M. Garven

A gyrotron-traveling-wave tube amplifier experiment with a ceramic loaded interaction region

The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE/sub 01/ mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

A TE/sub 11/ K/sub a/-band gyro-TWT amplifier with high-average power compatible distributed loss

Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain /spl sim/60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in K/sub a/-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the TE/sub 11/ mode and for stabilization employs a high-average power compatible diffractive loading technique.

High-Power Copper Gratings for a Sheet-Beam Traveling-Wave Amplifier at G-band

The design, fabrication, and electromagnetic cold testing results of an all-copper grating circuit intended for a G-band sheet-beam traveling-wave amplifier are presented. Fabrication was carried out via ultraviolet photolithography (UV-LIGA) using the SU-8 photoresists. Two cold test methods used to characterize the microfabricated circuits are reported and reveal excellent agreement with simulations. This type of all-copper grating also shows potential for use as a high-average-power sharp-cutoff filter.

Simulation and Optimization of Gate Temperatures in GaN-on-SiC Monolithic Microwave Integrated Circuits

This paper presents 3-D thermal simulation studies of GaN-on-SiC monolithic microwave integrated circuits (MMICs) containing multifinger micrometer-scale high electron mobility transistors (HEMTs). The heat spreading effect of HEMT source, gate, and drain metallizations on peak structure temperatures is examined. The impacts of a realistic die attach material and rear-of-die heat transfer coefficient on structure temperatures, and in particular on temperature nonuniformity, are examined. Variable gate finger spacing, in which the gate spatial positions are described by polynomials as a function of gate number, is investigated as a means for optimizing the temperature uniformity from gate-to-gate. A thermal simulation code with a parametric MMIC geometry-based mesh generator and a deformable mesh consistent with sequential movement of gate finger positions during optimization is employed for all of the studies. The code is multiscale with a sufficient resolution range to handle a multifinger HEMT structure while also including the MMIC die, die attach metallization, and a realistic heat transfer coefficient associated with microchannel coolers. A variable gate pitch geometry based on an optimized cubic polynomial demonstrates considerable advantage in temperature uniformity.

Measurements of intrinsic shot noise in a 35 GHz gyroklystron

Experimental measurements of electron beam shot noise in a 35 GHz, 225 kW, three-cavity gyroklystron have been obtained from both the input and output cavities. This intrinsic noise was studied in the absence of an applied carrier (i.e., at zero drive power). The spectrum of the noise emitted by the input cavity is found to have a Lorentzian shape, with peak noise power densities from the input cavity typically reaching 6.3×10−15 W/Hz (−112 dBm/Hz), and typical 3 dB bandwidths of 160 MHz. The output cavity noise spectrum is found to be equal to the input cavity noise spectrum multiplied by the measured linear frequency response of the gyroklystron. The measured noise levels at the input cavity are 0–5 dB lower than theoretical predictions for shot noise unaltered by collective effects. Furthermore, the input cavity noise power exhibits complex variations as a function of beam current, beam velocity ratio, and circuit magnetic field that are not predicted by present theory. Noise-to-carrier ratios expected...

Studies of electronic noise in gyroklystrons

Direct measurements of phase noise in a four-cavity, 35 GHz gyroklystron producing 50 μs pulses of 175–210 kW output power with 50–53 dB saturated gain are presented. The measurements were performed at a 10.7 MHz frequency offset from the carrier, where the noise is expected to be dominated by shot noise and where the extrinsic noise from the electron gun’s pulsed power supply is manageable. At an operating point with 70 kV beam voltage, 9 A beam current, and a beam velocity ratio of 1.3, a phase noise of −149±1 dBc/Hz was measured during the production of 180 kW output power at 50 dB gain. At a higher beam current of 10 A, the measured phase noise was −146±1 dBc/Hz during production of a 200 kW output power carrier with 53 dB gain. The directly measured phase noise levels were generally within 2–3 dB of the values expected on the basis of carrier-free noise temperature measurements. Overall, the measured gyroklystron noise levels are similar to those of conventional klystrons. Also presented are analytic...

Experimental studies of a gyro-TWT amplifier with a lossy ceramic interaction region

Summary form only given, as follows. Gyrotron traveling wave tube (gyro-TWT) amplifiers are under investigation for radar applications due to their high power and broad bandwidth capabilities in the millimeter wavelength range. The design and experimental study of a 35 GHz gyro-TWT amplifier operating in the circular TEO/sub 01/ mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided controlled loading of both the TEO/sub 01/ operating mode and the backward wave oscillations to ensure stable operation. The gyro-TWT circuit consisted of a lossy, linear amplification section that employed ceramic rings spaced to provide the required absorption. The final, nonlinear stage of amplification occurred in a short conducting wall section at the end of the interaction circuit.

9.5: Microfabrication of a 220 GHz grating for sheet beam amplifiers

A 50 watt CW 220 GHz sheet beam amplifier is under development at NRL. We report on microfabrication techniques used to construct a grating-based amplifier circuit using Ultraviolet Lithography (UV-LIGA) and Deep Reactive Ion etching (DRIE).

Broadband Microwave and W-Band Characterization of BeO-SiC and AIN-Based Lossy Dielectric Composites for Vacuum Electronics

The complex dielectric permittivity properties of lossy ceramic materials used in vacuum electronics are presented. The studies include broadband room temperature behavior in the 0.1-18 GHz range and in W-Band (75-110 GHz). Variable temperature measurements of selected materials at 94 GHz are also presented

Higher order mode excitations in gyro-amplifiers

In gyro-devices, a nonlinear output taper is often employed as the transition from the near cutoff radius of the interaction circuit to a much larger output waveguide. The tapers are usually designed to avoid passive mode conversion, and thus do not consider the effect of a bunched beam. However, recent simulations with the self-consistent MAGY code [Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)] indicate that higher order mode interactions with the bunched electron beam can substantially compromise the mode purity of the rf output. The interaction in the taper region is traveling wave in nature, and is strongly dependent on the residual beam bunching characteristics resulting from the upstream operating mode interaction. An experiment has been performed to quantify the rf output mode content from a Ka-band gyroklystron. The agreement between salient theoretical and measured rf output characteristics confirms the existence of higher order mode excitation in output tapers as predicted by theory.