Linear Beam Tubes Research Articles

Design of a simplified electron gun without BFE for linear beam tubes

Design of a simplified electron gun without BFE for linear beam tubes

Partially Grounded Depressed Beam Collector

A novel depressed beam collector concept is described that can be relevant for klystrons and other linear beam tubes. In addition to the usual collector element or elements biased negatively to recover unspent-beam energy, a grounded element is introduced to collect reflected primary and backsteaming secondary electrons that, on account of transverse deflection, can be prevented from returning toward the cathode. A theoretical and simulation analysis is presented leading to the estimations of efficiency for this configuration, with examples that apply to a recently demonstrated $L$ -band multibeam klystron. Efficiencies are predicted to be significantly higher for tubes employing partially grounded depressed collectors (DCs) than for tubes employing the conventional DCs.

An Analytical Approach for Obtaining User-Defined Axial Magnetic Field Profile of a PPM Focusing Structure

In this paper, a simple analytical approach for obtaining user-defined axial magnetic field profile of a periodic permanent magnet (PPM) structure used for focusing the electron beam in linear beam tubes is presented. Axially magnetized permanent ring magnet and single cell of a PPM structure are represented by equivalent solenoids, and their equivalent parameters are obtained from the dimensions and material properties of these magnet structures. Axial magnetic field for the finite-length PPM structure is obtained using the superposition of equivalent solenoids of all PPM cells. Finally, a methodology is presented to obtain a user-defined axial magnetic field profile in a PPM structure. Analytically computed axial magnetic field is compared with finite-element-method simulation and measured results and found to show a very good agreement.

An Improved Analysis of PPM Focusing Structures Including the Effect of Magnetic Saturation in the Iron Pole Pieces

In this paper, an improved analysis of the periodic permanent magnet (PPM) focusing structure for linear beam tubes is presented. An estimation of critical dimensions for the iron pole pieces to avoid magnetic saturation is made. The method is based on the permeance calculation of all possible flux paths using magnetic circuit theory. The analytically calculated axial magnetic field using the present approach is compared with measured and simulated results obtained by using the finite-element-method-based commercial software ANSYS. The agreement is very good.

English

A simple formula for plasma frequency reduction factor for a solid cylindrical electron beam in a metallictunnel has been developed by means of a 3-D curve fitting to the standard results of Branch and Mihran withaccuracy > 1.7 per cent, over the parametric regime of normalised beam-radius and beam-filling factor applicablefor linear-beam microwave tubes. An artificial neural network algorithm was used for the curve-fitting followingthe approach of universal approximation. The formula is simple and amenable to easy computation, even usinga scientific calculator. Defence Science Journal, 2008, 58(6), pp.768 -770 , DOI:http://dx.doi.org/10.14429/dsj.58.1705

An Improved Representation of AC Space-Charge Fields in Steady-State Simulation Codes for Linear-Beam Tubes

Accurate evaluation of the ac space-charge electric field is required in order to obtain accurate predictions for linear-beam-tube performance using steady-state simulation codes. The space-charge field is commonly calculated under the assumption that the beam current density and space-charge fields are all periodic in time and locally periodic in axial distance. In this brief, it is shown by example that there are cases in which it is important, in both the small and large signal regimes, to include departures from this local spatial periodicity assumption. We find in the cases that we have studied that it is sufficient to keep a single additional term, proportional to the spatial derivative of the bunched beam current density, in the expression for the space-charge electric field. The resulting expression is much faster to evaluate numerically than a full convolution integral over all axial wavelengths. The improved representation is suitable for use in both small- and large-signal linear-beam simulation codes. It has been implemented and tested in the CHRISTINE large-signal TWT simulation code

Effects of a Finite Axial Magnetic Field on the Beam Loading of a Cavity

The effect of a finite axial magnetic field on the beam loading of a cavity is evaluated. The calculation extends Branch's classic paper on ballistic bunching in that both the conductive and susceptive components of the beam-loaded admittance are computed, for general values of axial magnetic field. Also included is a comparison of the analytic formulation with a two-dimensional particle-in-cell simulation. This paper suggests that the finite axial magnetic field used in linear beam tubes (typically exceeding 1.5 /spl times/ the Brillouin field) would only modify the beam-loaded admittance by about 20%, from that computed under the assumption of an infinite axial magnetic field.

Characterization of Relaxation–Oscillation Noise in Continuous-Wave Traveling Wave Tubes

Extremely low noise levels are frequently required in radar and communication systems that utilize microwave tubes. A noise waveform frequently found in continuous-wave (CW) linear-beam microwave tubes is a relaxation-type oscillation that has a period typically on the order of several milliseconds. This paper provides an extensive experimental characterization of this relaxation-oscillation phenomenon by recording the corresponding signal modulations on the output of several solenoid-focused CW traveling wave tubes. The characteristics of these amplitude and phase modulations were studied during systematic variations of tube operating parameters that included magnetic field, cathode voltage, depressed collector voltage, heater filament voltage, and input drive level. A measurement technique was also developed with the capability to obtain detailed information on the growth of the relaxation-oscillation noise waveforms along the beam interaction region by comparing reflected and output noise waveforms.

