Based on a quasioptical approach and direct particle-in-cell simulations, we study dynamics of oversized relativistic surface-wave oscillators (SWOs) of the Cherenkov type with 2D periodical corrugated structures of cylindrical geometry. Such corrugation allows significant rarefication of the spectrum of modes with different azimuthal indices. As a result, selective excitation of a mode with a given azimuthal index is possible. Azimuthal index of the generated mode depends on the voltage rise time. For short (nanosecond scale) rise time, generation of an azimuthally symmetric mode can be realized. For longer (hundreds nanoseconds to microseconds) rise time, the modes with high azimuthal indexes would be excited. These conclusions are supported by the experiments where Ka-band SWOs with 2D corrugated structures were realized based on the 300 keV=100 A=4 μs thermionic accelerator SATURN. For an oversize factor of 16, stable narrow-band generation with output power of 1.5-2 MW was obtained at the frequency of 32.5 GHz corresponding to the mode with an azimuthal index of m ¼ 3. The project of Ka-band subgigawatt power SWOs operating at the azimuthally symmetric mode based on 500 keV=4 kA=20 ns high current explosive-emission accelerator SINUS-6 is under development.
Self-modulation regimes of generation in a powerful 10-micros X-band backward-wave oscillator were studied theoretically and experimentally. The sequence of the self-modulation patterns and corresponding bifurcation values observed as the current was increased were in good agreement with the results of simulations. It was found that at a current of 120 A chaotic self-modulation set in at a power of 2 MW and a relative spectral width of 4%.
We present the results of studies of a gyroklystron with the TE 53 output mode. A 30-dB gain is obtained at a frequency of 30 GHz for an output power of 5 MW, efficiency 25%, pulse duration 0.4 ms, and amplification bandwidth 40 MHz.1. At present, the development of multimegawatt gyroklystrons is mainly aimed at the international program of creating new-generation electron-positron colliders [1]. The majority of such gyroklystrons [2-6] use the TE 01 or TE 02 modes as operating ones for which the axisymmetric electric field on the cavity wall is zero, which is favorable from the electric-strength viewpoint. However, such modes fail to eliminate the risk of thermal "fatigue" of the cavity walls in the pulsed-periodic regime. Moreover, the TE 0p modes (with axisymmetric fields) compare unfavorably with modes of other types if the radial index p increases. Therefore, the high-order TE mp modes with nonzero azimuthal indices m, which have long been used in gyrotrons [7], are rather attractive. Both electric and magnetic fields of such modes on the cavity walls are relatively small compared with the fields in the zone of interaction with the electron beam.The possibility of creating a high-order nonsymmetric-mode gyroklystron is supported by the results of our previous studies such as development of the method allowing us to reach a high-frequency electricfield intensity of 150 kV/cm on the electrodynamic-system wall [8], creation of a magnetron-injection gun forming a helical electric beam (with an accelerating voltage of 280 kV and a current of 60 A) with a pitch factor of 1.3 for the case of acceptable electron spread over transverse velocities [9], and testing the output cavity in the active-oscillator mode where the output power exceeding 10 MW is obtained at a frequency of 30 GHz for the TE 53 mode and the radiation duration equal to the supply-voltage duration [10,11]. 3 4 5 2 1 7 10 8 9 6 Fig. 1. * zaitsev@appl.sci-nnov.ru † Deceased.
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