Ben Held

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.

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.

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.

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.

Design and Optimization Electron Guns and Depressed Collectors Using the MICHELLE code in the ANALYST Environment

Electron gun and multistage depressed collector design remain significant tasks in new vacuum electron device development. The ability to accurately predict device performance continues to improve. Recent work has been done on improving the ability for 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 various procedures for optimization of electron guns as well as multistage depressed collectors.

Modeling and design of high-power single-beam and multiple-beam Inductive Output Tubes

The Inductive Output Tube (IOT) is today the device-of-choice for terrestrial UHF broadcast applications due to the IOTs high efficiency with linearity and compact size. The accelerator community is also making the transition to IOT technology for a number of high-power UHF and L-band applications as a result of these benefits. Although the IOT appears to be quite simple, the actual operation of the device is quite complex and difficult to analyze quantitatively. Consequently, we are investigating the physics of the beam-wave interaction of the IOT with the goal of achieving significantly higher power operation. The time-domain electrostatic PIC code MICHELLE, in conjunction with the Analyst® suite of electromagnetic codes, were used to model the cathode-grid-anode structure that comprise the input cavity. Our investigation has led to the discovery of a mechanism responsible for intra-bunch charge formation. Time-domain PIC results of this effect will be shown. We will also present simulation results of the large-signal beam wave interaction in the output cavity using the code TESLA. Examples of single beam and multiple-beam (MB) IOT designs will also be shown.

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.

High-power Multiple-Beam IOT design

The Multiple-Beam (MB) Inductive Output Tube (IOT) has been proposed for UHF applications requiring from 500 kW to 1 MW of average power. Our team of Scientists and Engineers are developing and validating a suite of computation tools for the design of these advanced devices. The cornerstone of this effort is the development of a circuit-based beam-loading model in the Finite Element Electrostatic PIC code MICHELLE [1]. Results of our validation effort will be shown. We are also applying these tools, along with codes TESLA [2] and Analyst [3], to a design study of a number of five, six, ten and twelve-beam MB IOTs, for average power levels ranging from 500 kW to 1 MW. Results from this effort will be described.

Multi-stage depressed collector design using MICHELLE in ANALYST

A four-stage depressed collector is designed with MICHELLE using the optimization tool in ANALYST. The effect of slow or “true” secondary electrons (SSE), back-scattered primary electrons (BSE) and different geometries are studied in the modeling of multi-stage depressed collectors. Details of the design and optimization philosophy along with user settings in MICHELLE and ANALYST will be discussed. Results of a collector that was designed, built and tested at CPI, will be presented.

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.