Experimental study on Ka-band harmonic-multiplying two-stage tapered gyro-TWT

Summary form only given. An experimental setup of two-stage tapered harmonic multiplying gyrotron traveling-wave tube is being prepared to achieve high power millimeter waves. X-band input frequencies can be multiplied and amplified by the interaction at the 3^rd harmonic of electron cyclotron frequencies with practical, inexpensive, low frequency driving sources. The required magnetic field can be also reduced to one-third of the magnetic field at the fundamental harmonic interaction. To suppress unwanted oscillations, a linearly tapered two-stage rectangular waveguide at the fundamental TE₁₀ mode is used at the expense of low coupling between RF waves and electrons. A previous experimental demonstration at NRL showed a 20% bandwidth at saturated gain of 25 dB with 16% efficiency when the predicted velocity spreads of 4% are assumed at the fundamental harmonic interaction of Ka-band. In this study, however, under the 3^rd harmonic multiplying interactions, a self-consistent nonlinear calculation shows a saturated gain of 23 dB, an electronic efficiency of 10%, and a bandwidth of 3% on the assumption of 4% axial velocity spreads of electrons when an axis-encircling electron beam at a moderate beam voltage of 30 kV and a current of 1 A is employed.