Dimitrios Panagos

The MICHELLE three-dimensional electron gun and collector modeling tool: theory and design

The development of a new three-dimensional electron gun and collector design tool is reported. This new simulation code has been designed to address the shortcomings of current beam optics simulation and modeling tools used for vacuum electron devices, ion sources, and charged-particle transport. The design tool specifically targets problem classes including gridded-guns, sheet-beam guns, multibeam devices, and anisotropic collectors, with a focus on improved physics models. The code includes both structured and unstructured grid systems for meshing flexibility. A new method for accurate particle tracking through the mesh is discussed. In the area of particle emission, new models for thermionic beam representation are included that support primary emission and secondary emission. Also discussed are new methods for temperature-limited and space-charge-limited (Childs law) emission, including the Longo-Vaughn formulation. A new secondary emission model is presented that captures true secondaries and the full range rediffused electrons. A description of the MICHELLE code is presented.

Delayed photo-emission model for beam optics codes

Future advanced light sources and x-ray free electron lasers require fast response from the photocathode to enable short electron pulse durations as well as pulse shaping, and so the ability to model delays in emission is needed for beam optics codes. The development of a time-dependent emission model accounting for delayed photoemission due to transport and scattering is given, and its inclusion in the particle-in-cell code MICHELLE results in changes to the pulse shape that are described. The model is applied to pulse elongation of a bunch traversing an rf injector, and to the smoothing of laser jitter on a short pulse.

Modeling emission lag after photoexcitation

A theoretical model of delayed emission following photoexcitation from metals and semiconductors is given. Its numerical implementation is designed for beam optics codes used to model photocathodes in rf photoinjectors. The model extends the Moments approach for predicting photocurrent and mean transverse energy as moments of an emitted electron distribution by incorporating time of flight and scattering events that result in emission delay on a sub-picosecond level. The model accounts for a dynamic surface extraction field and changes in the energy distribution and time of emission as a consequence of the laser penetration depth and multiple scattering events during transport. Usage in the Particle-in-Cell code MICHELLE to predict the bunch shape and duration with or without laser jitter is given. The consequences of delayed emission effects for ultra-short pulses are discussed.

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.

RF density-modulated electron source simulations with MICHELLE

Summary form only given. New models have been developed and implemented in the MICHELLE Finite-Element Electrostatic Particle-in-Cell code1 in support of modeling RF photocathodes and IOTs (inductive output tubes). In the case of photocatodes, low emittance, high current density sources are required to achieve the small beam size needed for high frequency vacuum electronic devices and, in particular, low emittance sources are demanded for high power free electron lasers (FELs). Emission models are of particular importance in the emittance-dominated regime, where emission non-uniformity and surface structure of the cathode can have an impact on beam characteristics and situations that depend on beam quality (e.g., halo). We have been developing comprehensive time-dependent photoemission models that account for laser and cathode material and surface characteristics and adapting them to develop emission models for inclusion into beam simulation codes2. In addition to the photoemission effects, including the effects of thermal field emission and modeling dark current are key to predicting beam quality and performance degradation due to beam tails and halo.

Modeling emission processes in the Finite-Element MICHELLE gun & collector simulation code

The MICHELLE code [1], [2] 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.

The MICHELLE electron gun and collector modeling tool

Summary form only given, as follows. The new conformal grid MICHELLE 3D Electron Gun and Collector modeling code has been used in a variety of design modeling efforts to test it s capabilities and show it s robustness. This MICHELLE code specifically addresses shortcomings of beam optics simulation and modeling tools, and targets problem classes including gridded-guns, sheet-beam guns, multi-beam devices, and anisotropic collectors. Additionally, features like improved emission models, including new thermionic, Childs law, and secondary emission models are discussed. As part of this effort, a new, comprehensive secondary emission model has been developed for MICHELLE in order to achieve an accurate beam collection design capability. The basic physics model in the code is based on the equilibrium steady state application of the electrostatic PIC approximation employing both hexahedral and tetrahedral grid systems. The code employs a multiblock architecture, allowing flexibility in gridding and memory use, as well as hardware architecture adaptability. The current version supports a multiblock, conformal, structured, hexahedral grid. The multiblock, unstructured, the tetrahedral version of the code is currently under development. The structure of the code is the following: The code employs a basic infrastructure that handles generic tasks like grid definition and memory management, down to basic utility functions including basic IO, post-processing IO, and vector algebra. New algorithms have been developed and are being implemented for calculation of particle trajectories on finite element grids. The code supports third-party CAD and Gridding tools allowing the user to make use of current in-house software. In addition, a Post Processor has being developed that is tuned for the tasks of modeling and analyzing output for gun and collector simulations. The code allows magnetic field profiles input from ANSOFT s Maxwell 2D/3D, and other similar software.

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).