Alan H. McCurdy

Experimental investigation of a high power, two-cavity, 35 GHz gyroklystron amplifier

Experiments on a two-cavity gyroklystron amplifier are performed to demonstrate high power coherent radiation amplification at 34.95 GHz. Experiments show a saturated efficiency of 37%, a bandwidth of 0.36%, and a gain of 23.6 dB corresponding to peak radiation output power of 210 kW. Experimental results are in good agreement with large signal simulations. Calculations also show that a stagger-tuned three-cavity circuit increases the bandwidth to more than 0.9%.

Design and Performnance of a Three-Cavity Gyroklystron Amplifier

The Naval Research Laboratory (NRL) is investigating the gyroklystron amplifier (GKA) concept. The purpose of this program is to assess GKA characteristics in order to determine if such a device is capable of achieving the high efficiency, gain, and output power predicted by numerous studies. The first in a series of tubes to examine GKA behavior has been fabricated and is currently under testing. This tube utilizes three rectangular cavities operating in the TE101 mode. Design goals included 100-kW output power, 30-percent efficiency, 40-dB gain, and operation at a center frequency of 4.5 GHz. Test results have shown reasonable agreement with theoretical predictions. In particular, efficiencies as high as 33 percent have been observed with a saturated output power of 52 kW. Bandwidths of 0.4 percent have been measured. In addition, the device can be operated under certain conditions as a phase-locked oscillator. These results suggest that the desirable characterstics which have been predicted for the GKA can be realized with a microwave tube.

Design and analysis of a coaxial coupler for a 35-GHz gyroklystron amplifier

A single rectangular TE/sub 10/ feed four-slot coaxial coupler is designed and built for excitation of a TE/sub 011/ cylindrical cavity mode for use in high-power millimeter-wavelength gyroklystron amplifiers. A high degree of mode purity is obtained and matching of the cavity to the input line is studied. A model based on the mode-matching technique and dipole radiators has been formulated to predict operation of this coupler. The resulting numerical code is capable of finding resonant frequency and cavity bandwidth in a small fraction of the time taken by more general finite-difference/finite-element design tools. The model can be extended to self-consistently include an electron beam, and the model is compared to a coupler design based on Hewlett-Packards High-Frequency Structure Simulator code. The coupler has been successfully used in a high-power gyroklystron-amplifier experiment.

PIC code simulation of pulsed radiation in a tapered closed-cavity gyrotron

MAGIC, a two-and-one-half-dimensional particle-in-cell (PIC) code, has been used to investigate mode locking in closed-cavity gyrotrons. A cavity, with equally spaced modal frequencies, composed of a radially tapered section and a straight section of waveguide, is designed, built, and cold-tested. Experimental cold test results agree well with the MAGIC PIC code simulations. Using a sinusoidal current density modulation with an amplitude of 5% of the dc current and frequency of 280 MHz, the simulation results show radiation output in a train of narrow pulses (full width at half maximum /spl sim/1 us) at a 280 MHz repetition rate. Though the gyrotron does not appear to be mode locked, uniform pulse trains of 30-50 pulses can be obtained.

A mode-matching technique for mode coupling in a gyrotron cavity

The mode-matching technique (MMT) is used to compute the electromagnetic fields, stored energy, and input admittances of a gyrotron cavity coupled to one or more waveguides. The method is based on matching the cavity and waveguide eigenmodes across the cavity apertures and accommodates cavity walls of finite conductivity. The MMT is used in the gyrotron problem because fields in and near the aperture must be computed accurately, and because the eigenmode decomposition is advantageous for inclusion of an electron beam. Irrotational modes are part of the complete set of orthogonal vectors required to expand an H-field in an open cavity, but were excluded in most gyrotron literature; here, this is corrected. The MMT is numerically implemented for cavities of rectangular and circular cross section. Coupling between different modes in a gyrotron cavity through external and ohmic losses is demonstrated. A coupled (complex) cavity gyrotron design is analyzed using MMT. The energy and modal spectra of the cavity are computed, demonstrating the mode selective properties of the design.

