Broadband design for 203-GHz TE02/TE03 gyrotron backward-wave oscillator

This study proposes a broadband interaction structure for a fundamental harmonic TE₀₂ 203-GHz gyrotron backward-wave oscillator (gyro-BWO). Unlike the present terahertz tunable gyrotron which achieves the tunability by interacting with a series of high order axial modes, the gyro-BWO operates with single (fundamental) axial mode in frequency tuning and has the advantage of smooth and broadband tuning. The interaction structure is designed to be low resonance for a reflective gyro-BWO (forward-wave extraction), which eliminates the major difficulty of a normal gyro-BWO, i.e. mode converter for backward-wave extraction. Numerical investigations indicate that the reflective TE₀₂ gyro-BWO is capable of generating a peak power of 2.8 kW corresponding to 22.6% interacting efficiency with a 3dB-tuning-bandwidth of 7.3 GHz at beam current I_b = 0.5 A and beam voltage V_b = 25 kV. For high power handling capability, the interaction structure could be scaled up and operate with TE₀₃ mode using same electron beam. The reflective TE₀₃ gyro-BWO yields a peak power of 2.5 kW and 5.7 GHz tuning bandwidth.