D. Panagos

The new 3D electron gun and collector modeling tool: MICHELLE

Progress on a new three-dimensional electron gun and collector design tool is reported. This new simulation code is 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, multi-beam devices, and anisotropic collectors, with a focus on improved physics models. 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.

Advanced electron guns and depressed collectors design and optimization using the MICHELLE / ANALYST Environment

Next generation vacuum electron devices under development for millimeter and sub-millimeter wavelengths are often characterized by very small features that must be very precisely designed and manufactured for proper tube function and longevity. In this regime the need for automated physics-based optimization to aid the designer in meeting device performance specifications is much more critical than in larger, lower frequency devices where prototyping and experimentation are more readily performed. Recent work has been done on improving the ability of modeling and design simulation environments to aid the designer in finding optimum configurations. As the simulation tools have improved to enable first-pass design success in some cases, the potential benefits of optimization techniques become even more significant. This paper discusses methods for optimization of electron guns as well as multistage depressed collectors.

Electrostatic time-domain PIC simulations of RF density-modulated electron sources with MICHELLE

There is a significant level of effort by SAIC and BWR, funded by ONR & JTO, to enhance the three dimensional (3D) finite-element (FE) electrostatic time-domain (ESTD) particle-in-cell (PIC) code MICHELLE to provide modeling and simulation of the interaction of electron emission sources in the presence of electromagnetic cavity fields. These enhancements have direct importance to the free-electron laser (FEL) based high-energy laser community, providing the capability of modeling advanced photo-emission RF electron guns and the input cavity of high-power UHF inductive output tubes (IOT), both of which are needed for the FEL injector. We will discuss these code modifications, enhancements to our emission models, recent results of benchmarking against electromagnetic codes, as well as the limitations to the model.

Application of the 2D/3D MICHELLE code to electron gun and collector modeling

The focus of the development program is to combine modern finite element techniques with improved physics models. The code employs a conformal mesh, including both structured and unstructured mesh architectures for meshing flexibility, along with a new method for accurate, efficient particle tracking. New particle emission models for thermionic beam representation are included that support primary emission, and secondary emission is handled with an advanced model.

Status of the MICHELLE code and applications to RF guns

Summary form only given. The MICHELLE two-dimensional (2D) and three-dimensional (3D) steady-state and time-domain particle-in-cell (PIC) code has been employed successfully by industry, national laboratories, and academia and has been used to design and analyze a wide variety of devices, including RF photoemitters, RF gated emitters, multistage depressed collectors, gridded guns, multibeam guns, annular-beam guns, sheet-beam guns, beam-transport sections, and ion thrusters.

19.1: 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 and e-beam lithography 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 previously could not have been readily modeled. This presentation gives an overview of recent applications, capabilities, and the current status of MICHELLE. A gun optimization problem will be presented and the effects of different modeling parameters and meshing techniques will be discussed.

Applications and current status of 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. Its primary development focus has been for DC electron guns and depressed collectors, however, it has other applications such as RF electron guns, ion thrusters, photocathodes, etc. Its 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. In particular, application to time-dependent problems and optimization will be illustrated

Applications and status of the finite-element MICHELLE gun & collector simulation code

The MICHELLE two-dimensional (2D) and three-dimensional (3D) steady-state and time-domain particle-in-cell (PIC) code has been employed successfully by industry, national laboratories, and academia and has been used to design and analyze a wide variety of devices, including multistage depressed collectors, gridded guns, multibeam guns, annular-beam guns, sheet-beam guns, beam-transport sections, and ion thrusters. Recent work has included time dependent application for rf guns and photoemission gun applications. Time domain effects in RF guns and collector modeling has shown to be an important effect to capture. Thermionic emission in IOT guns and guns operating in the transition from space charge limited to temperature limited regimes require special attention to accurately predict the gun performance. Modeling thermal beams through guns and into transport regions multiplies the run time by a factor of about the increase in the number of particles to support a thermal beam distribution. As a result, a parallel version of MICHELLE is being developed to hold run times down. In the typical vacuum tube, the beam entering a collector is often in one of two states; as a DC beam or as a spent beam resulting from an RF interaction. The DC beam entering the collector is easily treated by the steady-state algorithm. This regime of operation often gives the highest peak power loads, which is an important design constraint to manage in the development process. However, MICHELLE can apply its time-domain ES PIC model to the bunched beam as well. In this case, the spent beam from an interaction region (from an interaction code or a PIC code) is brought into the collector as a function of time. One RF period of the beam is required and it is repeated and injected into the collector domain until a time-dependent steady-state has been achieved. Examples of new capabilities and applications will be presented.

Collector design with time dependent spent beam using the Michelle code

Summary form only given. The MICHELLE two-dimensional (2D) and three-dimensional (3D) electrostatic steady-state and time-domain particle-in-cell (PIC) code has been employed successfully by industry, national laboratories, and academia and has been used to design and analyze a wide variety of devices, including multistage depressed collectors, gridded guns, multibeam guns, annular-beam guns, sheet-beam guns, beam- transport sections, and ion thrusters. Its 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. Time domain effects in collector modeling has shown to be an important effect to model. Spent Beam Collectors, whether they are for energy recovery or just a beam dump, are often large devices with very a large ratio of incoming beam radius to device size. Required mesh resolution with such disparate spatial scales makes time-domain electromagnetic modeling not efficient and not the method of choice in many cases. Because of the demanding meshing requirements, MICHELLE with its finite-element meshing capability has been found to be an effective simulation tool for time-domain collector design. In the case of high-average power compatible collector design, mitigating potential virtual cathode behavior of the time-dependent beam is important. In particular, for high- perveance fundamental-mode MBK collector designs, attention must be paid not only on the energy spectrum of the spent beam but also on the time-dependent nature of the bunched beam upon entering the collector. This time- dependent behavior, if not properly taken into account, can lead to substantial and unexpected particle reflections, since the depth of the depression is a function of the peak RE beam current. We will present as an example the time-dependent spent beam collector modeling of an NRL high average power multiple beam klystron (MBK) collector.

Optimization of Electron Guns and Collectors using the 2D/3D Michelle and Anlayst Finite-Element Codes

This paper discusses various procedures for optimization of electron guns as well as multistage depressed collectors. The MICHELLE code has been interfaced to the ANALYST analysis package, which provides comprehensive support for finite-element electromagnetic analysis, including embedded computer-aided design (CAD) software, automated meshing, and both visual and numerical result processing.