Electron and ion noise in microwave tubes

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.

The effect of space charge on electron-wave instabilities near cut-off frequencies of the slow-wave structure

For a linear-beam microwave tube, the effect of space charge on signal generation and amplification in the electron flux-electromagnetic field system near a cut-off frequency of a slow-wave structure is studied numerically. In a parameter plane, regions of operation regimes are delineated. It is demonstrated that the space charge facilitates the oscillation near the higher cut-off frequency, but hampers that near the lower cut-off frequency.

Field emitter array development for high frequency applications

Radio frequency vacuum microelectronics combines the advantages of electron transport in vacuum with gated electron emission structures derived from solid-state microfabrication. The advent of practical gated vacuum emitters of micron size will have a strong impact on rf source technology. Next-generation rf amplifiers incorporating microfabricated emitters will allow amplifier designs that minimize the need for high-voltage power supplies, complex modulating circuitry, and the heavy magnets common with linear beam tubes. One attractive application for such efficient compact amplifiers is as the vacuum power booster of a microwave power module, a device that combines a solid-state preamplifier with a low-gain vacuum amplifier for efficiency. Program goals are to create a 50 W 10 GHz amplifier with 10 dB gain and efficiency exceeding 50%. We will survey the requirements placed on field emitter arrays for performance in a microwave amplifier as determined by various analyses, describe the present status of the performance of ring cathodes designed for inductive output amplifiers, i.e., the narrow-band, cavity-based klystrode under development at Varian/CPI and the wide-band, helix-based twystrode under development at NRL, and identify the challenges that remain before this new rf source technology is realized.

Computer simulations of low noise states in a high-power crossed-field amplifier

A large body of experimental data has been accumulated over the past 15 years or so on the remarkable ability of both magnetrons and CFA's to operate under certain conditions at noise levels comparable to those achieved in linear beam tubes. SAIC's MASK code, a 2 1/2 -D particle-in-cell code, has been able to compute total, integrated noise power to an accuracy of /spl plusmn/ a few dB in a high-power CFA, operating with a typical intra-pulse spectral noise density of /spl sim/47-50 dB/MHz. Under conditions that produced low noise (/spl sim/60-100 dB/MHz) in laboratory experiments, the MASK code has been, until now, unable to reproduce similar results. The present paper reports the first successful production of a very low noise state in a CFA simulation using the MASK code. The onset of this low noise state is quite sudden, appearing abruptly as the current is raised to a point near which the cathode operates as nearly emission limited. This behavior is similar to that seen in an experimentally observed transition between low noise and high noise operation in the SFD-266, a Varian [CPI] low noise CFA. Some comments are made concerning the nature of the noise as observed in the simulation and in the laboratory.

Charting the microwaves waters. A short overview in table form for quick and easy referencing

The author presents tables of microwave frequencies, solid state devices and their applications, linear beam tubes (O-type), crossed field tubes (M-type), and applications of microwave electron tube devices. >

Analysis and design of microwave amplifiers employing field-emitter arrays

An analysis of the microwave ( f≳1 GHz) properties of field-emitter arrays (FEAs) and several representative medium power (10–100 W) microwave amplifiers employing FEAs is presented. The FEA analysis is limited to parallel-plate structures having discrete pointlike vertical emitter tips and gate apertures aligned to each tip. A transmission line analysis of wave propagation in this structure is presented and used to evaluate the geometries and materials needed for microwave operation. This analysis is used to investigate the performance capabilities and emitter requirements of both modulated-emission linear beam tubes and microdevices based on FEAs. Specific microtriode designs are used to investigate practical problems such as space charge and thermal effects. Competitive performance should be achievable in gated-emission linear beam tubes by using FEAs that perform at levels previously reported by several laboratories. Existing FEA technology (currents of 10 μA per emitter, transconductances of 1 μS per emitter, 1 μm oxide thickness, and 3 μm emitter spacing) is suitable for use in cavity klystrodes(r) at frequencies through 10 GHz, and in moderately bunched beam (bunch width of 180°), octave-bandwidth traveling-wave-tube applications through 3 GHz. Extending the operating frequency and/or reducing the bunch width will require a larger ratio of transconductance to current. Microtriodes operating at 10 GHz will benefit from a modified FEA structure and improved emitter performance. An extra acceleration electrode must be added above the gate aperture to alleviate problems due to space charge between the gate and collector, and the gate oxide thickness must be increased to at least 2 μm. A FEA incorporating these features and capable of producing 5 μS and 100 μA per emitter could generate 130 W from a 5-mm-wide device with 8.6 dB gain, 7% bandwidth, and 36% power added efficiency. To allow higher gain and wideband operation, the transconductance at a given current must be increased. A FEA capable of producing 5 μS at only 10 μA per emitter would result in a microtriode with more than 1 octave bandwidth, 45 W output power, 10 dB gain, and 34% power added efficiency. Anode-to-case temperature differences of less than 100 °C appear possible in this device if BeO is used as the dielectric.

