In the quasi-optical gyrotron with a Fabry-Perot cavity, one can inject the electron beam at an oblique angle with respect to the optical axis of the cavity. This angle has a great influence on the gain mechanism, Doppler frequency shift, and saturation length of interaction. Numerical simulations show that two regimes of operation exist when the angle changes from 0 degrees to 90 degrees . When the angle is small (including 0 degrees ), the Doppler-shifted regime is identified with small negative optimized detuning and small optimized E-field amplitude. When the angle is large (including 90 degrees ), the gyrotronlike regime is identified with large negative optimized detuning and large optimized E-field amplitude. The quasi-optical gyrotron is quite efficient for harmonic operation.
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