Yu. K. Kalynov

High-harmonic gyrotron with sectioned cavity

High-harmonic large-orbit gyrotrons require long-length operating cavities because of both a weak electron-wave coupling and relatively low electron currents. Since diffraction Q factors of such cavities are very high, a large fraction of the radiated power is dissipated in Ohmic losses. A sectioned klystronlike cavity can be a way to combine a long electron-wave interaction region with a relatively low diffraction Q factor. In this paper, a design of a third-harmonic terahertz gyrotron is studied in detail and discussed. As compared to a regular cavity, the use of a sectioned microwave system provides an enhancement of the output rf power by several times along with the halving of the Ohmic losses.

Plasma creation by terahertz electromagnetic radiation

The results of experiments aimed at the study of the discharge in a focused beam of terahertz waves in argon under near-atmospheric pressures are presented. The range of electric fields and gas pressures, at which a breakdown occurs, is determined. The study of the discharge glow dynamics showed that the discharge starts at the maximum of the terahertz wave beam field and its front moved towards the radiation with the speed of about 105 cm/s into the region of the fields being significantly weaker than the breakdown value. Measurements of the ratio of wave transmission through the discharge allow one to conclude that the density of the plasma produced in the discharge exceeds 1015 cm−3. Some features of the terahertz discharge are discussed.

Klystron-like cavity with mode transformation for high-harmonic terahertz gyrotrons

A novel cavity scheme of a gyrotron is proposed and investigated. As it provides low Ohmic losses and high mode selectivity, it can be especially prospective for realization in gyrotrons operating in the THz frequency range. Numerical simulations show that it allows three-fold increase in the efficiency of the low-relativistic 500 GHz fourth-harmonic gyrotron as compared to conventional two-section scheme with modes transformation.

Development of a Magnetic Cusp Gun for Terahertz Harmonic Gyrodevices

A magnetic cusp gun (MCG) is being developed to generate an axis-encircling electron beam, which is called the large orbit beam, which is going to drive a 0.396-THz fourth-harmonic gyrotron. Developing an MCG imposes crucial challenges on a simultaneously minimizing guiding center deviation and velocity spread of the electron beam, particularly because an ultrahigh magnetic compression ratio is unavoidable, as is the case for a terahertz (THz) gyrotron. The study of the electron dynamics in the MCG reveals that, close to the emitter, a pair of focusing electrodes are employed to construct a special focusing and accelerating electric field as a way to balance the space-charge influence and guiding center deviation. Investigation indicates that both the electron-beam generalized-angular-momentum spread and the guiding center distribution are the critical factors contributing to beam parameter spread. Intensive optimization generates a high-power MCG with a pitch factor of 1.5, the highest magnetic field of 4 T, minimum transverse velocity spread of 1.1%, and a beam current of 2 A. The key parameters exhibit excellent stability tuning over a wide range of beam current and magnetic field. These merits enable the harmonic gyrotrons or even the frequency-tunable THz gyrotrons to be developed.

Theory of gyro devices with thin electron beams (large-orbit gyrotrons)

It is shown for a precisely aligned electron beam of small thickness in a large-orbit gyrotron that high cyclotron harmonics (up to s⩽5) can be selectively excited with an electronic efficiency that is acceptable for some applications (1–10%). When the quality of the electron beam is high, the selective properties are maintained even for modes with high radial indices, raising hopes that this method can be used to obtain coherent radiation not only in the millimeter but also in the entire submillimeter wavelength range. A method for taking into account the space-charge effects is developed, and it is shown that the corresponding effects can be important at relatively small values of the electron pitch factor. The results of the analysis and preliminary experiments reveal the possibility of creating high-power compact sources of submillimeter radiation for the spectroscopy of various media, the diagnostics of dense plasmas, and some other applications.

Features of plasma glow in low pressure terahertz gas discharge

Investigations of the low pressure (1–100 Torr) gas discharge in the powerful (1 kW) quasi-optical terahertz (0.55 THz) wave beams were made. An intense afterglow was observed after the end of gyrotron terahertz radiation pulse. Afterglow duration significantly exceeded radiation pulse length (8 μs). This phenomenon could be explained by the strong dependence of the collisional-radiative recombination rate (that is supposed to be the most likely mechanism of electron losses from the low pressure terahertz gas discharge) on electron temperature.

A method for suppression of spurious fundamental-harmonic waves in gyrotrons operating at the second cyclotron harmonic

A typical problem of gyrotrons operating at high harmonics of the electron cyclotron frequency is the suppression of parasitic near-cutoff waves excited at lower harmonics. In this paper, a method for a significant improvement of the selectivity of the second-harmonic gyrotrons is proposed. This method is based on the use of quasi-regular cavities with short irregularities, which provide different effects on the process of excitation of the operating second-harmonic wave and the spurious fundamental-harmonic wave by the electron beam.

Gyrotron with a sectioned cavity based on excitation of a far-from-cutoff operating mode

A typical problem of weakly relativistic low-power gyrotrons (especially in the case of operation at high cyclotron harmonics) is the use of long cavities ensuring extremely high diffraction Q-factors for the operating near-cutoff waves. As a result, a great share of the rf power radiated by electrons is spent in Ohmic losses. In this paper, we propose to use a sectioned cavity with π-shifts of the wave phase between sections. In such a cavity, a far-from-cutoff axial mode of the operating cavity having a decreased diffraction Q-factor is excited by the electron beam in a gyrotron-like regime.

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 TE 3,7 and TE 3,6 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 TE 2,5 and TE 2,4 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.

Large orbit gyrotron at submillimeter waves

A successful operation of a large orbit gyrotron (LOG) at submillimeter waves has been demonstrated. The LOG utilized a 250 keV/4A/10 /spl mu/s axis-encircling electron beam produced by a thermionic-emission electron optical system. The third cyclotron harmonic, single-mode oscillations at the frequencies of 371 GHz and 414 GHz for TE/sub 3,8/ and TE/sub 3,9/ modes, respectively, with a power up to 10 kW have been measured. Using a cavity of smaller diameter, the LOG operation at the TE/sub 3,5/ mode at the frequency of 369 GHz with power up to 20 kW was achieved.