual beam-wave interaction structures in the quasi-optical spatial power combining array are arranged into a linear array. Instead of a single cathode, fifteen cathodes, each with its own focus electrode or, in other words, a total of 15 focus electrodes are also used to create a required large sheet of beam for the large quasi-optical spatial power combining array of 15 channels of individual beam-wave interaction structure. Although a single stage collector was initially designed, however, a multi-stage depressed collector will also be designed and implemented to improve the efficiency of this K-band high power TWT. The overall size of the Ku-band high power TWT is relatively small since the same vacuum envelope and electron beam focus optics are shared among the five beam-wave interaction structures. Design and fabrication of this Ku-band high power TWT will be presented to demonstrate the large quasi optical spatial power combining array for very high power MMW TWTs and with reasonable broad bandwidth.

Design of a 670 GHz Extended Interaction Klystron

The development of terahertz power amplifiers presents significant new challenges as it brings into focus design, fabrication, and measurement issues that are not important factors at lower frequencies. We will describe our design approach to meet these challenges with particular emphasis on a 0.67 THz Extended-Interaction Klystron (EIK) [1].

High performance parametric design optimization of RF devices

We present an integrated environment for large scale multi-parameter design optimization of RF devices based on AFRLs Galaxy Simulation Builder productivity tool for distributed high-performance computing, Sandia National Labs DAKOTA optimization library, and a suite of highly efficient GPU-based Electromagnetic codes developed at NRL in collaboration with Leidos, Inc. The environment allows for an end-to-end optimization cycle of an RF device to be set up, deployed, carried out, monitored and analyzed in a quick, user-friendly, robust, and flexible fashion using a diverse variety of high-end parallel computing resources.

Developments in parallelization and the user environment of the MICHELLE charged particle beam optics code

The next generation of the MICHELLE ES PIC code is to improve its parallelization and leverages a number of existing and emerging DOD HPC architectures and software including distributed memory clusters, multicore, and computational accelerators such as GPUs and Intel Xeon Phi co-processors. The ongoing project supported by the DOD HASI program also aims to build interfaces between MICHELLE and existing HPC tools such as CAPSTONE, GSB, ParaView, and VisIt for efficient design and optimization workflow. This paper reports on the latest progress and discusses applicable algorithms and implementations.

A high-performance distributed computing framework for parametric design optimization of RF devices

The design cycle of RF devices is greatly facilitated by the use of the “virtual prototyping” methodology based on high-fidelity computer simulations that are capable of predicting the RF devices performance in response to changes in its physical parameters. In particular, critical dimensions of the structure, or quantitative properties of the various electromagnetic components are routinely utilized in sensitivity analyses coupled with performance optimization. This type of process is well suited to semi-supervised global optimization. To this end we have integrated our RF simulation codes with several existing software tools for optimization and distributed high-performance computing code deployment and management, such as the DAKOTA toolkit [1], and AFRLs Galaxy Simulation Builder (GSB).

P2-35: Effects of MICHELLE thermal beam modeling on Ka-Band devices

The effect of thermal velocities on beam formation and PPM focused beam transport, including cathode edge emission, are studied. MICHELLE simulations of a Ka-Band electron gun are presented that include a thermal beam emission model for a hot cathode, and these are compared to “cold” beam simulation results. Details of the MICHELLE thermal beam model are described, including an explanation of different options for configuring the model in the user interface, and which settings were used. The effects of thermal velocity spread on beam-wave interaction in a Ka-Band CC-TWT are discussed with applications to interaction code simulations.

Status of the michelle code and applications

The MICHELLE code is a Finite-Element Electrostatic Particle in Cell code for application to 2D and 3D particle beam formation, transport, and collection. Although its initial development focus had been for DC electron guns and depressed collectors, other applications such as RF electron guns, ion thrusters, photocathodes, etc. have become a recent focus. The MICHELLE codes ability to manage large mesh sizes and large particle counts in complex geometries requiring the resolution of disparate spatial scales in 2D and 3D on desktop computers has allowed it to be applied to devices that could not have been readily modeled in recent years. This presentation gives an overview of recent applications, capabilities, and the current status of MICHELLE. A gun optimization problem for a THz application will be presented. The effects of different modeling parameters and meshing techniques will be illustrated.