Initial results from a harmonic ubitron experiment

Summary form only given. An experiment has been designed to study harmonic operation of the ubitron in Ku band with the first-harmonic interaction below cutoff. The first-harmonic interaction is studied and compared to the higher harmonics. The experiment is designed to interact with either the TE₀₁ or TE/TM₁₁ modes. The TE₀₁ mode interacts with the odd harmonics of the electron wiggle motion with no contribution from nonlinear bunching due to a first-harmonic interaction. The TE/TM₁₁ modes interact through the second-harmonic periodic position. The experimental parameters are V_b=55-250 kV, I _b= 10-50 A, L_w= 3 cm, B_w = 1-1.5 kG, waveguide=1.372×0.622 in (WR137), and f=15 GHz. The interaction waveguide is overmoded. Three-dimensional nonlinear simulations of the interaction predict unenhanced efficiencies of 3% for the third harmonic (at 55 kV) and 7% for the second harmonic (at 120 kV). Operation in the first harmonic requires a beam voltage of 250 kV with a predicted efficiency of 15%