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The article talks about a new methodology that can be used for the development of the periodic permanent-magnet quadrupole (PMQ) focusing lattices. These lattices are shown to lead to higher amplifier power densities and hence can be used for various linear electron-beam amplifier applications.

Several experimental studies are conducted to explain the eight-beam five-cavity S-band multiple-beam klystron (MBK) with a 6% bandwidth and 3.2 GHz center frequency. The device is shown to be highly accurate and efficient and hence can be used in hardware implementation.

Several comparative studies are conducted to explain the mechanisms involved in the circuit attenuation that is used in the helix traveling-wave-tube (TWT) simulation codes. The discussed ad hoc model is shown to be extremely efficient and accurate in determining the attenuation rates.

The large-signal code TESLA is used to analyze the impact of an experimental multiple-beam klystron (MBK) on the prediction of the interception current of the circuit. TESLA is shown to be extremely effective and efficient in explaining the dynamics of each beam separately and hence can be used in numerous other applications.

The article explains a new design approach and methodology for a broadband high-power S-band multiple-beam klystron (MBK) amplifier. The amplifier is shown to be extremely efficient and beneficial, as it is also capable of producing higher beam current.

A time-domain physics-based model is employed to explain the impact of the nonlinear memory effects on the digital communications in a klystron. The nonlinear memory effects are shown to be a result of the combination of high data rates and operation near saturation.

The article presents a theoretical analysis of the transport of the intense sheet electron beam in a uniform solenoidal magnetic field in high-power vacuum electronic devices. The solenoidal magnetic field is shown to be extremely advantageous, as it helps in transporting higher current density beams, as compared to the periodic magnetic transport.

We present an extension of the klystron simulation code TESLA to model multiple-beam klystrons (MBKs) in which interaction parameters may vary significantly from beam-tunnel to beam-tunnel. In earlier work, the single-beam code was applied to model the MBK by assuming that all electron beams and beam-tunnels were identical and all electron beams interacted identically with the fields of the resonant cavities, using averaged values of R/Q to represent interaction with each resonant cavity. To overcome the limitations of this approach and to take into account the effects from nonidentical beams and/or beam-tunnels, we have modified the code to use a parallel algorithm for multiple beams. The implementation of the parallel version of TESLA is based on the latest Fortran-95 version of the serial code and uses the message-passing interface library for communication. For testing and verification purposes, the new version of the code is applied to simulate an experimental four-cavity, eight-beam klystron amplifier, which was designed and successfully tested last year at the Naval Research Laboratory. The results of modeling using the new parallel TESLA and their comparison with experimental data are discussed in detail. Index Terms--Large-signal code TESLA, message-passing interface (MPI), multiple-beam klystron (MBK), parallel algorithm.

Mode interactions in coaxial gyrotrons are studied numerically using the Maryland gyrotron code (MAGY), which has recently been modified to treat coaxial structures. Simulations are focused on the study of the FZK 170 GHz coaxial gyrotron. This device is being considered for electron cyclotron plasma heating in International Thermonuclear Experimental Reactor; however, current performance is below expectations based on previous simulations. The mode competition process during the voltage rise includes at least six modes in two groups (corotating and counterrotating with an electron beam). Simulations show that the sequence of modes achieving a measurable level of output power during start-up depends on several electron beam parameters: voltage, current, beam radius, beam thickness, pitch angle, and spread in transverse velocity. MAGY simulations made with nominal parameters confirm the results of previous simulations. Two beam parameters are not well characterized: pitch angle and spread in transverse velocity. We found that the measured results are consistent with a decrease in the pitch angle and a 3% transverse velocity spread. Also, the effect of modes at the 2[[OMEGA].sub.0] cyclotron harmonic on the fundamental cyclotron harmonic modes is studied and discussed. Index Terms--Coaxial cavity gyrotron, mode competition, multimode simulations, plasma heating.

The problem of modeling slow and reflected electrons that appear during operation of high-power klystrons is discussed. Particles with axial velocity below some threshold are treated using an alternative algorithm to meet this challenge.

