Microwave Amplifier Tubes Research Articles

A microwave gyro amplifier with a ferroelectric cathode

A ferroelectric cathode is employed for the first time as the electron-beam source in a microwave amplifier tube. A PLZT 12/65/35 ferroelectric ceramic with a high dielectric constant (/spl epsiv//sub r//spl sim/4000) is used in a form of a hollow cathode. The tube is operated in poor vacuum conditions (2/spl times/10/sup -5/ Torr) at room temperature, in a mechanism of a cyclotron-resonance maser amplifier. The device operates near the waveguide cutoff frequency at 6927 MHz. A 22-dB electronic gain and a 25-W output power are measured in this experiment.

Field emitter array development for microwave applications. II

Microfabricated field emitter arrays are being used in an ongoing DARPA/NRL program as a means for gating or prebunching electrons in a microwave amplifier tube. The goals of the program are to demonstrate 10 dB gain at 50 W and 10 GHz in a gated Klystrode amplifier tube with 50% efficiency. The proposed cathode specifications call for 160 mA peak emission and 10 GHz emission modulation from an annular emitter array having a 600 μm outer diameter and an inner diameter to be determined by transconductance and capacitance requirements. Experimental results have shown an average array capacitance of 6 nF/cm2, and that a transconductance of 1 μS/tip can be achieved at emitter–tip loadings of 10 μA/tip. Calculations based on these results show that emitter arrays having 0.4 μm diam gate apertures on 1 μm centers, a 600 μm outer diameter, and a 560 μm inner diameter should meet the tube specifications. Such arrays have been fabricated and shown to have essentially the same characteristics (Fowler/Nordheim coefficients) as the test cathodes used to develop the design parameters. These microwave cathodes have been successfully modulated at 10 GHz rates in an experimental Klystrode amplifier tube at CPI (formerly the Varian Associates Microwave Power Tube Division), and microwave output power has been achieved. Ongoing trials are showing steady progress toward the program goals.

Field-emitter-array development for microwave applications

Microfabricated field-emitter arrays are being investigated as a means for gating or prebunching electrons in a microwave amplifier tube. The goals of the program are to demonstrate 10-dB gain at 50 W and 10 GHz in a gated klystrode■ amplifier tube with 50% efficiency. The cathode specifications call for 160-mA peak emission at 10-GHz rates from an annular emitter array having a 600-μm outer diameter and an inner diameter to be determined by cathode capacitance, emitter-tip loading, and transconductance (Gm) considerations. We have shown that an average array capacitance of 6 nF/cm2 and emitter-tip loadings of 10 μA/tip can be routinely achieved with Gm≊1 μS/tip. Calculations based on these results show that an array having 0.4-μm-diam gate apertures on 1-μm centers, a 600-μm outer diameter, and a 560-μm inner diameter would be a reasonable first design to meet the tube specifications. Such an array would have a predicted capacitance of 2.18 pF, a peak Gm of 29.4 mS, and would produce a peak emission of 160 mA with a tip loading of 4.4 μA/tip. The power dissipated in driving the gate at 10-GHz rates would be between 0.1 and 1.0 W.

Electron reflection and secondary emission characteristics of sputter-textured pyrolytic graphite surfaces

Low secondary and reflected primary electron emissions from the collector electrode surfaces are important for optimum collector efficiency and hence for high overall efficiency of microwave amplifier tubes used in communication satellites and in military systems. Ion sputter texturing effectively suppresses electron emission from the surface of pyrolytic graphite, a promising collector electrode material. In this paper we present secondary and reflected primary electron emission characteristics of sputter-textured pyrolytic graphite surfaces with microstructures of various sizes and densities. The microstructure with the lowest electron emission levels, which were less than those of soot, consisted of a dense array of tall thin spires.

Progress in low-noise microwave tube design

Low-noise microwave amplifier tubes are used primarily in microway relay and radar receivers. A commercial S-band low-noise travelling-wave tube is used as an example to illustrate the basic components, their philosophy and design. Particular stress is laid on the tube processing and the design of the cathode-first-anode region of the electron gun.The wave theories for travelling-wave-tube noise figures are reviewed, and the significance of the older theory in the light of recent results is discussed. While the generalized noise theory of longitudinal-wave devices is well in hand, theories for the region of multi-velocity flow are still in the early stages. Noise figures of other types of longitudinal-wave tubes should theoretically be as low as those of travelling-wave tubes, but the latter have enjoyed greater development.Empirical results from the work on travelling-wave tubes are stated, although they are not fully understood. Finally, suggestions are made as to promising areas for noise-figure improvement.