This article explores the history and diversity of this remarkable technology, with emphasis on advances in vacuum-electronic amplifiers, including the microwave power module (MPM), that have been enabled by the ongoing development of modeling and simulation tools These physics-based codes, enabled by rapid advances in computational hardware, allow simulation-based design and optimization of complex vacuum-electronic circuits The growth of satellite-based digital communications technology over the past decade has opened lucrative commercial opportunities for vacuum-electronic amplifiers This exciting new area requires the efficient production of high-frequency power and the ability to handle spectrally efficient modulations within stringent packaging constraints at affordable cost

Vacuum electronics for the 21st century

This paper reports the development of a fully 3-D finite-element (FE) design tool for the design of electron guns and collectors called the electron optics simulator (EOS). It is a module of the microwave tube simulator suite (MTSS) developed by the University of Electronic Science and Technology of China. EOS specifically targets problem classes including axisymmetric gun, gridded gun, multibeam gun, and multistage depressed collector. EOS is a steady-state trajectory code, which is a solver of electrostatic field and trajectory that takes into account secondary electrons, space-charge effects, and the external magnetic field. The simulator is dependent on the following two front-end features in MTSS: preprocessing that includes a solid modeling tool and a fully 3-D tetrahedral FE mesh automatic generator, and post processing for analysis and display of the results. A friendly interface is provided to design, test, optimize, and validate the electron optics systems. The aforementioned theoretical models are discussed in detail in this paper. In the end, some comparisons between simulation and experiment are presented.

Electron Optics Simulator: A Three-Dimensional Finite-Element Electron Gun and Collector Design Tool

Analysis of temperature-limited flow, space-charge-limited flow, and the transition between them using a simple planar diode with a thermionic cathode, in which the cathode surface has spatially nonuniform emission properties, is presented. Our theoretical results, which are derived from a model based on solutions to the Vlasov and Poisson equations, compare well with the results of particle-in-cell simulations. We find that the location and the shape of the knee in the anode current versus temperature characteristic (Miram or “rollover” curve) are significantly affected by non-uniformities in the space-charge density in the A–K gap, but are relatively unaffected by the electron motion parallel to the electrode surfaces. In particular, emission from an actively emitting region is strongly affected by the forces (or lack thereof) exerted by the space-charge of the electrons emitted by their neighbors. Perhaps, most remarkably, we find that the limiting current reaching the anode is approximately given by the classical 1-D Child–Langmuir law, even if a significant fraction of the cathode surface is non-emitting.

/pdf/effect-of-nonuniform-emission-on-miram-curves-3lifz5fge5.pdf

Effect of Nonuniform Emission on Miram Curves

Recent advancements made in millimeter-wave helix-traveling wave tubes (TWTs) at L-3 Communications Electron Technologies (L-3 ETI), Inc. (formerly Electron Dynamic Devices, Inc. and originally Hughes Electron Dynamics Division), in continuous-wave (CW) output power capability and the overall efficiency of Ka-band and Q-band devices are presented. The 8921HP, L-3 ETI's latest high-power communications Ka-band TWT model, demonstrates 250-300-W CW output power and 47% minimum overall efficiency with a two-stage collector over 27.5-31 GHz. Another Ka-band device, the 8922HP, developed for pulsed radar applications over 33.4-36 GHz, produces 230-W minimum output power over the bandwidth with 49% minimum overall efficiency. This device was developed for pulsed operations but has demonstrated CW power-handling capability up to 270 W. In Q-band, the 8925HP, derived from the current production 120-W Q-band helix-TWT (8905HP), significantly extends the CW output power capability, demonstrating 230-W minimum over 43.5-45.5 GHz. The beam focusing is improved in both the Ka-band and the Q-band TWT models, with saturated radio-frequency beam interception well below 1% of the nominal beam current of 95 mA. The devices can be operated in pulsed mode by using the focus electrode to cut off the beam. The electron gun typically requires a focus electrode voltage of -800 to -900 V with respect to cathode for beam cutoff.

Development of high-power Ka-band and Q-band helix-TWTs

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, Child's 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.

/pdf/the-michelle-electron-gun-and-collector-modeling-tool-3jc9sw17fx.pdf

The MICHELLE electron gun and collector modeling tool

A new 3D finite-element gun and collector modeling code, MICHELLE, is under development at SAIC in collaboration with industrial partners and national laboratories. This development program has been designed specifically to address the shortcomings of current beam optics simulation and modeling tools for vacuum electron devices. The program specifically targets problem classes including gridded-guns, sheet-beam guns, multi-beam devices, and anisotropic collectors. In particular, although there are 3D gun codes that exist today, their ability to address fine scale features is somewhat limited in 3D due to disparate length scales found in certain classes of devices. Additionally, features like advanced emission models, including thermionic Child's law and comprehensive secondary emission models also need attention for detailed modeling, and are included in the code. As part of this new effort, an algorithmic representation of a comprehensive model of secondary emission is being developed for this computational tool 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 new 3D electron gun and collector modeling tool: MICHELLE

The MICHELLE code is a new Finite-Element (FE) two-dimensional (2D) and three-dimensional (3D) electrostatic particle-in-cell (PIC) code that has been designed to address the recent beam optics modeling and simulation requirements for vacuum electron devices, ion sources, and charged-particle transport. Problem classes specifically targeted include depressed collectors, gridded-guns, multi-beam guns and sheet-beam guns. 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. This presentation focuses on two significant advances to MICHELLE over the past year; hybrid structured/unstructured mesh support, and a time-domain electrostatic algorithm.

Recent advances in the MICHELLE 2D/3D electron gun and collector modeling code

Summary form only given, as follows. A new 3D finite element gun and collector modeling code is under development at SAIC in collaboration with industrial partners and National Laboratories. This development program has been designed specifically to address the shortcomings of current simulation and modeling tools. In particular, although there are 3D gun codes that exist today, their ability to address fine scale features is somewhat limited in 3D due to disparate length scales of certain classes of devices. Additionally, features like advanced emission rules, including thermionic Child's law and comprehensive secondary emission models also need attention. The program specifically targets problems classes including gridded-guns, sheet-beam guns, multi-beam devices, and anisotropic collectors. The presentation provides an overview of the program objectives, the approach to be taken by the development team, and a status of the project.

A description of the new 3D electron gun and collector modeling tool: MICHELLE

RF guns and density modulated guns that encompass photocathodes and inductive output tube sources are difficult to model but continue to be at the forefront of solutions to many applications. Modeling these density-modulated beam sources requires a high degree of computational mesh resolution to resolve geometrical features, or simply fine spatial scale phenomena. This paper presents the application of the MICHELLE code to such devices.

Application of the Finite-Element MICHELLE Beam Optics Code to RF Gun Modeling

The presentation describes work done under a Navy contract to reduce the life cycle cost of an S-band coupled cavity device. End of life of these devices was determined to be caused primarily by cathode wear-out. Consequently, the thrust of the program has been to re-design the electron gun for longer life.

Gun life improvement program

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