Abstract
The gyrotron backward-wave oscillator (gyro-BWO) base on electron cyclotron maser is a promising millimeter/submillimeter source for high power capability and broad band tunability. However, the development of the gyro-BWO is hampered by the nonstationary oscillation. We will discuss the stability and tunability form the fundamental physics characters and verify by the Ka band gyro-BWO experiment. The beam-wave interaction forms the axial mode in the gyro-BWO. The number of regions of positive energy deposition rate determines the order of axial mode in the linear operating region. The field contracts to the beam entrance at the nonlinear operating region. The time-dependent particle-in-cell code is used to analyse the nonlinear behavior in the gyro-BWO. The results show the gyro-BWO exhibits the broad and stationary tunability at the shorter interaction length. And the self-modulation could be the reason forming the nonstationary oscillation in the gyro-BWO. We conducted the Ka band gyro-BWO experiment to verify the tunability at the shorter interaction length (3 cm) and analyse the stability at the longer interaction length (9 cm). The experimental results show that the gyro-BWO could broad and stationary operation at the 3 cm interaction length with maximum efficiency 28%, whereas the nonstationary oscillation hampered the tunability at the 9 cm length with 20% efficiency. The signal gating technique was used to analyse the spectra of the nonstationary oscillation. The uneven spatial distribution of the beam deposited energy causes the field energy to bounce back and forth within the feedback loop and modulates the oscillation amplitude and generates the equally spaced sidebands about a main peak in the output spectra.