MODE SELECTION BY PRIMING IN AN OVERMODED ELECTRON CYCLOTRON MASER

It has been found that an externally injected signal can strongly influence the steady‐state electromagnetic mode in which a pulsed electron cyclotron maser (ECM) operates. A careful experimental examination reveals that the process by which this occurs is similar to the oscillator phase priming noted previously in magnetron work. It is found here that the degree of mode control depends on the injected signal power level (relative to the ECM noise power level), frequency, and time of application. Mode control is obtained at power levels nearly four orders of magnitude below that of the steady‐state ECM output power and over frequency bandwidths several times that of the cavity resonance band. The optimum input signal is a circularly polarized wave, corotating with the electron cyclotron motion. The experimental results are compared with a quasilinear coupled‐mode theory. The theory is used to provide analytic predictions of the temporal mode evolution through third‐order coupling. The comparison with expe...

Calorimeter analysis and test for symmetric circular waveguide mode output

Summary form only given, as follows. To measure the radiated power from a high power millimeter wave device it is preferable to perform calorimetry. A calorimeter has been designed and tested for measurements on a 35 GHz gyroklystron amplifier experiment at NRL. The output radiation is in the form of a TE/sub 01/ circular waveguide mode in a highly overmoded guide (operating frequency is more than three times cutoff). The final calorimeter design is an inverted cone structure with power absorption by octyl alcohol. Low radiation reflection, mode conversion, and extraneous heat dissipation are obtained. Three levels of analysis are performed. A simple ray tracing theory allows reflection coefficient and mode conversion to be found by replacing the symmetric circular mode with an equivalent rectangular one. A finite difference code based on a planar circuit analysis provides the same information with a more rigorous treatment of the rectangular boundary conditions. Finally, HFSS simulations are carried out for a restricted number of candidate designs using the actual circular structure. Cold test results will be described in additions to details of the calorimeter construction and calibration.

Self-consistency in the quasi-linear theory of electron cyclotron masers

Abstract The rate equation coefficients in the quasi-linear theory of the electron cyclotron maser are calculated self-consistently by using the true oscillator frequency. Theoretical corrections to the growth rate, oscillation frequency and saturation are calculated and compared with experiment. It is found that the self-consistency requirement can be met in a simple way which substantially improves the theory.

Mode interaction through amplitudes and phases in a two-mode gyrotron oscillator

An analysis is made of temporal evolution of electromagnetic modes in a two-mode gyrotron oscillator characterized by phase and amplitude interaction through the terms linear in the oscillator power. The problem is solved in the context of amplitudes and phases which vary slowly compared with the period of oscillation. Specific reference is made to competition between TE/sub 11q/ modes in a closed cavity gyrotron. Qualitative features which are found include phase locking, beat frequencies, periodic pulling, and mode excitation. This work has applicability when the frequency separation between the modes is on the order of the frequency bandwidth of each mode, or the modes are equally spaced in frequency. Gyrotrons of this type include those with low-quality-factor modes or degenerate modes. Phase interaction in the case of equally spaced cavity modes is of importance in analyzing mode-locking phenomena. >

35 GHz gyroklystron amplifiers

Summary form only given. Experiments on two-cavity and three-cavity gyroklystron amplifiers are underway to demonstrate a 140 kW, 35 GHz coherent radiation amplification for radar applications. High power, high duty (10%) relevant gyroklystrons require a TE/sub 011/ cylindrical cavity mode, operating at a fundamental beam cyclotron mode. An electron beam of 70 kV, 6 A produced from a magnetron-injection-gun is injected into the cavities through a high compression magnetic field powered by a 13 kG superconducting magnet. Drift tubes consisting of lossy ceramic rings are designed to suppress undesired oscillations. A drive signal is injected into the first cavity through a mode selective coaxial coupler. A capacitive probe is placed directly before the input cavity to measure the beam velocity ratio. Large signal non-linear calculations predict that the two-cavity gyroklystron will produce a peak power of 140 kW, corresponding to efficiency of 33% and a saturated gain of 23 dB over a 0.35% bandwidth at /spl alpha/=1.5, /spl Delta/v/sub z//v/sub z/=15%, 13.3 kG, Q/sub 1/=150, and Q/sub 2/=200. In order to increase an amplifier bandwidth, a stagger tuned three cavity circuit is designed.