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Large-Orbit Gyrotron operation in terahertz frequency range

Terahertz gyrotrons are not wide-spread yet because of the strong magnetic field required when operating at fundamental and second cyclotron harmonics, and severe mode competition when operating at higher harmonics. These problems can be significantly mitigated in the Large Orbit Gyrotron (LOG), where all the electrons describe helical trajectories with the axes coinciding with the cavity axis or close to it.1 Due to such a configuration stronger coupling with operating modes and better mode selectivity at high harmonics can be obtained. After realization of a sub-Terahertz high-harmonic LOG with a 250-keV electron beam and operating frequency 0.4 THz2, a new 3rd-harmonic LOG with significantly lower electron energy of 80 kV and higher operating frequency of 1 THz has been successfully tested at the Institute of Applied Physics. To decrease particle velocity spread the new oscillator exploits a cusp electron gun where the electron transverse velocity is seeded near the cathode, in the region of a sharp reverse of the magnetic field. The transverse velocity achieves the operating value by compression in a magnetic field increasing 3,000 times up to 13.7 T. A beam with current 0.7 A, pitch-factor 1.4 and satisfactory velocity spread 0.3 has been obtained in such a system. It is important that the cusp gun properly operates in wide region of electron energies (50–80 keV) and magnetic fields (10.5–14 T). A rather long LOG cavity provides sufficient mode separation and single-mode operation at the 3rd and 2nd operating harmonics. The modes TE3,7 and TE3,6 with radiation frequencies 1.00 and 0.87 THz and output power 0.4 and 0.3 kW, respectively, have been radiated at the 3rd cyclotron harmonic. The modes TE2,5 and TE2,4 with frequencies 0.68 and 0.55 THz and power 1.8 and 0.6 kW have been observed at the 2nd cyclotron harmonic. The pulse duration and repetition rate are 10 µs and 0.1 Hz, respectively. An output quasi-optical mode converter transforms all the modes into Gaussian wave beams.