Input Cavity Research Articles

Design of a high-efficiency low-voltage axially modulated cusp-injected second-harmonic X-band gyrotron amplifier

We present the theoretical design of a second-harmonic small-orbit gyrotron amplifier which utilizes the interactions between a 35-kV 4-A beam and a TE/sub 011/ cavity to produce over 70 kW of amplified power at 9.9 GHz in a 1.83-kG magnetic field. One of the novel features of this device is that the electron gun produces an axially streaming annular beam which is velocity modulated by a short TM/sub 0n0/ input cavity. Perpendicular energy is imparted to the beam via a nonadiabatic magnetic transition at the end of a 13-cm drift region. An electronic efficiency of 53% is predicted with a large signal gain near 20 dB by a single particle code which takes into account nonideal effects associated with finite beam thickness and finite magnetic field transition widths.

The multiplet cavity: A buncher for broad-bandwidth klystron amplifiers

The bandwidth of a klystron output cavity scales (approximately) as p/sup 0.8/P/sup 0.2/, where p is the beam perveance and P is the beam power. For high-perveance (p>10 /spl mu/pervs) high-power (P>10 kW) electron beams, it is relatively straightforward to design a broad-band output cavity. However, the design of the input cavity of the broad-band klystron is in some ways more difficult. The purpose of the input cavity is to produce a velocity-modulated electron beam with a frequency-dependent modulation amplitude that will optimize the bandwidth of the entire klystron system, while providing a magnitude of velocity modulation large enough to minimize the length of the klystron. This paper shows how a multiplet (multiple cavity) buncher cavity can be designed to provide broad-band (>20%) operation while keeping short the drift section length of the klystron.

Two-cavity gyroklystron with self-excited input cavity

Abstract A new type of oscillator a two-cavity gyroklystron-oscillator, is proposed. In this device, auto-oscillations in the first cavity modulate the electron energy; in the drift space the electrons are bunched, and the bunches excite powerful oscillations in the second cavity. A simple theory for such an oscillator has been developed. The design parameters of a two-cavity tube operating in the TE011 the X-band have been calculated. As predicted by theory, the output power and efficiency are enhanced in comparison with those for a conventional gyrotron, and this is demonstrated experimentally.

Operation of a K-band second harmonic coaxial gyroklystron

Amplification studies of a two-cavity second harmonic gyroklystron with a coaxial input cavity and drift tube are reported. The inner conductor is supported by tungsten pins which intercept the beam, and it utilizes lossy dielectric rings to enhance mode suppression. A double-anode magnetron injection gun produces a 440 kV, 200–268 A, 1 μs beam with an average perpendicular-to-parallel velocity ratio near one. The TE011 input cavity is driven near 9.886 GHz and the TE021 output cavity resonates near 19.772 GHz. Peak powers near 30 MW have been achieved, although more easily reproducible peak powers hover closer to 20 MW. While stability is improved over previous devices, beam interception reduces the accessible range in parameter space and thus degrades amplifier performance. Pin erosion is also evident and qualitatively agrees well with theoretical predictions.

Beam-cavity interaction physics for mildly relativistic, intense-beam klystron amplifiers

We examine beam-cavity interaction physics relevant to mildly relativistic, intense-beam klystron amplifiers. This is an interesting but difficult regime of operation, because of the combination of high beam current and low voltage. The advantage of this regime is that it is relatively easy to access high beam powers (and potentially high microwave output powers) at relatively low beam energy. We calculate the effect of the extremely high beam loading in the input and idler cavities. The output cavity's shunt impedance must match the low beam impedance in order to prevent high output gap voltages that will reflect electrons back upstream. This leads to very low cavity Q factors (<10). We derive expressions for mode purity for such low-Q cavities and show that acceptable cavities can be designed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-power operation of a K-band second-harmonic gyroklystron.

Amplification studies of a two-cavity second-harmonic gyroklystron are reported. A magnetron injection gun produces a 440 kV, 200--245 A, 1 \ensuremath{\mu}s beam with an average perpendicular-to-parallel velocity ratio slightly less than 1. The ${\mathrm{TE}}_{011}$ input cavity is driven near 9.88 GHz and the ${\mathrm{TE}}_{021}$ output cavity resonates near 19.76 GHz. Peak powers exceeding 21 MW are achieved with an efficiency near 21% and a large signal gain above 25 dB. This performance represents the current state of the art for gyroklystrons in terms of the peak power normalized to the output wavelength squared.

