H. Guo

A novel highly accurate synthetic technique for determination of the dispersive characteristics in periodic slow wave circuits

A highly accurate (0.1-0.5%) synthetic technique for determining the complete dispersive characteristics of electromagnetic modes in a spatially periodic structure is presented. It was successfully applied for the cases of the fundamental (TM/sub 0(1)/) as well as higher-order (TM/sub 0(2)/, TM/sub 0(3)/) passband modes in a corrugated waveguide. This structure is commonly used in relativistic backward wave oscillators, traveling wave tubes, extended interaction oscillators, and a variety of multiwave Cerenkov generators. An appropriately shorted periodic structure resonates at specific frequencies. To measure these frequencies accurately and unambiguously, the authors used unique antenna radiators to excite pure modes in the circuit under test. An analytical technique for deriving the complete dispersion relation using the experimentally measured resonances is presented. This technique, which is based on the intrinsic characteristics of spatially periodic structures, is applicable to slow wave structures of arbitrary geometry. >

High efficiency, phase-locked operation of the harmonic-multiplying inverted gyrotwystron oscillator

The inverted gyrotwystron (phigtron) is a millimeter wave frequency-doubling amplifier that has been demonstrated to produce over 300 kW peak power at twice the input frequency (centered at f/sub in/=16.85 GHz and f/sub out/=33.7 GHz) over a 0.5% bandwidth with a saturated gain of 30 dB and efficiency greater than 35%. The device has also been studied both theoretically and experimentally in a different operating regime where frequency-doubled, phase-locked oscillation is possible. A signal, injected via a fundamental gyro-traveling wave tube input section, modulated a 55 kV, 10 A electron beam. After transit through a drift section, the prebunched electron beam produced phase-locked, second harmonic oscillations in a TE/sub 03/ mode output cavity. RF output centered at either of two frequencies, 34.42 and 34.62 GHz, with a maximum output power of 180 kW, an efficiency of 32% and a locked signal gain of 35 dB was measured. A theoretical prediction of locking bandwidth, a design overview, and the experimental results are presented followed by a summary and discussion of the results.

Latest progress in studies of harmonic multiplying gyro-amplifiers

There has been considerable interest in the development of high-power gyro-amplifiers, stimulated mainly by millimeter wave radar applications. For high performance radar, in addition to output power level, such parameters as gain, efficiency, phase stability, duty cycle, compactness and bandwidth have highly demanding specifications. The investigations of harmonic-multiplying gyro-amplifiers, a new kind of nonlinear relativistic electron device, which have been filed for patent application by H. Guo et al., are driven by the objective of developing coherent millimeter wave sources capable of basically satisfying the demands of advanced radar transmitters.

Novel method for determining the electromagnetic dispersion relation of periodic slow wave structures

A novel method for calculating the dispersion relation of electromagnetic modes in an arbitrary periodic slow wave structure is reported. In this method it is sufficient to know the frequencies corresponding to three special wave number values, with other points calculated using an approximate analytical expression. This technique was successfully applied to determine the dispersion relation of the TM01 mode in a sinusoidally corrugated slow wave structure. This structure is commonly used in relativistic high‐power backward wave oscillators and traveling‐wave tubes, and is expected to have many additional applications.

Initial experimental results of a new triplet harmonic-multiplying gyrotron amplifier (Gyrotriotron)

A new triplet gyrotron amplifier, which we call gyrotriotron, has recently been developed. The device consists of a gyroTWT input stage to modulate the beam at S/sub 1/=1 cyclotron harmonic, a four-unit clustered-cavity as intermediate stage to bunch the beam at S/sub 2/=2 cyclotron harmonic and a gyro-TWT output stage to extract energy from the bunched beam at S/sub 3/=2 cyclotron harmonic. Electronic coupling between neighboring stages is realized through ballistic beam bunching in the radiation-free drift space similar to what happens in a gyroklystron. Because S/sub 3/ = S/sub 2/ = 2S/sub 1/, the device is a harmonic multiplying gyrotron amplifier with a scheme of 1-2-2. The clustered-cavity employed in the gyrotriotron is a group of TE/sub 03/ mode-selected cavities, which are closely adjacent and without any coupling between them.

