Field-emitter-array development for microwave applications

Microfabricated field-emitter arrays are being investigated as a means for gating or pre-bunching 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 (C), emitter-tip loading, and transconductance (G_m) considerations. We have shown that an average array C of 6 nF/cm² and emitter-tip loadings of 10 μA/tip can be routinely achieved with G_m≈1μS/tip. Calculations based on these results show that an array having 0.4-μm-diameter 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 C of 2.18 pF and a peak G_m 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