Alexander N. Vlasov

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.

Overmoded GW-class surface-wave microwave oscillator

Results of theoretical and experimental studies of a GW-class, large diameter microwave oscillator are presented. The device consists of a large cross-section (overmoded), slow-wave structure with a unique profile of wall radius specifically designed to support surface waves and to provide a strong beam-wave coupling at moderate voltage (500 kV), an internal adjustable microwave reflector, a coaxial microwave extraction section, and a coaxial magnetically insulated field emission electron gun. In preliminary experiments carried out at 8.3 GHz, the power level exceeding 0.5 GW and efficiency of 15% have been measured calorimetrically.

Simulation of microwave devices with external cavities using MAGY

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.

High-power four-cavity S-band multiple-beam klystron design

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 . 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 (<1.8%), the average power density is well within the limits for liquid cooling for pulse lengths up to 1.3 ms.

Effect of the azimuthal inhomogeneity of electron emission on gyrotron operation

Gyrotrons are typically driven by electron beams produced by magnetron-type thermionic electron guns operating in the regime of temperature limited emission. Very often, the current density in such annular electron beams is azimuthally nonuniform. To describe the effect of this nonuniformity on gyrotron operation, the code MAGY [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)], which is widely used for modeling of slow and fast microwave sources, was properly modified. The results of numerical simulations demonstrate the effect of azimuthal inhomogeneity of the emission on the excitation of low- and high-frequency satellites of the operating mode and on the efficiency degradation. The calculations are done for parameters typical for megawatt-class, long-pulse, millimeter-wave gyrotrons, which are currently under development for electron cyclotron plasma heating and current drive experiments in controlled fusion reactors.

Large-Signal Multifrequency Simulation of Coupled-Cavity TWTs

We describe a steady-state large-signal model of coupled-cavity traveling-wave tubes in which the input and output signals are periodic functions of time that may be represented by Fourier series of finite length. The model includes both linear and nonlinear effects including circuit dispersion, reflections, intermodulation, and harmonic generation. The model uses a lumped element representation of the circuit and a 1-D disk model of the beam. Several favorable comparisons of model predictions with experimental measurements, including gain versus frequency and power transfer characteristics, are illustrated. The inclusion of nonlinear effects in this multifrequency model enables predictions of intermodulation products, as functions of the input power. An example of the computation of C3IM is illustrated.

Relativistic backward‐wave oscillators operating near cyclotron resonance

A linear and nonlinear, time dependent self-consistent theory for relativistic backward wave oscillator is developed. In this theory the transverse motion of the electrons is taken into account. Analytical and numerical analysis of the model equations near the cyclotron resonance gives a good estimate for the power drop due to the cyclotron absorption observed in many experiments and predicts an increase in the power under some conditions.

A Computationally Efficient Two-Dimensional Model of the Beam–Wave Interaction in a Coupled-Cavity TWT

A new computationally efficient 2-D model of the beam-wave interaction in coupled-cavity traveling-wave tubes (CC-TWTs) has been developed. The model provides self-consistent time-dependent solutions of Maxwells equations together with a fully relativistic solution of the electron equations of motion. The model is based on different treatments of the RF fields in the region occupied by an electron beam and in the region of the coupled-cavity structure. The RF fields inside the beam tunnel are represented as a sum of eigenmodes of the local cross section of the beam tunnel. The fields outside the beam tunnel are represented as a superposition of modes of an equivalent circuit with lumped capacitors, inductors, and resistors. The model has been implemented in the TESLA-CC code. The results of the code predictions agree well with measured data for a wideband CC-TWT operating in the Ka-band. The code also shows good agreement with predictions of the 1-D code CHRISTINE-CC in regimes in which a 1-D approximation is applicable. A numerical study of CC-TWT operation shows that, in the small-signal regime, the code is able to predict a gain enhancement due to transverse motion at focusing magnetic fields comparable with Brillouin equilibrium values, which is the major 2-D effect. In the large-signal regime, the code is also capable of treating cases in which the transverse displacement of electrons becomes large and of determining the dependence of the spent beam energy distribution on radial position.

Parallel Simulation of Independent Beam-Tunnels in Multiple-Beam Klystrons Using TESLA

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.

Demonstration of a High Power, Wideband 220-GHz Traveling Wave Amplifier Fabricated by UV-LIGA

We present the first vacuum electronic traveling wave amplifier to incorporate an interaction circuit fabricated by ultraviolet (UV) photolithography and electroforming, demonstrating over 60 W of output power at 214.5 GHz from a 12.1 kV, 118 mA electron beam. The tube also achieved an instantaneous bandwidth of ~15 GHz in G-band in the small signal regime. The all-copper circuit was fabricated in two layers using a UV-transparent polymer monofilament embedded in the photoresist to form the beam tunnel prior to electroforming. Effects arising from fabrication errors and target tolerances are discussed. This microfabrication technique and demonstration paves the way for a new era of vacuum electron devices that could extend into the 1-2 THz range with advances in high-current-density electron guns.