Carter M. Armstrong

Application of field emitter arrays to microwave power amplifiers

This paper describes the operation of a field emitter array (FEA) as the electron source of a traveling-wave tube (TWT) amplifier. Issues of beam control and focus at high current density and low magnetic field are addressed as well as issues relating to the inherent high emittance of the FEA beam and cathode protection from ion bombardment. Large signal, nonlinear RF-modulated FEA-TWT interaction simulations show circuit efficiencies that approach 50%, even for minimal bunching of average-to-peak current ratios of 0.7-0.9. Direct radio frequency (RF) modulation at the cathode is predicted to significantly improve linearity in the high-efficiency regime as well as reduce harmonic power levels. An unmodulated C-Band FEA-TWT was built to test the focusing approach as well as the robustness of the emitters in an operating vacuum device. The device uses a 1-mm diameter Spindt emitter with a custom-designed electron gun and helix circuit. The FEA-TWT has operated to date with a maximum current of 91.3 mA and shows 99.5% transmission under both drive and no-drive conditions. Output power of the device is 55.0 W at 1.5 GHz with a saturated gain of 23.4 dB and efficiency of 17%, and performs as predicted by simulation. During all operation, the FEA emission appears extremely stable, with no temporal variations observed at any time.

100 W Operation of a Cold Cathode TWT

Recent demonstration of low-voltage high-transconductance field emitter array operation holds promise for the successful development of reliable cold cathode vacuum electronics device technologies. This paper reports on the experimental studies of implementation of such field emitter arrays as the electron source for a moderate power traveling wave tube (TWT) operating in the C-band frequency regime. The cold cathode TWT has operated for over 150 h at duty factors up to 10%, beam currents up to 121 mA, and RF powers up to 100 W at 5 GHz. High cathode current densities of 15.4 A/cm2 were achieved concurrent with excellent beam control, resulting in 99.4% beam transmission under zero-RF-drive operating conditions and 97.3% transmission at maximum RF output power. The cathode is shown to operate with a 72% reduction in the operating voltage from the previous generation of emitters fabricated by SRI International, bringing the operating voltage for full current operation well below 100 V. Extensive device characterization and life testing has been performed, and interesting variation in cathode performance was observed during the high-duty high-current portion of the testing program. The results presented here represent the highest current, highest power, and highest duty factor ever reported for an RF vacuum device employing a field emission cold cathode electron source.

Experimental study of a Ka-band gyrotron backward-wave oscillator

A gyrotron backward-wave oscillator (gyro-BWO) has been operated and magnetically tuned over the frequency range 27-32 GHz. Tuning by varying the electron beam voltage was effective over a smaller frequency range ( Delta f approximately 1 GHz). Output power was as large as 7 kW, corresponding to a device efficiency of 19%. This large efficiency value was unexpected, and related analysis indicates it may be associated with the nonuniform magnetic field profile in the interaction region. >

Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of /spl sim/10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWTs with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (/spl alpha/), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies /spl plusmn/30/spl deg/ from fitted linear phase line. >

High-power cusp gun for harmonic gyro-device applications

A cusp gun, designed to generate an axis encircling beam with low-velocity spread for high-harmonic high-power (70 kV, 3.5 A) operation in gyrotrons and peniotrons, was successfully fabricated and hot tested at reduced voltage in a beam tester designed to measure beam perveance, size, thickness, velocity ratio (beam /spl alpha/), and, in the near future, beam ripple and velocity spread. A relatively thin, round, hollow beam was imaged on the Cerium glass witness plate. Beam transmission through the anode was 98.5%. The beam /spl alpha/, as determined by capacitive probe data, agreed well with simulations over a large range of operating parameters.

Experimental study of efficiency optimization in a three-cavity gyroklystron amplifier

An experimental study quantifying the effects of both penultimate cavity tuning and magnetic field profiling on the saturated efficiency of a three-cavity gyroklystron amplifier has been performed at a frequency of 4.5 GHz. As predicted by theory, it is observed that the penultimate cavity detuning in the gyroklystron is in the opposite direction from a conventional klystron for enhanced efficiency operation. This is a result of the opposite energy dependence of the bunching mechanism for the two interactions. Magnetic field profiling has also been shown to have a very strong effect on the amplifier efficiency, both through beam loading effects in detuning the penultimate cavity and through enhanced energy extraction in the output cavity. The measured efficiency of the gyroklystron is observed to improve from 5% to 21% by profiling the magnetic field along the device, and then from 21% to 29% by penultimate cavity detuning with an optimized magnetic field profile. >

A 100 Watt W-Band MPM TWT

L-3 EDD has developed a W-Band TWT with 100 W RF power over 4 GHz of bandwidth around 94 GHz suitable for MPM integration. The TWT employs an aperture grid modulated electron gun with mod-anode current control, a serpentine waveguide interaction circuit, and a single-stage depressed collector. Two TWTs have been built and tested to over 100 W pulsed output power. The first unit has been operated at high duty, producing 65 W CW output power and 75 W average pulsed power with reduced beam current. The TWT is designed for ease of manufacture, and is suitable for MPM integration, relying on conduction cooling and capable of operation to an altitude of 50k feet.

Linear theory of slow wave cyclotron interaction in double-ridged folded rectangular waveguide

A general dispersion relation for slow wave cyclotron traveling wave interaction in a folded waveguide is derived from a fluid dynamical treatment. The serpentine structure is formed by folding a double-ridged rectangular waveguide so that the orientation of the magnetic field changes (H-plane bend) instead of the conventional E-plane bend configuration. The H-plane bend structure has the potential for the production of high power, broad band radiation. For a cold beam, the linear theory predicts a gain of 2.0-2.5 dB/cm and a bandwidth of 20-25% in the millimeter wave frequencies. The bandwidth is sensitive to the axial velocity spread of the beam. The bandwidth decreases to 10% at /spl delta//spl upsi//sub z///spl upsi//sub z/=3%. Means to suppress the backward wave oscillations have to be applied for using the full bandwidth. >

Phase stability of gyroklystron amplifier

Expressions are derived for RF phase stability in conventional klystrons and gyroklystrons. Phase noise is found to depend on electron energy in a completely different way for the two types of devices, due to the inherent differences in gun dynamics and interaction mechanisms. In general, phase stability is better in gyroklystrons operating at voltage >

The vitality of vacuum electronics

Contrary to popular belief, vacuum electronics is alive and well. In fact with expanded activity in Asia and the Pacific Rim the field is more international than ever. Advances in materials, methodology, fabrication and technology are giving birth to a new generation of devices with higher power, higher efficiency and higher frequency[1], while strides in the companion technologies of high voltage power electronics and thermal-mechanical design are doing the same for integrated RF sources and compact amplifiers. Concerns with a diminishing supplier base are being addressed. Acknowledging the new generation of bright and energetic engineers joining our field, provided courtesy of our university partners, the continued vitality of vacuum electronics is not in doubt.