Multimode Gyrotron Research Articles

Influence of reflections on mode-competition processes in a high-power multimode gyrotron

Reflection of radiation from an output window may strongly affect operation of a gyrotron. In recent years, the control of gyrotron radiation by reflection has become a subject of active study. In this paper, we study the effect of reflection on mode competition in a multimode gyrotron. We consider the interaction of five modes with neighboring azimuthal indexes in a gyrotron with a fixed Gaussian axial field profile. We assume the mode spectrum to be nearly equidistant and set the spectral density parameter value as in high-power gyrotrons used for plasma heating. We analyze the domain of stability of the operating mode on the cyclotron frequency mismatch–normalized beam current parameter plane with and without the reflections. The possibility of suppressing spurious sideband modes and expanding the single-mode oscillation zone of the operating mode by injecting the reflected signal is demonstrated by numerical simulation. The most advantageous situation is when the designed mode is matched with the output window while the reflections of the parasitic modes are rather strong and the reflected waves return into the gyrotron resonator in the adverse phase reducing the Q-factors of the parasitic modes.

Locking of the Frequency of a Multimode Gyrotron by a Quasi-Monochromatic External Signal

We consider the influence of an external quasi-monochromatic signal on a multimode gyrotron in the case where the signal frequency is close to the frequency of the operating mode of the gyrotron. The influence of the mode competition on the generation regime at a harmonic frequency modulation is studied. The calculations were performed for a high-power 170 GHz gyrotron developed at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS). Depending on the parameters of the gyrotron and the external signal, various generation regimes were determined, including locking of the operating-mode frequency when the radiation frequency is close to the frequency of the external signal, beats of the frequency and amplitude of the operating mode, and excitation of spurious modes.

Нелинейная динамика многомодового гиротрона под воздействием внешнего квазимонохроматического сигнала.

Нелинейная динамика многомодового гиротрона под воздействием внешнего квазимонохроматического сигнала.

Possibilities of increase of gyrotron efficiency via frequency locking by external signal

Possibility of efficiency increase of a gyrotron, which frequency is locked by an external monochromatic signal, is studied. Dependencies of orbital efficiency on cyclotron frequency resonance mismatch for different currents of the electron beam are found. It is shown that in multi-mode gyrotron there is a possibility of shifting of frequency-locking zone to the region of parameters corresponding to the optimal efficiency, so efficiency may be significantly increased compared to the free gyrotron.

Terahertz Frequency- and Mode-Insensitive Broadband Quasi-optical Converter Antenna System

Recently emerged multimode gyrotron, a high-power broadband terahertz radiator, encounters the challenge of efficiently converting a series of operating whispering-gallery modes (WGMs) into free-space Gaussian beams. To this demand, we propose a frequency- and mode-insensitive antenna capable of broadband multimode converting. For a single mode, to achieve broadband operation, special reflector configuration and large-radius launcher guarantee the system high robustness to frequency-induced wave number variation. Furthermore, for a series of operating WGMs, in order to achieve multimode operation, high-order mode indices guarantees familiar field patterns and ray trajectories. In particular, high-purity Gaussian beams are simultaneously achieved in different WGMs of broad continuous bands, including 351–361 GHz for TE11,2 mode, 375–385 GHz for TE12,2 mode, and 398–410 GHz for TE13,2 mode. The results are verified by both the vector diffraction theory and the method of momentum. This kind of mode converter will promote the development of multimode gyrotrons and other antenna-feeder systems for high-power terahertz applications.