Performance characteristics of a high-power X-band two-cavity gyroklystron

The operating characteristics of a two-cavity X-band gyroklystron experiment are reported. Beam voltages and currents up to 440 kV and 200 A, respectively, are generated in 1 mu s pulses by a thermionic magnetron injection gun. Velocity ratios ( nu /sub perpendicular to // nu /sub z/) near one in the output cavity are used to achieve peak powers of 24 MW near 9.87 GHz. The maximum saturated efficiency of more than 33% occurs at a beam voltage of 425 kV and a current of 150 A. A large signal gain in excess of 34 dB is realized by operating the input cavity just below the start oscillation threshold. Details of tube stability and the dependence of amplification on magnetic field profile, input signal parameters, and various beam quantities are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design and computer simulation of a gyrotwystron

Abstract We have designed a 4.5 GHz gyrotwystron consisting of an input cavity, a stagger tuned intermediate cavity and an output waveguide operating in the TE°11, mode. Using a non-linear simulation code, we predict operation with a saturated gain of 22 dB, an efficiency of 22.5%, an output power of 60 kW and a bandwidth of ∼2%. The efficiency and gain are similar to those achieved in a three-cavity gyroklystron operating at the same frequency, but the bandwidth is about an order of magnitude larger. Changing the profile of the B field optimizes efficiency and gain but decreases the bandwidth.

Theory of the relativistic gyrotwistron

A generalized theory of the relativistic gyrotwistron, the device combining the elements of the gyroklystron and the gyro-traveling wave tube, is presented. A modulation of electrons in the input cavity is considered with the account of modulation in an electron axial momentum that is important for relativistic particles passing through a short cavity. A comprehensive study of large-signal operation of the output waveguide section in the cases of gyroresonance at the fundamental and second cyclotron harmonics has demonstrated a wide variety of electron bunching phenomena and the possibility of achieving high electron efficiency in a wide range of gyrotwistron parameters.

Methods for improving equidriving power-frequency characteristics of broadband high power klystron

Two methods for improving the equidriving power-frequency characteristics of broadband high power klystrons are presented. One is that a comb-line bandpass filter with some attenuation properties is inserted between the TWT driver and the klystron for compensating the gain-frequency characteristics of the klystron to get the required equidriving power-frequency characteristics. The other is that a reactive element is connected with the input cavity to change its resonance frequenciesfo andQL, and thus to improve the power-frequency characteristics of the klystron.

Design of an 85 GHz TE1,3phase-locked gyroklystron oscillator experiment

The design is described of an 85 GHz TE1,3 phase-locked gyroklystron oscillator experiment. The purpose of the experiment is to study the physics issues associated with ultra-high power highly overmoded phase-locked gyroklystron oscillators. The problems of mode conversion losses at cavity boundaries and self-oscillation of the input cavity presented the most challenging aspect of the design. A WKB type theory of mode conversion losses in highly overmoded waveguide cavities had to be developed to quantify design trade-offs. Three input cavity designs with varying degrees of design risk with respect to mode conversion losses were fabricated and cold tested. Two cavities with conservative designs were found to perform very close to expectations.

Mode conversion losses in highly overmoded waveguide cavities

A technique to quantify the effects of mode conversion losses in highly overmoded waveguide cavity resonators using the WKB method is presented. The WKB theory is accurate for weakly irregular waveguide cavities with small amounts of mode conversion. This approach to the problem has the advantage that the mode conversion losses in waveguide cavities can be quantified quickly and cheaply. The objective is to compute the amplitude and phase of a resonant mode in the cavity to zeroth order (i.e. without regard to mode conversion effects), and then use amplitude and phase of the computed zeroth-order fields to compute the amplitude of the converted mode. This approach is valid to the extent that the converted modes are not a major perturbation to the resonant mode. The results of the theory are presented in the context of the trade-offs associated with the design of an 85 GHz TE/sub 1.3/ phase-locked gyroklystron oscillator experiment. Mode conversion losses in the input cavity are a particular problem, and place limits on the amount of usable beam current (and therefore the output power capabilities of the gyroklystron). The isolation of the input and output cavities due to mode conversion must be carefully considered in any design. >

The input gap voltage of a klystron

Following the discovery that several large-signal klystron programs did not agree on the value of the RF voltage at the input gap of a klystron, the question was examined afresh. Available textbooks were found to make restrictive assumptions about matching and/or tuning of the input cavity, assumptions which are no longer acceptable. A formula was derived, in terms of cold test parameters and drive power, that correctly apportions the power between dissipation, reflection, and beam loading, thereby determining the RF gap voltage.

Singularities of radiation-flux absorption in a cylindrical cavity

The distribution of radiation absorbed by the inner surface of a cylindrical cavity is found by the Monte Carlo method for a different nature of the reflection and different ratios between the input beam and cavity diameters.

Optimum operational parameters of the ultrashort cavity laser

The influence of input parameters and cavity parameters of the ultrashort cavity laser is discussed in terms of numerical calculation of the rate equations of this laser. The optimum parameters of the ultrashort cavity laser are obtained. The results of this letter will provide some theoretical guides for designing and adjusting picosecond tunable dye lasers with ultrashort cavities.

