A Ka-Band Phigtron With A Novel Coupled Ball-Cavity As Output

Abstract

The harmonic multiplying gyrotron can reduce the drive frequency as well as the magnetic field by a factor equal to the harmonic number, but mode competition constitutes the main difficulty for the device adopting the traveling wave as output. To avoid the mode competition, we design a Ka-band Phigtron (phase coherent harmonic multiplying inverted gyrotwystron) with a novel output in the form of coupled ball-cavity which is characterized by high mode selectivity, high power and extended interaction. The device uses a magnetron injection gun (MIG) to produce its electron beam. The drive signal is applied via a Ku-band input coupler. By amplification of the TE01 drive wave through the fundamental harmonic (s=1) cyclotron in the gyro-TWT section, the signal at harmonic of the drive frequency is nonlinearly generated in the electron beam. The amplified wave is absorbed in a matched load at the end of the gyro-TWT section, but all harmonic components in the beam current continue to evolve in the drift section due to ballistic bunching. The growth process of harmonic components in the beam current is a nonlinear aspect of cyclotron maser bunching, and provides the basis for harmonic-multiplication. The entrance to the output ball-coupled cavity is placed at the position where the second harmonic component in the current density reaches a value near its maximum. Thereby, a cavity mode is excited which is resonant at twice the frequency of the drive signal. This cavity mode grows rapidly through the second harmonic (s=2) cyclotron maser interaction. The Ka-band output power is axially extracted and travels to the vacuum window while the spent electron beam dumps into the beam collector. The Phigtron is powered by a modulator which provides a flat pulse of 80 kV with a variable pulse length of 2 us~1 ms and a repetition rate of 20~1000 Hz. The MIG produces a beam current up to 16 A. Six independent DC current supplies power the water-cooled solenoid magnets with total weight of 350 kilogram that allow for considerable variation in the axial magnetic field profile. The field in the interaction region is increased from 0.56 to 0.68 T over a 55 cm distance. The voltage applied to the intermediate anode is about 39 kV. A Ku-band, pulsed, helix TWT provides the input power. Output power is measured by a calibrated calorimeter. Output frequency is measured by a calibrated wavemeter. Measurement gives 33.5 dB gain and 1.5% continuous bandwidth (500 MHz) around 34.1 GHz as well as an amplification range of 1 GHz from 33.5 to 34.5 GHz. High-power spectral peak at 33.75 GHz gives a value of 200 kW with corresponding efficiency of 20% and gain of 35 dB. No spurious oscillation was observed, a fact we attribute to the merits of both using a relatively low order mode combined with the s=l interaction in the input section and the use of a ball-coupled cavity as the output section. In summary, we have demonstrated experimentally a highly overmoded, millimeter wave source in a Phigtron configuration with frequency/harmonic multiplication and a novel ball-coupled cavity for stable operation.