The results of the experimental measurements of the ratio frequency phase as a function of cathode voltage for an eight-beam four-cavity multiple-beam klystron (MBK) is presented. The low values for the phase-pushing factor are a benefit of the MBK's high-perveance operation that results in a shorter circuit length relative to single-beam devices of comparable power.

Large signal code is used to study the traveling-wave tubes (TWTs) and its results are compared with that of a transverse TWT of the same dc beam power. The results have shown that the transverse TWT retains its advantages under large signal conditions.

We report here on a methodology to use frequency-domain physics-based nonlinear simulations of traveling-wave tubes (TWTs) in block time-domain models. We present examples using an improved version of the large-signal code CHRISTINE and a two-block baseband time-dependent model with a feedback loop that takes into account reflections internal to the TWT. The novelty in this approach is that it enables us to relate digital performance to the physical characteristics of the device, and it has the potential to significantly impact the analysis of TWT amplifiers for high data-rate applications. Index Terms--Block model, CHRISTINE, digital, helix, high-power amplifier, nonlinear model, reflections, traveling-wave tube amplifier (TWTA).

We present a general discussion of multiple-gap cavity characteristics and stability issues along with the detailed design of a five-cavity, broadband, high-power multiple-beam klystron (MBK). The predicted performance characteristics of this S-band fundamental-mode klystron circuit include a 3-dB bandwidth of 6.7% with peak power ~600 kW and an electronic gain of 33 dB. The circuit is powered by a 32 A, 45 kV eight-beam electron gun (Nguyen et al., 2004) whose design has been successfully tested in a previous narrowband MBK circuit (Nguyen et al., 2004), (Abe et al., 2005). The present circuit was designed and optimized with the three-dimensional particle-in-cell code MAGIC. Index Terms--Broadband circuit, multiple-beam klystron (MBK), multiple-gap cavity, two-gap cavity.

Two distinct types of couple-cavity structure, a C-band double staggered ladder, and an L-band slot coupled structure, were analyzed using the comparative analysis of the Curnow and Malykhin-Konnov-Komarov (MKK) circuits. The basis of comparison is the level of accuracy with which the models reproduce the cold circuits phase velocity and characteristic impedance of the structures as functions of frequency.

A new model for calculating the ac space charge in a linear-beam traveling wave tube (TWT) large-signal simulation code is described when the true three-dimensional (3-D) geometry of the interaction circuit is taken into account. The ac space charge fields generated by a set of test currents placed inside the periodic interaction structure is characterized using the 3-D electromagnetic simulation code CTLSS.

The reflection of waves at the termination of a beam loaded traveling wave structure is presented. The presence of a bunched beam in the structure induces additional currents and voltages on the structures, which gives rise to an additional reflection at the termination.

A report on the advances in the development to the MICHELLE electron gun collector design tool is presented. The MICHELLE code is a new finite-element (FE) two-dimensional and three-dimensional electrostatic particle-in-cell code designed to address the recent beam optics modeling and simulation requirements for vacuum electron devices, ion sources, and charged-particle transport.

The effectiveness of predistortion techniques for traveling wave tube (TWT) linearization is studied using single-tone, two-tone, and quadrature-amplitude-modulation signals. Results from a series of predistortion linearization experiments for five TWTs covering L-, C-, Ku-, and Ka-bands and including both Helix- and coupled-cavity TWTs were also presented.

This paper presents the design of a high repetition rate electron gun to be employed in future high-power broadband multiple-beam amplifiers operating in S-band. Special emphasis is placed on the geometric layout of the gun to ensure high repetition rate operation at moderate beam switching voltages, good beam optics, and optimal beam packing for long life, efficient beam-wave interaction and broad bandwidth. The resulting gun design employs an eighteen-beam magnetically shielded cathode topology capable of generating a total current of 41.6 A (evenly divided among eighteen beams) at a voltage of 42 kV. In the beam tunnels, each beamlet has a radial fill factor of 48%. The nominal beam modulation voltage is 5.2 kV. With an estimated gun capacitance of 150 pF, a repetition rate in excess of 10 kHz is feasible with the current state-of-the-art in modulator technology. Index Terms--Electron gun, multibeam, multiple-beam amplifier (MBA), multiple-beam klystron (MBK).

