Recent developments to the MICHELLE electron gun and collector design tool are reported in this paper. The MICHELLE code is a new finite-element (FE) two-dimensional and three-dimensional electrostatic particle-in-cell 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, multibeam guns, sheet-beam guns, and ion thrusters. The focus of the development program is to combine modern FE 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, with an advanced secondary emission model. This paper reports on three significant advances to MICHELLE over the past year; hybrid structured/unstructured mesh support, a time-domain electrostatic algorithm, and an ion plasma model with charge exchange.

Recent developments to the MICHELLE 2-D/3-D electron gun and collector modeling code

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the ...

100 years of the physics of diodes

This article gives a brief overview of the common vacuum electronic tube amplifiers used in high-power transmitters. Only three types of devices [travelling wave tubes (TWTs) including helix and coupled-cavity types, microwave power modules (MPMs), and klystrons] are covered, with emphasis on the recent advance in the millimeter-wave band.

Vacuum tube amplifiers

A technological breakthrough is embodied in the successful demonstration of an extended interaction klystron (EIK) amplifier, which has produced over 7.5 kW of peak output power at W-band (94 GHz). An efficiency of ${\sim}{17\%}$ has been achieved with a depressed collector. The EIK is driven by a 20-kV, 4-A sheet beam in a permanent magnet solenoid, with 99% beam current transmission from gun to collector. Key features that contribute to the success of this device are: 1) tight beam focusing and correspondingly narrow beam tunnel, which are made possible by the solenoidal focusing and which provide high interaction impedance and high gain per unit length and 2) the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.

Demonstration of a Multikilowatt, Solenoidally Focused Sheet Beam Amplifier at 94 GHz

In this paper, the transport of intense sheet electron beams in a uniform solenoidal magnetic field in high-power vacuum electronic devices is theoretically examined with the 3-D beam optics code MICHELLE. It is shown that a solenoidal magnetic field can be an effective transport mechanism for sheet electron beams, provided the beam tunnel is matched to the beam shape, and vice versa. The advantage of solenoidal magnetic field transport relative to periodic magnetic transport resides in the feasibility of transporting higher current density beams due to the higher average field strength achievable in practice and the lower susceptibility to field errors from mechanical misalignments. In addition, a solenoidally transported electron beam is not susceptible to voltage cutoff as in a periodic magnetic focusing system; hence, device efficiency is potentially higher.

Intense Sheet Electron Beam Transport in a Uniform Solenoidal Magnetic Field

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 (Child's 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 MICHELLE three-dimensional electron gun and collector modeling tool: theory and design

ARGUS is a three-dimensional, electromagnetic, particle-in-cell (PIC) simulation code that is being distributed to U.S. accelerator laboratories in collaboration between SAIC and the Los Alamos Accelerator Code Group. It uses a modular architecture that allows multiple physics modules to share common utilities for grid and structure input., memory management, disk I/O, and diagnostics, Physics modules are in place for electrostatic and electromagnetic field solutions., frequency-domain (eigenvalue) solutions, time- dependent PIC, and steady-state PIC simulations. All of the modules are implemented with a domain-decomposition architecture that allows large problems to be broken up into pieces that fit in core and that facilitates the adaptation of ARGUS for parallel processing ARGUS operates on either Cray or workstation platforms, and MOTIF-based user interface is available for X-windows terminals. Applications of ARGUS in accelerator physics and design are described in this paper.

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Applications of the ARGUS code in accelerator physics

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

The 3D finite-element gun and collector modeling code, MICHELLE, has been under development at SAIC in collaboration with industrial partners and national laboratories. This development program has been designed 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, with a focus on improved physics models. The code includes both structured and unstructured grid systems for meshing flexibility. Advances have been made in the areas of accurate particle tracking through the mesh and beam emission methods, including new models for thermionic, temperature-limited, Child's law and secondary emission.

/pdf/the-michelle-electron-gun-and-collector-modeling-tool-41z3dateai.pdf

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.

W. Krueger

Papers

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

Applications of the ARGUS code in accelerator physics

The MICHELLE electron gun and collector modeling tool

The MICHELLE electron gun and collector modeling tool

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