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

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

In this paper a design procedure of a 9kV, 0.45A, with a beam waist of 1.5 mm electron gun, without control grid, operating under the space-charge limited condition, to be used in a C-Band Traveling-Wave Tube (TWT), is presented. During the design, three electron gun geometric parameters were varied, namely: the cathode-anode distance, the anode height and the focusing electrode angle, in order to study the electron beam behavior. To cut the electron beam current, a negative voltage was applied on the focusing electrode. It was verified that a 2.5kV biasing negative voltage in the focusing electrode, decreased the electric current from 0.45 A to 1.2mA. All gun simulations were done using the EGUN code.

An electron gun design for a C-band TWT

The EGUN code is used to model a 0,7µPerv electron gun with control and shadow grids under space-charge limited flow. The model parameters will be used to implement a pulsed traveling-wave tube (TWT). The simulation main goal is to obtain an electron gun to yield a 2.0 mm beam-waist with a 6.5 compression ratio. This work presents the sensitivity analysis for the following input geometric quantities: the cathode radius disc; the electrode focus angle; the cathode-to-anode distance; the grid-to-cathode distance; and the grid voltage. It was observed a decreasing of the gun perveance and the current when the grid voltage was turned on if it compared to the gun design without grids. The results also show that a shadow-grid physically modeled that led to a gun with lower current/perveance. Moreover, a 3-Dimensional (3D) viewer of the EGUN output current density data is implemented. Last, a final assembling stage electron gun experimental setup is presented.

Modeling of a gridded electron gun for traveling wave tubes

This thesis presents the theory, design, and experimental demonstration of coherent microwave generation at 2.4 GHz in a metamaterial loaded waveguide using a 490 keV, 84 A, one microsecond pulse length electron beam that produced more than 5 MW of microwave power. Three different metamaterial structure designs named MTM1, MTM2, and MTM3 were tested with design frequencies of 2.8, 2.4, and 3.7 GHz, respectively. The waveguides were loaded with two metamaterial plates that were machined with complementary split ring resonators with periods ranging from 5 to 10 mm. The metamaterial waveguides supported two distinct modes: a symmetric mode that occurs when the two metamaterial plates were excited in phase, and an antisymmetric mode that occurs when the metamaterial plates were excited out of phase. The electron beam propagated on axis between the metamaterial plates. The output radiation was studied for solenoid magnetic field values in the range 350 to 1600 G and for beam voltages from 350 to 500 kV. The best results were found in a 370 mm long structure using the MTM2 design, where output power levels of up to 5 MW were obtained at 400 G in the antisymmetric mode at a frequency near 2.39 GHz. The frequency tuning vs. magnetic field for operation at a power level exceeding 1 MW was consistent with that predicted by an anomalous Doppler shifted resonance condition, W = k~v, Q/7. At magnetic fields above 750 G, the microwave output switched to the symmetric mode at a frequency near 2.44 GHz, but the power level dropped drastically to below 100 W. In contrast to the antisymmetric mode, the frequency tuning of the symmetric mode was consistent with that predicted by a normal Cherenkov resonance, w = k~v,. CST PIC simulations predict the observed output frequencies and the switch between modes at 750 G. However, the CST simulations also predict multi-megawatt power levels in both modes, which was observed in the antisymmetric mode, but not the symmetric mode. The discrepancy between the symmetric mode output power of the simulations and experiment Is unexplained. To the authors knowledge, these are the first reported experimental results of high power (> 1 MW) microwave generation from an electron beam interacting with a metamaterial structure. The results are important for the development of new microwave

High power microwave generation using an active metamaterial powered by an electron beam

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

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The MICHELLE electron gun and collector modeling tool