This paper describes the detailed design of an eight-beam electron gun for use in S-band multiple-beam amplifiers operating in the fundamental mode. The gun operating voltage is 45 kV with a total beam current of 32 A, evenly divided among the beamlets. Each individual beam has a perveance of 0.42 [micro]pervs making a total beam perveance of 3.35 [micro]pervs. The optimized electron gun is singly convergent using a four-fold symmetry with the four inner and four outer emitters interlaced 90[degrees] apart. The emitter current density has been kept below 10 A/[cm.sub.2] (space-charge limited). The cathode is magnetically shielded and the longitudinal magnetic field in the interaction region is in the range of 1.1-1.8 kG. The design of the magnetic focusing system minimizes beam corkscrewing as well as electron interception on the tunnel walls. Beam optics simulations of the gun indicate excellent beam transport characteristics with a final beam-to-tunnel radial fill factor of less than 0.45. The primary computational tools used in the design process were the three-dimensional gun code MICHELLE, and the magnetostatics code MAXWELL-3D. Index Terms--Electron gun, klystron, MICHELLE, multibeam, multiple beam.

We develop a methodology for the design of multiple-cavity klystron interaction circuits. We demonstrate our approach with the detailed design of a collector and a four-cavity circuit for a multiple-beam klystron (MBK) operating in the fundamental mode at a center frequency of 3.27 GHz (S-band). These elements are designed to be used with a 32-A 45-kV magnetically shielded eight-beam electron gun currently under fabrication [1]. Upon integration of the gun, circuit, and collector, the MBK will be used for beam transport and beam-wave interaction studies and to validate developmental design codes and design methodologies. The device has a predicted gain of 33 dB at a peak pulsed output power of 750 kW with a corresponding electronic efficiency of 52%. For the present design, broad bandwidth is not a design objective, and the 3-dB bandwidth is 2.5%. Downstream of the output cavity, the magnetic field profile and the interior surface profile of the collector are carefully shaped to minimize the space-charge potential depression at the entrance to the collector, minimizing reflected electrons. The maximum calculated instantaneous power density on the walls of the collector is approximately 55 kW/[cm.sup.2]; at low duty cycles ( Index Terms--Collector, klystron, multibeam, multiple-beam amplifier (MBA), multiple-beam klystron (MBK).

We demonstrate the effectiveness of the Telegraphist's Equations Solution for Linear-beam Amplifiers code for simulation of a range of klystron amplifier designs. Simulation results are compared to both particle-in-cell (PIC) simulations and experimental data, with good agreement obtained in each case. Specific features and optimizations included in the model are discussed and shown to allow a significant reduction in simulation times over the general PIC approach. Index Terms--Klystrons, microwave amplifiers, simulation.

A fully two-dimensional (2-D) dc space charge model has been implemented in a large-signal traveling-wave amplifier code. The simulation algorithm takes an iterative approach by alternately solving the Poisson equation and the beam trajectory equations to converge toward a self-consistent steady-state solution. This approach is similar to that employed in steady-state gun codes. However, it is well known from gun simulations that the iterative algorithm can be slow to converge. We have found the slow convergence is due to a convective numerical instability. To speed up convergence, we implemented and tested stabilization schemes based on mixing one-dimensional and 2-D Poisson potentials during the iteration cycles. These schemes are shown to accelerate convergence considerably. The fully 2-D dc space-charge model permits accurate treatment of the axial dc space-charge field in the computation of the large signal gain and efficiency, taking into account the fast variation of beam parameters along the device axis. Therefore, it can be applied to a mismatched beam with large scalloping motion. The methodology of incorporating dc space charge is general and could be incorporated in other large signal codes. Index Terms--Electron beams, iterative methods, numerical analysis, numerical stability, simulation software, space charge, traveling-wave amplifiers, traveling-wave tubes (TWTs).

Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain ~60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in [K.sub.a]-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the T[E.sub.11] mode and for stabilization employs a high-average power compatible diffractive loading technique. Index Terms--Dielectric, gyro-traveling wave tube (gyro-TWT), loss, millimeter-wave amplifier.

