K.T. Nguyen

MAGY: a time-dependent code for simulation of slow and fast microwave sources

We present the newly developed Maryland Gyrotron (MAGY) code for modeling of slow and fast microwave sources. The code includes a time-dependent description of the electromagnetic fields and a self-consistent analysis of the electrons. The calculations of the electromagnetic fields are based on the waveguide modal representation, which allows the solution of a relatively small number of coupled one-dimensional partial differential equations for the amplitudes of the modes, instead of the full solution of Maxwells equations. Moreover, the basic time scale for updating the electromagnetic fields is the cavity fill time and not the high frequency of the fields. The equations of motion of the electrons are formulated within the framework of the guiding-center approximation and solved with the electromagnetic fields as the driving forces. Therefore, at each time step, a set of trajectories are calculated and used as current sources for the fields. We present two examples for the operation of the code, namely the two-cavity gyroklystron and the backward-wave oscillator (BWO). These examples demonstrate the possible usage of the code for a wide variety of electron-beam systems.

Experimental studies of a four-cavity, 35 GHz gyroklystron amplifier

The experimental results from a four-cavity, Ka-band gyroklystron amplifier operating in the(TE)/sub 011/ cylindrical mode at the fundamental of the cyclotron frequency are presented. A peak output power of 208 kW at 31.90 GHz, with a 3-dB bandwidth of 178 MHz (0.5%), an electronic efficiency of 30%, and a saturated gain of 53 dB was obtained with a 72-kV, 9.6-A electron beam at a magnetic field of 12.95 kG and a measured beam velocity ratio of 1.36. The magnetic field was found to have a strong influence on the power-bandwidth tradeoff. At 13.35 kG, a peak output power of 174 kW at 31.90 GHz, with a 3-dB bandwidth of 240 MHz (0.7%), an electronic efficiency of 25%, and a saturated gain of 56 dB was obtained with the same beam parameters. The gyroklystron amplifier was unconditionally stable at these operating points and stability studies are reported. Experimental results were found to be in excellent agreement with large signal simulations.

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.

TWT stability for frequencies near a band edge

Using a simple model we derive a universal condition for oscillation in a traveling wave tube for frequencies near a band edge, when the adjacent band gap is small. The condition is expressed graphically as a value for the maximum allowable (stable) small signal gain as a function of the size of the band gap. As an example of the application of the general condition, a simple upper bound on the E-plane offset of a beam tunnel in a folded-waveguide TWT is obtained.

Sheet electron beam millimeter-wave amplifiers at the Naval Research Laboratory

To meet the need to transmit increasingly massive volumes of data, both the defense and commercial sectors are turning to higher operational frequencies to take advantage of larger signal bandwidths while concurrently requiring increased amplifier power to achieve the necessary signal-to-noise ratios over large transmission distances. In response to these needs, the last decade has seen a leap in performance of a variety of millimeter-wave devices. The Naval Research Laboratory (NRL) is the principal U.S. Department of Defense R&D center focused on the development of the science and technology behind new millimeter-wave high power solid-state and vacuum electronic devices. Selected examples of NRLs research projects are described with an emphasis on high power millimeter-wave vacuum electronic devices.

MMW to upper-MMW vacuum electronics research at NRL

The Vacuum Electronics Branch of the US Naval Research Laboratory (NRL) is actively engaged in research and development of key technologies for high-power MMW to upper-MMW amplifiers. This work includes the development and application of physics-based modeling and simulation tools, the design and development of high-perveance sheet electron beams and associated slow-wave and standing-wave interaction structures, research on high-current-density emitters, and the development and application of precise microfabrication techniques and thermal management schemes. An overview of these activities will be presented, including a discussion of the technological advances required to achieve order-of-magnitude increases in amplifier performance and a summary of the approaches being pursued and their status.

Large-Signal Code TESLA: Improvements in the Implementation and in the Model

We describe the latest improvements made in the large-signal code TESLA, which include transformation of the code to a Fortran-90/95 version with dynamical memory allocation and extension of the model for more accurate treatment of slow and reflected particles

Using Large Signal Code TESLA for Wide Band Klystron Simulations

Large signal klystron simulation code TESLA has been developed to be suitable for simulations of wide band klystrons with two-gap two-mode resonators. The results of TESLA simulations for NRL S-bans MBK with extended bandwidth have been compared with predictions of electromagnetic code HFSS and PIC code MAGIC

TESLA: simulation code for multiple beam klystrons

TESLA (Telegraphists Equations Solution Linear beam Amplifiers) is a new code designed to simulate linear beam vacuum electronic devices with cavities, such as klystrons, interaction klystrons, twistrons, and coupled cavity amplifiers. The model includes a self-consistent, nonlinear solution of the three-dimensional electron equations of motion and the solution of time-dependent field equations. Also, fields in the external cavities are modeled with circuit like equations and couple to fields in the beam region through boundary conditions on the beam tunnel wall.

Circuit Design of A High Power, S-Band Eighteen-Beam Klystron

Circuit design for a high-power fundamental-mode multiple-beam klystron (MBK) is presented. This S-band circuit will be powered with a 42 kV, 41.6A, eighteen-beam electron gun currently under development. The circuit is comprised of six cavities. Three of which have two gaps to achieve desired gain and bandwidth in a reasonably short circuit length. Simulations with MAGIC-3D indicate peak RF power of 740 kW and 3-dB instantaneous bandwidth of> 11% is feasible in a circuit length of approximately 22 cm. Electronic efficiency and gain are 42% and 34-dB, respectively.