Computer simulation of electron flow in linear-beam microwave tubes

Abstract The computer simulation of electron flow in linear-beam microwave tubes, such as a travelling-wave tube (TWT) and klystron, is used for designing and optimising the electron gun and collector and for analysing the large-signal beam-wave interaction phenomenon. Major aspects of simulation of electron flow in static and rf fields present in such tubes are discussed. Some advancements made in this respect and results obtained from computer programs developed by the research group at CEERI for a gridded electron gun, depressed collector, and large-signal analysis of TWT and klystron are presented.

Effect of boundaries on the space charge potential in coupled cavity travelling wave tubes

In the theory of microwave linear beam tubes, it has hitherto been assumed that the space charge potential for a cylindrical electron beam passing through a concentric, earthed, conducting tunnel broken by interaction gaps can be calculated by supposing that the tunnel is continuous. In this paper, the effects of the gaps are computed for both an electron beam and a disc of charge. It is shown that for a uniform electron beam these effects can be represented by replacing the actual boundary with an equivalent continuous tunnel, the radius of which is termed the effective tunnel radius. An empirical equation has been derived from which the effective tunnel radius can be calculated to an accuracy of better than ±3%. The effective tunnel radius is a function of the tunnel radius, the gap length and the gap separation but is independent of the beam radius. For a disc of charge, it is found that the effective tunnel radius depends on the position of the disc relative to the gaps but that the average value of the effective tunnel radius as the disc moves through the gapped tunnel is equal to the value computed for a uniform electron beam. This average value of the effective tunnel radius is independent of the disc thickness.

1. Microwave vacuum devices

The paper is a survey of a few of the advances made in microwave tube design and technology during the past few years. Subjects covered are improvements in thermionic emitters, the attainment of good frequency bandwidth in klystrons, the achievement of high frequency agility rates in magnetrons, the latest techniques of gridded gun design in linear beam tubes, and a brief description of cyclotron wave devices for high power millimetre wave generation.

A novel electrostatic-focusing depressed collector for linear beam tubes

In a theoretical study, various types of retarding electrostatic fields are compared with respect to their usefulness in a depressed collector. Without space charge, fields having focusing properties lead to higher collector efficiencies η coll than defocusing fields. For a special case, the hyperbolic field, η coll is given as a function of the angular beam spread, beam diameter, and initial beam energy. η coll increases with 1) decreasing angular beam spread, 2) increasing ratio of collector size to beam diameter, 3) increasing initial beam energy, and 4) the number of collector stages. Experimentally, a 3-stage collector of the hyperbolic field type was studied in connection with a PPM-focused high-power Dual Mode TWT, having a pulse-up ratio of 8 dB. At 50-, 75,, and 100- percent depression, the tube efficiency was 26 percent in the low-power mode and 34 percent in the high-power mode, corresponding to collector efficiencies of 76 and 71 percent, respectively.

High-power linear-beam tubes

High-power klystrons, coupled-cavity traveling-wave tubes (TWT's), and hybrid tubes, all of which utilize the microwave cavity as the basic circuit element, are described. These amplifiers are used in communications, radars, electronic countermeasures, and other applications at power levels from a few hundred watts to megawatts, at frequencies from ultrahigh frequency on up, and are particularly suited for high average powers. High gain, 30 to 60 dB, is normally achieved, and bandwidth usually lies in the 1-30-percent range. Elementary theory of operation is described, together with design considerations and systems interface information. Typical tube designs and data are presented. Recent developments are discussed, including high-efficiency techniques (to 75 percent), improvements in bandwidth, periodic focusing, and beam control electrodes. Most of the basic design techniques are well developed, and emphasis is being shifted to improvements in the detailed performance characteristics such as gain and phase resolution ripples, noise, and the sensitivities to operating voltages and currents.

RF defocusing of ion-focused electron beams

Ion-focusing of electron beams in high-power linear-beam microwave tubes is considered, and a qualitative discussion of the defocusing mechanism of an externally applied RF power is presented. The primary mechanism of beam breakup is the space-charge force of bunched electrons which limits the use of ion-focusing in high-power tubes.