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

A 94-GHz overmoded traveling-wave tube (TWT) has been designed, fabricated, and successfully tested. The TWT operates in the rectangular TM31 mode of the cavity, while lower order modes are suppressed using selectively placed strips of lossy dielectric. The 87-cavity TWT circuit was directly machined from Glidcop, a dispersion-hardened copper. The TWT was tested in a 0.25 T solenoidal magnetic field in 3- $\mu \text{s}$ pulses. Operating at a voltage of 30.6 kV with 250 mA of collector current, the TWT was zero-drive stable and achieved 21 ± 2 dB linear device gain with 27-W peak output power. Taking into account 3 dB of loss in both the input and output coupling circuits, the gain of the TWT circuit itself is 27 ± 2 dB with 55 W of saturated circuit output power. Using the 3-D particle-in-cell code CST Particle Studio, the linear circuit gain was estimated to be 28 dB and the saturated output power 100 W, in good agreement with the experimental results. The measured bandwidth of 30 MHz was significantly smaller than the predicted value of 250 MHz. The overmoded TWT is a promising approach to high-power TWT operation at W-band and to the extension of the TWT to terahertz frequencies.

An Overmoded W-Band Coupled-Cavity TWT

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

This work reports some initial results of a 2-D electron gun design code (XMGUN) based on the finite-element method (FEM). Using the Galerkin weak formulation, the nodal analysis, and the first-order elements, the Poisson equation was solved for the electron gun electrostatic potential. The nonrelativistic particle paths were numerically calculated by a fourth-order Runge-Kutta method. An iterative scheme was repeated until the electron paths convergence was achieved under full space-charge limited condition. In order to validate the algorithm, the focusing properties of a 2-D Pierce electron gun with planar symmetry were studied. The quality of the beam was evaluated based on the particle's final position, the transit time, and the particle energy evaluations. Using these three parameters, a good agreement was found between the theoretical and calculated results. Absolute current density errors of less than 1% were found, even with a coarse discretization of the domain. The XMGUN tool was used to design a 30-kV, 7.1-A, and 1.37-μPerv axis-symmetric high-power electron gun for use in vacuum microwave devices. The figure of merit used as reference to measure the quality of the electron beam was the normalized transverse velocity.

The XMGUN Particle Path FEM Code

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.

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

An important problem in the design of RF linacs is the coupling between the waveguide that feeds RF power into the accelerator and the cavity through which the beam is being accelerated. The designer needs to know the coupling coefficient, the frequency shift, and the external Q due to the waveguide. In addition, the details of the field geometry in the vicinity of the aperture are important in the design. The simulation code ARGUS has been employed in a collaboration between SAIC and AccSys Technology, Inc. To model the external Q of the drift tube linac (DTL) tanks in the injector of the Superconducting Super-Collider (SSC). (The drift tube linear accelerators are designed and built by AccSys Technology.) As the coupling aperture (iris) size and shape is changed, the coupling factor changes. This paper presents results of numerical simulations produced to aid in determining the optimum iris size for power coupling to the tank. A comparison of the simulation results will be made with results from experimental data. >

Frequency Domain Determination of the Waveguide Loaded Q for the SSCL Drift Tube Linac

Summary form only given. The Cold-Test, Large-Signal Simulation code (CTLSS) is being developed to provide an electromagnetic simulation tool that is designed to interoperate with large-signal codes employed in microwave and millimeter-wave vacuum electron device design. Recently, CTLSS has been redesigned as a conformal-mesh code to increase simulation accuracy in complex structures. The new code solves Maxwell's equations using a multi-block, structured, non-orthogonal-grid for two classes of problems: (1) eigenvalue problems and (2) driven-frequency problems. The numerical formulation in CTLSS is based on a generalization of the Finite Integration Technique to non-orthogonal grids. The iterative solution methods, based on Jacobi-Davidson and QMR, were selected to handle non-Hermitian operators that arise when treating lossy materials. Using a conformal mesh provides greater accuracy and efficiency when modeling complex structures, compared with the stairstep discretization of structures, due to improved boundary representation and significantly improved convergence characteristics. The conformal representation of boundaries also permits accurate determination of wall-losses. This paper will summarize the current status of CTLSS development. Examples related to the design of helix TWTs and coupled-cavity TWTs will be presented.

Cold-test, large signal simulator: a conformal-mesh electromagnetic field solver

Frequency domain determination of the waveguide loaded Q for the SSCL drift tube linac

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