A recent theory for achieving stability in high-power gyrotron traveling wave (gyro-TWT) amplifiers has been verified. By keeping the interaction length shorter than the critical oscillation length for the competing modes, stability was achieved. The experiment also verified the theoretical prediction that harmonic gyro-TWT's can produce higher power because their weaker interaction yields a higher threshold electron current for oscillation. The second-harmonic gyro-TWT single-stage amplifier stably generated 207 kW, nearly twice the level achieved by fundamental-harmonic gyro-TWT's at a comparable voltage.

• Received 7 August 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.4322