R.H. Kyser

Gain broadening in an inhomogeneous gyrotron traveling wave amplifier

A wideband low-voltage millimeter-wave gyrotron traveling-wave amplifier (gyro-TWT) that incorporates precise axial tapering on both the magnetic field and the interaction circuit for broadband operation is discussed. The amplifier operates in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency at a beam voltage of 33 kV and a beam current of 1.6 A. By using an electron beam with an alpha of around 0.6, small signal gain in excess of 16 dB is observed over a wide instantaneous frequency range (33%, 27-38 GHz), in good agreement with theory. Work on the large signal characterization of the amplifier is reported. Nonlinear calculations indicate saturated gains of around 16 dB with approximately 30% bandwidth and 10% efficiency.<<ETX>>

Investigation of the stability of a tapered gyro-TWT amplifier

The stability of a wideband single-stage tapered gyrotron traveling wave tube amplifier (gyro-TWT) operating in Ka-band (27-38 GHz) has been investigated. The amplifier operates at 33 kV, 1.6 A in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency. Small signal gain in excess of 20 dB has been demonstrated over a wide (33%) frequency range with this device. Recent narrowband large signal measurements have demonstrated a saturated efficiency of around 10%, corresponding to a peak power of 5 kW at 35 GHz. Attempts to increase the amplifier gain and efficiency further have been limited, however, by the onset of oscillations due to insufficient output circuit match. A stability study has been performed to assist in the optimization of the present device, and to aid in the design of a follow-on two-stage experiment. The study has focused on the parametric dependence of the start oscillation current threshold on the velocity ratio, the magnetic field detuning, and the output port reflection coefficient.<<ETX>>

Broadband operation of a two-stage tapered gyro-TWT amplifier

A two-stage Ka-band gyro-TWT amplifier experiment operating in the TE/sub 10/ rectangular tapered waveguide mode at the fundamental cyclotron frequency is underway at the Naval Research Laboratory. The objective of this program is to demonstrate the stable wideband operation of a low voltage tapered gyro-TWT having high gain and good gain uniformity. Initial experiments operating at 33 kV and 1.5 A have demonstrated a linear gain of 30 d8, saturated gain of 26 dB over an 8% 3 dB-bandwidth, and maximum efficiency of 15%. The operation is compared with theory.<<ETX>>

A Broadband Gyro-TWTA Experiment

A tapered reflection-type gyrotron amplifier(33kV, 1.2A) has been designed and built. The amplifier incorporates precise axial tapering of both the magnetic field and the interaction circuit for stable broadband operation(27-34 GHz). Cold test results show that the tube presents an excellent RF match over the entire operating band. This device will be used to explore the performance characteristics of low voltage operation.

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.

Experimental study of GYRO-BWO

An experiment has been carried out at NRL to test the potential of a gyrotron backward wave oscillator (Gyro-BWO) as a high power tunable mm-wave source. Voltage and magnetic tunability have been demonstrated in Ka-band. The initial operation of the device has shown >15% efficiency over the 27-29 GHz frequency range and > 5% up to 35 GHz. An improved experimental set-up is under preparation which will allow a detailed comparision with linear theory (1).

Input coupler for a 35 GHz gyroklystron

Summary form only given. Gyroklystron amplifiers at 35 GHz are currently being designed and operated at the Naval Research Laboratory. To couple the input signal, in a relatively broadband way, into the desired TE/sub 011/ circular cavity mode, a coaxial cavity is constructed around the right circular input cavity. The two cavities are coupled by slots cut in the shared wall. Significant analysis and cold test has been done on this device. Theoretical analysis includes a circuit model, a combined mode-matching/dipole aperture theory, and HFSS simulations. The design goals are for a resonant frequency of 35 GHz, Ohmic quality factor of over 1000, total quality factor of near 175 with over 99% mode purity. Cold test results indicate the success of the design work. Sensitivity of the cavity resonance to slot orientation, cavity length, and aperture alignment is studied. A particular problem is encountered in attempting to match in the input drive signal to the cavity. Limits on attainable match are found using conventional filter theory. The final design for the cavity will be described as well as initial tests with an injected electron beam.

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.

35 GHz gyroklystron amplifier development at NRL

Summary form only given, as follows. Experiments on a two-cavity gyroklystron are underway to demonstrate a 140 kW, 35 GHz gyroklystron amplifier, operating at a fundamental beam cyclotron mode and a TE/sub 011/ cylindrical cavity mode. A high power electron beam of 70 kV, 6.6 A is produced from a magnetron-injection-gun which is optimally designed for the TE/sub 01/ mode at 35 GHz. Drift tubes consisting of lossy ceramic rings (80% BeO, 20% SiC) are designed to suppress undesired oscillations. A drive power is injected into the first cavity through a multi-hole coaxial coupler. A capacitive probe is placed directly before the input cavity to measure the beam velocity ratio. Large signal non-linear calculations predict a peak efficiency of 30% (extracted power=140 kW) and a saturated gain of 20 dB over a 0.3% bandwidth at /spl alpha/=1.5, /spl Delta/v/sub Z//v/sub Z/=20% at 13.3 kG and Q/sub 1/=Q/sub 2/=200. Design parameters and initial hot-test results of the amplifier will be presented.

Second harmonic gyro-peniotron oscillator experiment

Summary form only given, as follows. The approach under study at NRL for efficient millimeter wave operation at reduced values of the magnetic field is based on the harmonic gyro-peniotron interaction occurring in slotted waveguide structure with a high quality axis encircling beam. An efficient second harmonic TE/sub 21/ gyro-peniotron oscillator with a rising-sun slotted waveguide structure has been designed utilizing an axis-encircling beam (70 kV and 3 A). The hybrid particle-in-cell code predicts an efficiency of 60% at electron beam velocity ratio of 1.4 with cold beam. The simulated performance was optimized where B/B/sub grazing/=1.015, the quality factor of 600, and cavity length of 2 cm. Experiments are underway to demonstrate the high efficient gyro-peniotron interaction and correlation with the quality of the electron beam.