The physical mechanism responsible for the phenomenon of power holes in forward wave linear beam amplifiers is demonstrated. Mathematical models of these mechanisms are implemented in the large signal helix traveling-wave tube (TWT) simulation code CHRISTINE 3D.

In this paper, we demonstrate the use of a power margin as a figure-of-merit for evaluating the performance and optimizing the design of traveling-wave tube amplifiers (TWTAs) used in digital communication applications with multilevel modulations. The power margin is a system-level measure that balances both device efficiency and nonlinear distortion and provides a more direct prediction of the system-level performance of power amplifiers than device-level measures such as device efficiency or error-vector-magnitude. We calculate the power margin for M quadrature amplitude modulation for an existing TWTA to demonstrate the setting of an optimal amplifier operating drive level according to the criterion of the maximum power margin. The power margin can also be used to compare the performance of different traveling-wave tube (TWT) configurations. We compare the calculated power-margin performance for helix TWT circuits optimized with different optimization goal functions using the helix TWT design code CHRISTINE. The goal functions used in the optimization of the TWT circuits include AM/PM optimization, complex gain optimization, efficiency optimization, and a new digital goal function optimization. The digital goal function is shown to provide an enhanced power margin compared to the other three goal functions and demonstrates the potential of TWT device design optimization from a system perspective. Index Terms--Communication systems, microwave circuit optimization, microwave power amplifiers (PAs), nonlinear distortion, satellite communication, traveling-wave tubes (TWTs).

In nearly any application of a microwave tube, noise characteristics are important figures of merit. This paper reviews and presents new results of recent studies of such noise. The areas discussed are ion noise in linear beam tubes, one of the oldest studied noise phenomena, and electron noise in gyroklystrons, one of the newest areas of interest. Index Terms--Electron noise, ion noise, microwave tube noise.

The gyro-traveling wave-tube (gyro-TWT) is a microwave amplifier with simultaneous high power, high frequency and broad bandwidth capabilities. The design technique associated with the ceramic loading scheme using the 35 GHz gyro-TWT is described.

A self-consistent large-signal beam-field interaction model for vacuum electronic microwave sources with external cavities is described. The model includes a self-consistent solution of the three-dimensional equations of electron motion and the time-dependent field equations. The RF fields are decomposed into the fields inside the beam region and the fields inside outer resonators. The RF fields inside the beam region are represented as a superposition of local waveguide modes. The RF fields inside resonators are represented as a sum over resonator modes. The various modes are coupled together due to gaps connecting cavities with each other and with the beam region. The numerical implementation of the model requires additional analytical steps to obtain an effective, convergent, and stable numerical solution. The modified version of the code MAGY has been tested by a comparison with known results and also with measured data. Index Terms--Cavity resonators, klystrons, large signal models, MAGY, vacuum electronic devices.

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-gnns, 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. Index Terms--Beam formation, computer-aided design (CAD), electron gun, MICHELLE, multistage depressed collector, object oriented design, voyager.

The maximum achievable gain in individual sections of helix-type traveling-wave amplifiers is limited by the requirement that the device be stable with respect to the excitation of spurious modes. The excited modes may be of two types: backward waves, which are absolutely unstable, or forward waves, which are unstable in the presence of reflections and regenerative amplification. Whether a specific device is unstable depends on a number of details that must be computed numerically. We will present a model, used in the traveling-wave tube simulation code CHRISTINE, in which stability is determined including a number of important effects. These are: the placement of severs, their reflection and transmission coefficients, the profile of attenuation along the interaction length, the presence of a driven signal, and the coupling of forward and backward waves due to asymmetries in the helix support structure. Asymmetries result in a stopband near the '[pi]' point formed by the coupling of the forward and backward waves. For this case, a model is developed to evaluate the maximum stable length. Index Terms--Backward-wave oscillator (BWO), stability, traveling-wave tube (TWT).