Theoretical investigation on a multifrequency multimode gyrotron at <italic>Ka</italic>-band

In this paper, a multifrequency, multimode gyrotron has been designed, which can operate at 28, 29, 31, and 32 GHz with corresponding modes TE0.3, TE+5.2, TE−3.3, and TE+6.2, respectively. For all operating status, the mode competitions have been investigated carefully with the help of a new time-dependent, multimode, self-consistent code, which is built on the trajectory approach. For the operating at 28 GHz, it also has been simulated by CST Particle Studio. They have similar results when comparing two results. For the operating at other status, the changes of electron parameters caused by the changed dc-magnetic field have been analyzed in detail. In the analyzes, the guiding center radius and Larmor radius would not be influenced severely, but the velocity ratio would be influenced seriously, such as, the velocity ratio of operating at 32 GHz would be reduced to about 0.9787, which caused a severe reduction in the efficiency. In order to alleviate the influences, a compensation magnetic coil (CMC) has been used in the magnetron injection gun region, in which the changes of electron parameters have been analyzed too. The simulation results show that the electron efficiency of operating at 32 GHz can be increased from 24.6% to 31% by applying CMC. The multimode multifrequency gyrotron operated at ka -band gyrotron has been investigated in theory, which can provide new possibilities in high-power millimeter source development.

Numerical Research on a 170-GHz Time-Dependent Multimode Coaxial Gyrotron With Outer Corrugation

In this paper, a coaxial gyrotron with outer corrugation is considered. Its multimode electron equation of motion is derived from the Lorentz equation, and its time-dependent amplitude and phase equation for the $\mathrm {TE}_{mn}$ mode is obtained from the wave equation. Using the time-dependent multimode beam–wave interaction equations, the time evolution processes of the time-dependent multimode coaxial cavity gyrotron with outer corrugation are investigated by numerical calculation. Results show that a 170-GHz coaxial gyrotron with outer corrugation can start up and operate stably at the $\mathrm {TE}_{34,19}$ mode and its competing modes are suppressed under different initial field amplitude conditions. By the calculation of the time-dependent multimode and single-mode coaxial gyrotron with outer corrugation, it is found that the output power and the output efficiency of the $\mathrm {TE}_{34,19}$ mode are 1.856 MW and 32.59%, respectively. These studies show the mode competition processes of the coaxial gyrotron with outer corrugation at the beginning start-up stage.

Mode Competition Effect on Frequency Locking of a Multimode Gyrotron by a Monochromatic External Signal

We study frequency locking of the gyrotron operating mode by a monochromatic external signal under conditions of competition of several modes. The calculations were performed for a megawatt-power 170-GHz gyrotron with the operating TE28.12 mode. Mode interaction of both equidistant and nonequidistant spectra is considered. Frequency detuning of the modes is comparable to the cyclotron resonance bandwidth. Influence of the external-signal power on the boundaries of the frequency locking zones is studied. The possibility to achieve a high orbital efficiency and broadband frequency tuning in the frequency-locking regime is considered.

Zones of Frequency Locking by an External Signal in a Multimode Gyrotron of a Megawatt Power Level

Zones of Frequency Locking by an External Signal in a Multimode Gyrotron of a Megawatt Power Level

Frequency and phase stabilization of a multimode gyrotron with megawatt power by an external signal

We have numerically studied capturing of the oscillation frequency and phase of the TE28.12 operating mode by an external monochromatic signal for a multimode gyrotron with a close to real regime of activation. It is shown that, even at a low external-signal power, in a wide range of the magnetic field and voltage, a highly efficient single-mode generating is set at the frequency of the external signal.

Enhanced Stability of Second- and Fourth-Harmonic Gyrotrons Driven by a Frequency-Doubled Prebunched Beam

There is currently considerable interest in operating gyrotrons at the second and higher cyclotron harmonics in order to access the near-terahertz regime while reducing magnetic field requirements. High-frequency gyrotrons have successfully operated at the second harmonic. However, competition from the fundamental harmonic increasingly limits operation in the higher order modes needed for the near-terahertz regime. Savilov recently proposed a scheme for frequency-doubled phase bunching of gyrating electron beams in a waveguide resonator formed from Bragg reflectors and with the drive frequency equal to the cyclotron frequency. The advantages of phase prebunching at twice the cyclotron frequency include suppression of the fundamental harmonic, enhanced second-harmonic operation, and increased likelihood of fourth-harmonic operation. We have investigated the use of this phase bunching technique to enhance higher harmonic operation in gyrotron oscillators with annular beams. We compute the frequency-doubled bunching produced by a Bragg-type prebunching cavity and use a large-signal, multimode, and multiharmonic gyrotron oscillator code to simulate the effect of this bunching on a highly overmoded output cavity. Regimes of stable operation are predicted for the second- and fourth-harmonic point designs.

