220-GHZ sheet-beam extended-interaction klystron design

The extended interaction klystron (EIK) is a compact amplifier that is well-suited to millimeter-wave and submillimeter-wave operation, having achieved state-of-the-art performance of 9 W CW at 218 GHz with a tightly focused 12-kV round beam. By utilizing an EIK circuit in a sheet-beam topology, our goal is to produce more than an order of magnitude higher power than can be generated with a round beam. This talk describes the design of a four-cavity 220-GHz EIK circuit, completed with the code MAGIC-3D based on our sheet-beam EIK cavity topology. This circuit includes an input cavity to initiate beam velocity modulation, two synchronously tuned idler cavities to enhance beam modulation, and an output cavity for RF power extraction. The input and output circuits are nearly identical with eight gaps each and an external coupling waveguide, which yields a Q_ext, ~ 300. The idler cavities have seven gaps with no external coupling. All cavities operate in the 2p-mode. The circuit is driven by a 520 mA, 16.5 kV sheet beam with an aspect ratio of 19. The beam is transported using a 9 kG solenoidal magnetic field. With these beam parameters and the large number of gaps, this circuit configuration effects a very efficient beam modulation. Output power of 453 W is achieved in MAGIC-3D with an input power of 25 mW, corresponding to an electronic gain of 41.6 dB in a circuit length of about 1.2 cm. Our sheet-beam EIK cavity topology also facilitates fabrication. Two sets of 220-GHz OFHC copper cavities have been fabricated with conventional machining techniques for evaluation purposes. The surface finish and precision appear to be within specified tolerances. This is particularly impressive in consideration of the fact that the width of the interaction gaps is only slightly larger than a typical human hair (~ 75 microns). Cold testing is currently underway at NRL. Cold test results will be presented at the conference. Details on beam formation and transport will also- be presented and discussed.