MODE COMPETITION AND STARTUP IN CYLINDRICAL CAVITY GYROTRONS USING HIGH-ORDER OPERATING MODES

The problem of mode competition in cylindrical cavity gyrotrons is con sidered. The normalized variable equations are used to calculate the o scillation regions of possible operating modes in the energy-velocity- pitch-angle plane. The analysis is self-consistent and includes the ef fect of changing beam current, pitch angle, and energy during the star tup phase. The time evolution of beam parameters during startup is com puted for several types of startup methods and used to determine the o scillating cavity modes during startup. Depending on the type of start up chosen, the cavity can be made to oscillate in several modes or in a single chosen operating mode-even for high-order modes where many ot her possible operating modes exist, Some startup methods are seen to b e less favorable than others, allowing for oscillation of unwanted mod es and some methods are seen to be more sensitive to small beam/cavity misalignment. The accessibility to the high-efficiency hard-excitatio n region can also be determined and is seen to depend on the startup s cenario. The startup analysis is linear whereas the stablility and int eraction efficiency computations are fully non-linear. The method is g eneral and can be applied to any operating mode, with the mode competi tion analysis specifically useful for high-order modes where the spect rum is dense. The analysis of the accessibility to the hard-excitation region is applicable to high- and low-order operating modes. Both q = 1 and q = 2 longitudinal mode numbers are considered.