Absolute instabilities in the gyrotron traveling wave amplifier are investigated with a simulation approach which models electron and wave dynamics in the sever and distributed-loss section. Distributed wall losses are shown to be far more effective than the sever in stabilizing these instabilities. Physical interpretations are given and theoretical predictions are verified by a K"-band experiment which achieved 62 kW peak power with 12% bandwidth, 21% efficiency, and 33 dB saturated gain through the use of a mechanically adjustable magnetron injection gun.PACS numbers: 42.52.+x, 85.10.Jz The gyrotron traveling wave tube amplifier (gyro-TWT) promises a new generation of high power, broadband, millimeter-wave amplifiers.

The gyrotron traveling-wave tube (gyro-TWT) amplifier is known to be highly susceptible to spurious oscillations. This study develops a simulation approach to analyze the stability of a coaxial-waveguide gyro-TWT with distributed wall losses. The interplay among the absolute instabilities, the gyrotron backward-wave oscillations, and the circuit parameters is analyzed. Simulation results reveal that the distributed wall losses effectively stabilize spurious oscillations in the coaxial gyro-TWT. Furthermore, the wall resistivity of the center conductor is shown to be an additional effective mechanism for suppressing oscillations. Under stable operation conditions, the coaxial gyro-TWT with distributed losses is predicted to generate 435kW in the Ka band with 31% efficiency, a saturated gain of 45dB, and a bandwidth of 1.86GHz (≈5.8%) for a 70kV, 20A electron beam with an α(=ν⊥∕νz)=1.0 and an axial velocity spread of Δνz∕νz=5%.

Distributed wall loss is proposed to enhance the stability and tunability of a W-band TE01 gyrotron backward-wave oscillator (gyro-BWO). Simulation results reveal that loss effectively suppresses the unwanted transverse modes as well as the high-order axial modes (HOAMs) without degrading the performance of a gyro-BWO that operates at the fundamental axial mode. Linear and nonlinear codes are used to calculate the interaction properties. The effects of the distributed loss on the starting currents of all of the modes of interest are discussed in depth. The interacting structure is optimized for stability. The calculated peak output power is 102kW, corresponding to an efficiency of 20%. The 3dB tuning bandwidth is 1.8GHz, centered at 94.0GHz when using 5A and 100kV electron beam.

The magnetron injection gun is capable of generating relativistic electron beam with high velocity ratio and low velocity spread for a gyrotron backward-wave oscillator ͑gyro-BWO͒. However, the velocity ratio ͑␣͒ varies drastically against both the magnetic field and the beam voltage, which significantly limits the tuning bandwidth of a gyro-BWO. This study remedies this drawback by adding a variable trim field to adjust the magnetic compression ratio when changing the operating conditions. Theoretical results obtained by employing a two-dimensional electron gun code ͑EGUN͒ demonstrate a constant velocity ratio of 1.5 with a low axial velocity spread of 6% from 3.4-4.8 Tesla. These results are compared with a three-dimensional particle-tracing code ͑computer simulation technology, CST͒. The underlying physics for constant ␣ will be discussed in depth.

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