Frequency-multiplying gyrotron traveling-wave tube amplifier

The frequency multiplication was theoretically predicted and verified by experimental observation in a two-stage tapered gyrotron traveling-wave tube amplifier. The electron cyclotron maser interaction at harmonic frequencies was utilized between traveling electromagnetic waves in the two-stage tapered waveguide and the helical electron beam, under the presence of a reduced guiding magnetic field by harmonic operation. A 30 kV, 1 A axis-encircling electron beam from a cusp gun was chosen for both the frequency multiplication and the power amplification. For the theoretical predictions, a small-signal theory was used to investigate the spatial growth rate of the device as a guideline for large-signal simulations. Device performances such as the saturated output power, harmonic frequency multiplication, and electron bunching behavior were numerically analyzed using the large-signal theory and compared with those from particle-in-cell (PIC) code simulations. The dependencies of a bunching parameter on a beam velocity ratio, an interaction length, and an input drive power were analytically derived using a ballistic bunching theory. These theoretical analyses on the frequency-multiplying gyro-TWT were the basis of the experimental development. In the proof-of-principle experiment, the theoretical prediction of frequency multiplication from an X-band drive signal to a Ka-band output signal through the third harmonic electron cyclotron maser interaction in the two-stage tapered gyro-TWT was verified. The measured Ka-band output frequencies from 31. 8 to 36.0 GHz were three times of the input drive frequencies from 10.6 to 12.0 GHz. The measured output power was about 20 dBm due to a reduced beam current of 160 mA. The power amplification, however, is expected when the beam emission is improved, and it is now underway.