A new triplet gyrotron amplifier, the gyrotriotron

Summary form only given. A new triplet gyrotron amplifier, which we call gyrotriotron, has recently been developed. The device consists of a gyro-TWT input stage to modulate the beam at S/sub 1/=1 cyclotron harmonic, a four-unit clustered-cavity as intermediate stage to bunch the beam at S/sub 2/=2 cyclotron harmonic and a gyro-TWT output stage to extract energy from the bunched beam at S/sub 3/=2 cyclotron harmonic. Electronic coupling between neighboring stages is realized through ballistic beam bunching in the radiation-free drift space similar to what happens in a gyroklystron. Since S/sub 3/=S/sub 2/=2S/sub 1/, the device is a harmonic multiplying gyrotron amplifier with a scheme of 1-2-2. The clustered-cavity employed in the gyrotriotron is a group of TE/sub 03/ mode-selected cavities, which are closely adjacent and without any coupling between them. HFSS simulation of a sub-unit of such a mode selective clustered-cavity is given. This is a first use of cluster cavity in a gyrotron. The proof-of-principle gyrotriotron with Ku-band input, Ka-band TE/sub 04/ mode output, predicted peak output power of 500 kW and 5% bandwidth, has been fabricated and is being tested in the Harmonic Gyrotron Lab at University of Maryland.

Experimental proof-of-principle results on a mode-selective input coupler for gyrotron applications

Proof-of-principle results for a mode selective input coupler are presented. Transmission and reflection measurements for the TE/sub 02/ cylindrical waveguide mode are given along with the output mode pattern. The results show good agreement for the cutoff frequency, mode pattern general behavior and variation with frequency for signals above cutoff. A maximum passband of 1.2 GHz (/spl sim/7%) has been achieved. Comparisons with theory for overall frequency response (from 15 to 18 GHz) and mode pattern characteristics (at 17.5 GHz) are also presented. The design and concept are promising for harmonic gyrotron-traveling-wave-tube amplifier and phase-locked gyrotron oscillator applications. >

Axial mode locking in a harmonic-multiplying, inverted gyrotwystron

When in microwave sources, an open waveguide operating at frequencies close to cutoff is used as a resonator, a number of modes with the same transverse structure, but different axial distribution can be excited by an electron beam. The width of the resonance curves of these modes broadens as the number of axial variations grows. This leads to overlapping of these curves at large axial indexes. As a result, phase locking of such modes may occur. A theory describing axial mode locking in gyrodevices is developed. The results of theoretical analysis are compared to a harmonic-multiplying, inverted gyrotwystron experiment. It is shown that such phase-locked operation in a set of modes with overlapping resonance curves can significantly enlarge the bandwidth of gyrodevices. Furthermore, this technique may be broadly applicable to other devices, which employ cavities with overlapping modes.

Studies of a three-stage inverted gyrotwystron

We have studied by numerical simulation, a new three-stage configuration of the inverted gyrotwystron, which operates with harmonic-multiplying mechanism. The simulations based on the analytical theory predicts a highly efficient and high-phase stability operation of the device, and gives results consistent with those measured in experiment. Related investigations of electron bunching, output power-frequency response, drive-gain dependence, and phase fluctuations due to beam voltage rippling are also presented.

Theory of clustered-cavity gyroklystron

An analytical theory of a new device configuration, a clustered-cavity gyroklystron, is developed. The device considered has two clusters of cavities: an input cluster and an output cluster. The results show that, by using a cluster cavity concept, the bandwidth of gyroklystrons can be enlarged significantly without sacrifice of gain or efficiency which may lead to the development of a new type of high power, moderate bandwidth millimeter-wave amplifier. The theory has also been used to analyze the effect of stagger tuning between cavity frequencies within a single cluster, as well as between different clusters on the bandwidth and gain of the device.