Experimental studies of low-voltage, grating TWT amplifiers

Abstract

Recent results are presented from experimental studies of spontaneous emission and linear gain in a low-voltage, sheet-beam-compatible, grating TWT amplifier. A round probe beam from a 10 kV, 0.25 A Pierce gun electron source is utilized for 14 GHz amplifier experiments. From the spontaneous emission measurements we obtain the following valuable informations:(a) measurements of the grating-induced spontaneous emission spectrum which can be correlated with the theoretically predicted linear gain curves, (b) measurements of broad-band noise emission which are critical for amplifier sensitivity characterization, and (c) measurements of spontaneous emission in the backward-wave regime to correlate with start oscillation predictions. The noise emission spectra have been correlated with the most dominant experimental factors including guide magnetic field intensity and body currents. Measurements of linear gain are also compared with the theoretical prediction in both forward and backward-wave regimes. The slow-wave structure consists of a uniform grating that adiabatically tapers to a smooth wall at both ends for impedance matching between the conventional TE/sub 10/ rectangular guide mode and the TE/sub x10/ hybrid slow-wave mode of the grating circuit. Small loop antennas are inserted at each end in smooth-walled rectangular guide sections for input and output signal coupling with the TE/sub 10/ mode. Broadband, high-vacuum-compatible, SiC waveguide attenuator fins provide excellent suppression of unwanted end reflections and prevent the system from self-oscillation in the forward-wave regime.