Observation of nonlinear phenomena in ferromagnetic transmission resonance

In this paper we present the nonlinear effects we obtained so far through ferromagnetic-transmission-resonance experiments in iron and nickel. The experiments were performed in the geometry in which both the applied and microwave fields are mutually parallel and also parallel to the sample surface at the input cavity. In this geometry, we have found two interesting transmitted waves through ferromagnetic samples several microns thick. One is polarized parallel to and has the same frequency as the applied microwave field. The other, polarized perpendicular to the applied field, has a frequency half that of the applied field and is generated in the ferromagnetic sample. The results are compared to the nonlinear theory presented in the previous paper. The agreement between the experimental and theoretical results is reasonable. The theory shows that the nonlinear phenomena are very sensitive to the exchange stiffness constant. It is thus hoped that these phenomena will be useful in determining the exchange stiffness constant, and ultimately the exchange integral, as a function of temperature. Further, they might provide new information on spin-relaxation processes.

Low-Frequency Klystron for Accelerator Applications

Recent advances in computer simulation of the electron-field interaction in the high-power klystron have resulted in major improvements in klystron performance and a better agreement of calculated and measured characteristics. Designs for two klystrons to drive the four Alvarez sections of the LAMPF accelerator are presented in this paper. The output powers for the two klystrons are 0.5 and 3.0 MW at 201 MHz. The length required for the ?-MW klystron is only 3.25 m, and the 3-MW klystron requires 3.75 m due to the higher power collector. A brief description of the computer codes and logic used to achieve these designs is presented. Full-scale test cavities for the ?-MW klystron have been built, and the input cavity is being built to validate the fabrication procedures. The low-frequency, high-power problems in klystron technology are discussed. General scaling laws are derived to produce efficient, compact, narrow-band klystrons with peak output power in the megawatt range. As an example, the salient design features of a 3-MW, 100-MHz klystron are presented. A novel feature of this design is a cavity which is simultaneously excited at two frequencies. Use of such cavities could reduce construction costs and significantly improve the performance of low-frequency klystrons, which should make them more attractive as components in future heavy ion accelerators.

Interaction of Electrons with Cavities

The radiation emitted by an electron traversing an output cavity tuned to a single frequency after having traversed an excited input cavity is calculated using quantum field theory. It is found that the emission has maxima when the output cavity is tuned to a harmonic of the inputcavity frequency, the result being very similar to classical klystron theory. The emitted power is independent of the state of the field in the input cavity under any realizable conditions, so that this provides no mechanism for determining the state of this field.

Large-signal behavior of distributed klystrons

A theoretical large- and small-signal analysis of the behavior of standing-wave-type distributed circuit klystrons, based on a confined flow model, is described. A two-cavity klystron with a π-mode double-gap distributed output circuit and a conventional single-gap input cavity was selected as a specific example for study. The field distribution for the experimental cavities was measured and a conversion efficiency of 52 percent was predicted. The effects of the output cavity position relative to the input cavity, the output cavity gap angle, and the space charge were analyzed theoretically. The theoretical results were compared with measurements made on a precision demountable klystron with identical beam and circuit parameters, but with a Brillouin-focused beam rather than a confined-flow beam. A load efficiency of 35 percent, a conversion efficiency of 42 percent, and small-signal gains up to 15 dB were measured. The corresponding figures for the conventional single-gap output circuit were 24 percent, 28.6 percent, and 9 dB. The measured effects of changes in the tube parameters were in general agreement with the theoretical predicted results. The study indicates that the π-mode double-gap cavity is a very practical output structure yielding high efficiency with a very short interaction length. It should prove useful in medium- and high-power klystrons.

Microwave Magnetoelastic Resonances in a Nonuniform Magnetic Field

A new magnetoelastic resonance effect is described. A dc magnetic field was applied in the long direction of YIG slabs (1 cm long, 0.5 cm wide, and 0.1 or 0.07 cm high), each end of which was inserted in a stripline cavity. Under appropriate conditions a pulse of microwave power applied to one cavity resulted in a pulse in the second cavity delayed by several microseconds. Small variations were observed in the amplitude of the delayed pulse as the dc field was varied. Similar variations with the same periodicity (∼1 Oe) were also observed in the undelayed reflection from the input cavity. The amplitude variation is attributed to the excitation of magnetoelastic resonances in the nonuniform magnetic field near the endface of the sample. Because of the strong coupling between spin waves and elastic waves, the turning point (at which kspin waves=0) and the endface of the sample define a magnetoelastic quasicavity whose resonances give rise to the effect noted. According to this interpretation the separation of adjacent peaks should be δH = |H′dem|λ/2, where H′dem is the gradient of the demagnetizing field evaluated at the crossover point (kspin wave = kphonon) and λ the wavelength of transverse elastic waves. Measurements have been made showing the dependence on dc field, slab thickness, and frequency. Agreement with theory is satisfactory.