Experimental Verification of Frequency Multiplication in Two-stage Tapered Gyro-TWT

A Ka-band harmonic multiplying two-stage tapered gyro-TWT has been developed. The third harmonic interaction of electron cyclotron frequencies is used to multiply X-band signals to Ka-band. A previous similar experimental demonstration at NRL showed a 20 % bandwidth at saturated gain of 25 dB with 16 % efficiency when the predicted velocity spreads of 4 % were assumed at Ka-band with the fundamental harmonic interaction. This work showed the linearly taperpd rectangular waveguide at both stages could produce enhanced gain, gain uniformity, and efficiency. Here, for a harmonic multiplying gyro-TWT, a self-consistent large-signal calculation predicts a saturated gain of 23.8 dB, an electronic efficiency of 12 %, and a bandwidth of 4.1 % on the assumption of 4 % axial velocity spread of the electron beam when an axis-encircling electron beam at a moderate beam voltage of 30 kV and a current of 1 A is employed. For a proof-of-principle, experimental setup is prepared. An electromagnet up to 5 kG and 13 sub-magnets to control the tapered magnetic field are constructed and tested. An axis-encircling electron gun with a single cusp is operated at the beam voltage of 30 kV and the beam current of 1 A. Interaction circuits are fabricated and tested. A 0-dB directional coupler shows a good performance as HFSS predicts. For the first time, triple frequency multiplication from 11.4GHz to 34.2GHz has been experimentally verified in the high power Ka-band gyrotron traveling-wave tube amplifier. Hot test for optimum output power is presently underway. Both experimental and theoretical works including a Particle-In-Cell Code MAGIC3D on this device will be presented in details. The separate abstract on the velocity spreads measurement is also submitted.