A set of optimization goal functions designed to improve the efficiency and linearity performance of helix traveling-wave tubes (TWT) is described. These goal functions were implemented in the CHRISTINE suite of large-signal helix TWT codes along with a steepest-descent optimization algorithm to automate the process of circuit parameter variation and to facilitate the rapid exploration of alternative TWT designs. We compare the predicted power, efficiency, and linearity of four different helix TWT circuits, each developed according to a different set of optimization criteria. Out of these designs, a single design was selected to be further developed for use in C-band high-data-rate communications experiments. The detailed design of this linearized TWT with a predicted 1-dB small-signal bandwidth of 1.2 GHz, small-signal centerband gain of 35.7 dB ([f.sub.c] = 5.5 GHz), and centerband saturated output power of 52 dBm (158.5 W) is presented. Index Terms--Digital modulation, helix traveling-wave tube, high-data-rate communications, large-signal code, optimization, traveling-wave tube (TWT).

A time-dependent nonlinear analysis of a coupled-cavity traveling-wave tube (CCTWT) is presented. The coupled-cavity structure is modeled by a set of equivalent circuit equations where the equations for currents and voltages are coupled to the nearest neighbor cavities. Input and output coupler models as well as sever cavities are included in the formulation. The electron dynamics are treated using the three-dimensional Lorentz force equations although the RF field representation is an analytic model based on cylindrically symmetric geometry. The magnetic focusing fields are also cylindrically symmetric and can be either a solenoid or a periodic permanent magnet stack. The space-charge fields are found by mapping charge to a two-dimensional grid (r, z) and solving Poisson's equation by a finite difference grid formulation. The circuit and Lorentz force equations are integrated in time in a self-consistent manner. The formulation is capable of treating multiple drive frequencies and the associated intermodulation products as well as oscillations and backward wave instabilities. Hence, the model can be used to perform stability analyses. Furthermore, the cavity parameters can be varied to model dynamic velocity tapering for efficiency enhancement. The simulation is applied to the analysis of a sample C-Band CCTWT, and comparisons with measured performance of a Ka-Band CCTWT at Communications and Power Industries, Palo Alto, CA, are made. Index Terms--Coupled cavity traveling-wave tubes (CCTWTs), large-signal analysis, nonlinear simulation, time-domain simulation, traveling-wave tubes (TWTs).

The results of a focused program to develop high-power W-band gyro-amplifiers, which culminated in the demonstration of record average output powers from amplifiers in this band, are described. Following an experimental and theoretical study of low-duty prototype amplifiers, two high-average power devices were designed, built, and demonstrated. The first high-average power amplifier achieved 10.1-kW average output power at 33% efficiency in the T[E.sub.01] mode at 93.8 GHz. The instantaneous bandwidth was 420 MHz and the saturated gain at the 10.1-kW point was 32 dB. The second high-average power gyroklystron, designed for improved bandwidth, demonstrated 10.2-kW average power at 31% efficiency with 700-MHz instantaneous bandwidth and 33-dB saturated gain. The measured results of the low-duty prototype amplifiers and the high-average power gyroklystrons are described in detail. In addition, theoretically predicted results for a high-average-power W-band gyrotwystron amplifier, which is currently in construction, are presented. Index Terms--Amplifier, gyro-amplifier, gyrotron, millimeter wave, W-band.

The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular T[E.sub.01] mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with--3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation. Index Terms--Ceramic loading, electron beam, gyro-traveling wave tube (gyro-TWT), millimeter-wave amplifier.

This paper explores the recent history and diversity of this remarkable technology, with emphasis on recent advances in the more traditional device types (traveling-wave tube and klystron), as well as more recent innovations such as the microwave power module, inductive output amplifier, fast-wave devices, ultrahigh-power sources, and RF vacuum microelectronics. These advances can be credited to a combination of device innovation, enhanced understanding gained through improved modeling and design, the introduction of superior materials and sub-assembly components and the development of advanced vacuum processing and manufacturing techniques. Index Terms--Cathodes, free-electron laser, gyrotron, high-power microwaves, klystron, microwave power module, numerical simulation, traveling-wave tube, TWT, vacuum electronics, vacuum microelectronics.

A steady state, three-dimensional multifrequecny large signal model of helix traveling wave tubes is described and implemented.

The design of a Ka-band gyrotron traveling wave tube amplifier with a wide range of operating capabilities is described.

The large-signal, one-dimensional multifrequency code CHRISTINE is used to simulate the noise-power ratio for a helix traveling wave tube.