MULTIMEGAWATT RELATIVISTIC HARMONIC GYROTRON TRAVELING-WAVE TUBE AMPLIFIER EXPERIMENTS

The first multimegawatt (4 MW, eta = 8%) harmonic (w = s Omega(c), s = 2, 3) relativistic gyrotron traveling-wave tube (gyro-twt) amplifier experiment has been designed, built, and tested; Results from this exp erimental setup, including the first ever reported third-harmonic gyro -twt results, are presented, Operation frequency is 17.1 GHz, Detailed phase measurements are also presented, The electron beam source is SN OMAD-II, a solid-state nonlinear magnetic accelerator driver with nomi nal parameters of 400 kV and 350 A, The flat-top pulsewidth is 30 ns, The electron beam is focused,using a Pierce geometry and then imparted with transverse momentum using a bifilar helical wiggler magnet, The imparted beam pitch is alpha = beta(perpendicular to)/beta(parallel to ) approximate to 1, Experimental operation involving both a second-har monic interaction with the TE(21) mode and a third-harmonic interactio n with the TE(31) mode, both at 17 GAz, has been characterized, The th ird-harmonic interaction resulted in 4-MW output power and 50-dB singl e-pass gain, with an efficiency of up to similar to 8% (for 115-A beam current), The best measured phase stability of the TE(31) amplified p ulse was +/-10 degrees over a 9-ns period, The phase stability was lim ited,because the maximum RF power was attained when operating far from wiggler resonance, The second harmonic, TE(21) had a peak amplified p ower of 2 MW corresponding to 40-dB single-pass gain and 4% efficiency , The second-harmonic interaction showed stronger superradiant emissio n than the third-harmonic interaction, Characterizations of the second - and third-harmonic gyro-twt experiments presented here include measu rement of far-field radiation patterns, gain and phase versus interact ion length, phase stability, and output power versus input power.