E.L. Wright

Simulation of microwave devices with external cavities using MAGY

The simulation code MAGY developed at the University of Maryland and Naval Research Laboratory is able to describe the self-consistent nonlinear interaction between the electromagnetic fields of simply connected axisymmetric structures and electron beams. The code has been used effectively primarily for the design and simulation of gyro-devices. The main advantage of the MAGY code is its capability to find both steady state and slowly evolving dynamic solutions with minimal computational effort. The goal of the present work is to develop the formulation to be capable to model vacuum electronic devices with external cavities while keeping all the useful features of the MAGY code.

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.

End-to-end analysis using MICHELLE and TESLA codes

The 2.5D large-signal code TESLA is capable of accurate and efficient modeling of linear-beam vacuum electronic amplifiers, such as klystrons and multiple-beam klystrons (MBKs), extended interaction klystrons (EIKs), inductive output tubes (IOTs) and coupled cavity traveling-wave tubes (CC-TWTs). The demanding computational accuracy in the designs of modern vacuum amplifiers requires a monolithic approach in the modeling of the entire device, so called end-to-end analysis. In this work we will report a new approach to perform such analysis. This approach is based on using the gun-collector code MICHELLE and the latest version of the code TESLA. We added in TESLA the capability to import and process detailed electron beam distributions given by the gun-code MICHELLE. The given initial distribution of particles can be de-populated to keep their total number reasonably low to take advantage of the performance advantages of the TESLA model. After the TESLA large-signal simulation, the spent-beam information can be exported back into MICHELLE to perform the collector simulation. To support currently available time-dependent simulations by MICHELLE we are working to include in TESLA the time-dependent electron beam data processing as well. We will show TESLA simulations of a few different devices with electron beam distributions imported from the gun-code.

RF density-modulated electron source simulations with MICHELLE

Summary form only given. New models have been developed and implemented in the MICHELLE Finite-Element Electrostatic Particle-in-Cell code1 in support of modeling RF photocathodes and IOTs (inductive output tubes). In the case of photocatodes, low emittance, high current density sources are required to achieve the small beam size needed for high frequency vacuum electronic devices and, in particular, low emittance sources are demanded for high power free electron lasers (FELs). Emission models are of particular importance in the emittance-dominated regime, where emission non-uniformity and surface structure of the cathode can have an impact on beam characteristics and situations that depend on beam quality (e.g., halo). We have been developing comprehensive time-dependent photoemission models that account for laser and cathode material and surface characteristics and adapting them to develop emission models for inclusion into beam simulation codes2. In addition to the photoemission effects, including the effects of thermal field emission and modeling dark current are key to predicting beam quality and performance degradation due to beam tails and halo.

Low-voltage (≤ 20 kV) sheet beam with solenoidal focusing for w-band extended interaction klystron (EIK)

Low-voltage, high-perveance sheet beams are attractive for compact, high-power millimeter wave (MMW) amplifiers, because significantly higher beam current and RF power can be accommodated in the larger cross sectional area (as compared to conventional pencil beam devices). Furthermore, solenoidal focusing offers important advantages over the more popular periodic permanent magnet (PPM) focusing because of the higher field (and hence higher current density) that can be achieved with existing permanent magnets and the absence of orbital stability limitations.[1,2] To demonstrate these advantages, a beam stick (gun, magnet, beam tunnel, and collector) is being fabricated to generate and transport a 19.5-kV, 3.5-A sheet electron beam (0.32 mm × 4 mm) in a uniform 8.5-kG magnetic field. This beam is designed to power a 94-GHz extended interaction klystron (EIK) at ∼10-kW peak power. Simulations using MICHELLE [3] show that this beam can be transported with substantially no wall interception through a 0.4-mm-high beam tunnel over distances of 4–5 cm, approximately 4 times the length of the EIK interaction circuit.[4] A fundamental requirement for achieving this stable transport in a uniform magnetic field is avoidance of E×B drift effects (e.g., diocotron instability and beam tilting). We have suppressed these effects by utilizing an essentially planar magnetic field topology and by injecting a very uniform beam into the magnetic field in such a way that the electron guiding centers lie along the midplane and the beam edge is approximately matched to a self-electric-field potential contour in the beam tunnel. Key features of the beam stick and our design methodology will be described.

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.

High-power S-band fundamental-mode eight-beam klystron and gun design

Summary form only given. In multiple-beam amplifiers, beamlets are transported in individual beam tunnels but interact with RF fields in a common interaction region. This approach enables the designer to have the best of both worlds: individual beamlets can have low perveance which is conducive to efficient bunching and beam transport, leading to higher gain, electronic efficiencies, and average power, while the aggregate beam current can be high, facilitating high beam and RF power, and also broad bandwidth. A fundamental mode multiple-beam amplifier is described in this paper and we report on the design of a beam forming system for high-average-power broadband S-band multiple-beam amplifiers, developed at the Naval Research Laboratory (NRL). These amplifiers utilize eight individual electron beams and operates in the fundamental TM/sub 01/ mode.

19.1: Status of the MICHELLE code and applications

The MICHELLE code is a Finite-Element Electrostatic Particle in Cell code for application to 2D and 3D particle beam formation, transport, and collection. Although its initial development focus had been for DC electron guns and depressed collectors, other applications such as RF electron guns, ion thrusters, photocathodes and e-beam lithography have become a recent focus. The MICHELLE codes ability to manage large mesh sizes and large particle counts in complex geometries requiring the resolution of disparate spatial scales in 2D and 3D on desktop computers has allowed it to be applied to devices that previously could not have been readily modeled. This presentation gives an overview of recent applications, capabilities, and the current status of MICHELLE. A gun optimization problem will be presented and the effects of different modeling parameters and meshing techniques will be discussed.

Inductive output tube modeling and simulation code development

The inductive output tube (IOT) is the preferred technology for a number of applications requiring tens to hundreds of kilowatts of RF power at UHF and L-band frequencies and has been proposed for applications requiring as much as a megawatt CW. Although conceptually simple, modeling and simulation of the physics of the electron beam formation region within the IOT input cavity presents a significant challenge due to the disparate scales of the components. To address the needs of manufacturers of high-power IOTs and support the development of high-power Multiple-Beam (MB) IOT technology, our team is developing the tools necessary to allow end-to-end modeling and simulation, and design optimization, of the beam-wave interaction of these devices. The finite-element time-domain electrostatic PIC code MICHELLE [1], in conjunction with the Analyst® [2] suite of electromagnetic codes, are undergoing extensive modifications to provide modeling capability of the cathodegrid-anode structure that comprise the input cavity, while the beam wave interaction of the output cavity will be performed using the code TESLA [3]. The latest results of this effort will be shown.

Applications and current status of the finite-element MICHELLE gun & collector simulation code

The MICHELLE code is a Finite-Element Electrostatic Particle in Cell code for application to 2D and 3D particle beam formation, transport, and collection. Its primary development focus has been for DC electron guns and depressed collectors, however, it has other applications such as RF electron guns, ion thrusters, photocathodes, etc. Its ability to manage large mesh sizes and large particle counts in complex geometries requiring the resolution of disparate spatial scales in 2D and 3D on desktop computers has allowed it to be applied to devices that could not have been readily modeled in recent years. This presentation gives an overview of recent applications, capabilities, and the current status of MICHELLE. In particular, application to time-dependent problems and optimization will be illustrated