We describe high-harmonic gyrotrons with axis-encircling electron beams developing on the basis of two experimental setups. The 30 keV / 0.7 A CW gyrotron is developed for the spectroscopy applications. Recently, selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics were observed in a series of experiments. Quasi-regular cavities with periodic phase correctors are designed to improve the operation at the third harmonic, as well as to achieve the fourth-harmonic operation at frequencies of up to 0.65 THz. The pulsed 80 keV / 0.7 A gyrotron is aimed to provide high-power (hundreds Watts) microwave pulses at the third cyclotron harmonic at frequencies close to 1 THz. Recently, a sectioned cavity with a decreased diffractive Q-factor was experimentally tested at this setup. Now we study possibilities to increase the peak level of the output power up to the level of several kW in order to use this gyrotron in plasma applications.
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