Frequency Tuning in Short-Wave Gyrotrons with Irregular Cavities
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Radiophysics and Quantum Electronics, Vol. 62, No. 11, April, 2020 (Russian Original Vol. 62, No. 11, November, 2019)
FREQUENCY TUNING IN SHORT-WAVE GYROTRONS WITH IRREGULAR CAVITIES I. V. Osharin and A. V. Savilov ∗
UDC 621.385.69
We study the process of frequency tuning of a cyclotron resonance maser with a quasiregular sectioned cavity, which is based on gyrotron excitation of a higher longitudinal mode. It is shown that a regime can be achieved, in which the generation frequency varies fast (during one pulse) with no variation in the power of the output radiation.
1.
INTRODUCTION
At present, gyrotrons are the most advanced sources of coherent radiation, which operate in the subterahertz frequency band at relatively high values of the average power. The benefits of gyrotrons are that they are relatively small and easily available to users. This is determined mainly by application of moderately relativistic electron beams in these devices. Several gyrotrons operating in the continuouswave and pulsed generation regimes in the frequency band 0.2–1.3 THz have recently been implemented in experiments successfully. Here, one should mention both gyrotrons with the conventional (tubular) configuration of electron beams, which operate at the first or second cyclotron harmonics [1–10], and gyrotrons with large electron orbits [11–15], where higher cyclotron harmonics can be excited. The possibility of making frequency-tunable gyrotrons is of interest for many applications. A wellknown method for such tuning is passing over to operation at higher longitudinal cavity modes [2, 4, 16], when the increase in the operating value of the cyclotron frequency of electrons results in that excitation of the lowest longitudinal mode in the regime of gyrotron resonance (i.e., without Doppler conversion of the electron cyclotron frequency or its harmonic) is replaced with excitation of higher longitudinal modes in the regime of the resonance similar to that in the backward-wave oscillator (BWO), i.e., with the Doppler conversion of (decrease in) the cyclotron frequency. As a rule, a drawback of this method is the sharp drop in the output radiation power, when the generation switches from the lowest longitudinal mode to higher modes, which is explained by both the lower diffraction Q-factor of higher longitudinal modes and the decrease in the efficiency of the electron-wave interaction for the waves excited in the BWO regime. This work studies the process of frequency tuning for the gyrotron based on excitation of a quasiregular sectioned cavity with a periodic set of short irregularities (phase correctors) [17–19]. Such a cavity consists of several regular sections separated by short widenings, which ensure the phase shift equal to π between the sections. This corresponds to the change in the sign of the electron-wave coupling factor. In this situation, a higher longitudinal mode, rather than the lowest quasicritical cavity mode, occurs in the gyrotron-type cavity. At the same time, the quasicritical longitudinal mode is excited in the
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