Abstract
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 $300\text{ }\text{ }\mathrm{keV}/100\text{ }\text{ }\mathrm{A}/4\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ 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 $m=3$. The project of Ka-band subgigawatt power SWOs operating at the azimuthally symmetric mode based on $500\text{ }\text{ }\mathrm{keV}/4\text{ }\text{ }\mathrm{kA}/20\text{ }\text{ }\mathrm{ns}$ high current explosive-emission accelerator SINUS-6 is under development.
Highlights
For spatially extended relativistic electron beams of sheet and tubular geometry, the use of two-dimensional (2D) distributed feedback is beneficial for providing spatial coherence of radiation and can be exploited in order to increase the total radiation power in the microwave generators [1,2]
Experimental studies of free electron masers (FEMs) based on the novel feedback mechanism have been performed in Ka-band at the University of Strathclyde (Glasgow, UK) [3] and in W-band at the Budker Institute of Nuclear Physics (Novosibirsk, Russia) [4,5] in collaboration with the Institute of Applied Physics RAS
Note that for the first proof-of-principle experiments, the interaction length was chosen to exceed the optimal value to deliberately ensure the excess of the starting conditions for self-excitation of the oscillator. This coincides with the results of 3D simulations, which demonstrated the possibility to enhance the output power with a certain reduction in the interaction length in the realized surface-wave oscillators (SWOs)
Summary
For spatially extended relativistic electron beams of sheet and tubular geometry, the use of two-dimensional (2D) distributed feedback is beneficial for providing spatial coherence of radiation and can be exploited in order to increase the total radiation power in the microwave generators [1,2]. Such 2D feedback can be realized in planar and coaxial 2D Bragg structures (resonators) having double-periodic corrugation.
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