Observation of Mode Competition in an 11.4-GHz Magnicon Amplifier

This paper describes the observation of pulse shortening in an 11.4-GHz magnicon amplifier due to competition with a parasitic gyrotron mode. This effect was observed following the installation of a new electron beam collector. The parasitic mode occurred only at high operating powers, when the beam transverse momentum was high, and the growth of the parasitic mode caused the power in the output pulse to drop substantially. We analyze the competition between the gyrotron and magnicon modes in the output cavity using a time-dependent multimode gyrotron simulation code that has been specially modified to model competition between synchronous magnicon and nonsynchronous gyrotron interactions, and compare the results to experimental observations. We also analyze the effect due to the change in the collector.

Mode competition in fourth-harmonic magnicon amplifiers

In the magnicon RF amplifier, the drive and gain cavities are cylindrical deflection cavities which operate in a rotating TM/sub 110/-mode and spin up an electron beam to high transverse momentum. As a result of the rotating-mode interaction, the electron beam entry point into the output cavity rotates about the axis at the drive frequency. The gyrotron-like output cavity can be operated at m times the drive frequency by using a mode with an azimuthal index of m, as this mode rotates at m/sup -1/ times its RF frequency, thus maintaining synchronism with the electron beam. Previous frequency-multiplying magnicons have used m=2; in this paper it is shown that magnicons with m=4 may be practical, provided one also operates the output cavity at the m/2 harmonic of the cyclotron frequency. Operation at higher harmonics lowers the frequency of the deflection cavities allowing lower RF fields, reducing cavity breakdown problems; lower magnetic fields, reducing magnet cast and complexity; and a larger electron beam, relaxing beam quality constraints. On the other hand, higher order azimuthal-index magnicon modes interacting at higher order cyclotron interactions are subject to competition with nonsynchronous (gyrotron) modes and are more sensitive to electron beam scanning angle spread. A time-dependent multimode gyrotron code has been modified to examine competition in the output cavity between the phase-synchronous operating mode and other nonsynchronous modes which interact via the conventional gyrotron interaction. Calculations have been carried out for fourth-harmonic magnicons with TM/sub 410/ and TE/sub 411/ mode output cavities and include the effects of mode competition with nonsynchronous modes as well as a spread in entry-point angles of the scanning electron beam.

Theory of competition between synchronous and nonsynchronous modes in a magnicon output cavity

In the magnicon amplifier, a scanning electron beam drives a synchronous fast-wave interaction in a cylindrical output cavity. The output cavity is designed to support a synchronous transverse magnetic (TM) waveguide mode, usually the TM210 mode. However, a number of other transverse electric (TE) or TM modes can be excited in the cavity via a nonsynchronous, gyrotron-type interaction. To investigate the possibility of competition from these nonsynchronous modes, a multimode gyrotron simulation theory and code have been adapted to the magnicon configuration. The gyrotron theory and corresponding code have been generalized to include a synchronous TM mode as well as nonsynchronous TE modes. Proper phase averaging between the modes, and between the modes and the beam electrons, is critical to accurate mode competition calculations. In nonsynchronous interactions this is achieved by averaging with respect to electron entrance time and the orbit guiding center angle. The synchronous mode interaction is invariant with respect to these two averages; however, it is affected by the scanning angle spread, which is included via a third average over scanning angles. Calculations have been carried out to model a second-harmonic X-band magnicon experiment, which is currently underway at the Naval Research Laboratory (NRL). The output cavity has been optimized for the TM210 mode at 11.4 GHz or twice the drive frequency (ωd=5.7 GHz). The principal competing mode is the TE121 mode. The simulations show that nonsynchronous mode interactions are suppressed by the synchronous interaction if the scanning angle spread is sufficiently small (≤90° in the NRL configuration).