Tianzhong Zhang

Theoretical study on mode competition between fundamental and second harmonic modes in a 0.42 THz gyrotron with gradually tapered complex cavity

In this paper, the nonlinear dynamics of mode competition in the complex cavity gyrotron are studied by using multi-frequency, time-dependent theory with the cold-cavity longitudinal profile approximation. Based on the theory, a code is written to simulate the mode competition in the gradually tapered complex cavity gyrotron operating at second harmonic oscillation. The simulations tracking seven competition modes show that single mode oscillation of the desired mode TE17.4 at 150 kW level can be expected with proper choice of operating parameters. Through studying on mode competition, it is proved that the complex cavity has a good capability for suppressing the mode competition. Meanwhile, it is found that TE17.3 could be excited in the first cavity as a competition mode when the gyrotron operating at large beam current, which leads to that TE17.3 and TE17.4 with different frequencies can coexist stably in the complex cavity gyrotron with very close amplitudes. Thus, the complex cavity might be used for...

Investigation of the Influence of Electron Beam Quality on the Operation in 0.42-THz Second Harmonic Gyrotron

High-power terahertz gyrotrons always operate at high-order modes to avoid damage from Ohmic heating. However, the spectral densities of these modes are very high, especially when gyrotrons operate at high-order harmonics. Therefore, the self-excitation condition can be satisfied for several modes simultaneously. Correspondingly, in order to determine which mode will be excited first in the designed terahertz gyrotron, the starting currents of competing modes should be calculated more accurately to predict the operating status. In this paper, an existing linear theory is generalized to take into account the influences of the field profile, finite beam thickness, and velocity spread. Starting currents are calculated for the operating and the most dangerous competing mode in a 0.42-THz second harmonic gyrotron with a complex cavity, which is recently under development at the Terahertz Science and Technology Research Center. The calculations show that the radial and velocity spreads play a critical role in the starting current and prove that the measured output terahertz radiation in the designed device is generated from the interaction between the desired mode and the electron beam.

Study of Beam-Wave Interaction in a 170-GHz Confocal Gyrotron Traveling-Wave Tube

Confocal gyrotron traveling-wave tube (gyro-TWT) is a novel gyrotron amplifier capable of operating in higher order modes while generating high power at the same time. So far, confocal gyro-TWT has been studied by MIT both in theory and experiment at 140 GHz. In this paper, a center frequency of 170-GHz TE06-mode confocal gyro-TWT is designed theoretically, and it features high power and broadband capabilities. Theoretical investigation predicts that, when the magnetic injection gun is optimized to generate an electron beam of 65 kV, current of 7 A, with a pitch factor of 1.2, this gyro-TWT is of an excellent performance. At a given operating magnetic field of 6.11 T, with an input power of 10 W, a saturated gain of at least 30 dB, 3-dB bandwidth of over 4 GHz and highest efficiency of 17.1% are achieved.

Investigation on Mode Competition Between the Fundamental and Second-Harmonic Modes in the Complex Gyrotron

Stable operation in a single mode is an important goal of high-power gyrotron. Both multimoding and switching into unwanted modes can lead to lower efficiency. Based on the multimode, multifrequency, and time-dependent fixed field theory, a numerical code has been written for simulating the mode competition in the complex cavity gyrotron operating at the second harmonic, which is confirmed to be correct by comparison with the self-consistent simulation. With the help of the code, the mode competition in the designed complex gyrotron with the frequency of 0.42 THz is simulated by tracking several competition modes without considering ohmic losses, whose influence is roughly estimated. Through mode competition simulations, it is verified that the complex gyrotron with gradually tapered cavity has a good capability to suppress the mode competition, especially the competition from the fundamental modes. Meanwhile, the simulations show that the single mode oscillation of the desired mode TE17.4 at 150-kW level can be expected when the optimal operating condition is that the beam voltage is 50 kV, the beam current is 6 A, and the applied magnetic field is 7.98 T.

Analysis of the Photonic Bandgaps for Gyrotron Devices

The global bandgaps of photonic crystal are theoretically analyzed in this paper. The electromagnetic-wave propagation characteristics of photonic bandgap (PBG) structures, which is used in the the millimeter-wave, submillimeter-wave, and terahertz regime vacuum electronic devices and accelerators, were numerically simulated using the finite-element method software high-frequency structural simulator. The dispersion curves of the lattices in different rod radius to-rod spacing ratios and the global bandgaps for the general 2-D PBG structures formed by triangular and square arrays of metal rods were simulated. A mode map that shows the relationship between the structures and the contained modes was plotted and a 220-GHz metallic PBG resonator operating in TE04 mode was designed for a gyrotron device to verify the theoretical and numerical simulations, and a comparison of mode density and quality factor between the PBG resonator and the equivalent cylindrical resonator has been carried out.

Theoretical Study of Mode Competition in Terahertz Gyrotron With Second Harmonic Oscillation

The problem of mode competition can present a major hurdle for gyrotron to achieve stable and efficient operation, especially when the gyrotron operates at electron cyclotron harmonics. Therefore, an effective theoretical investigation of the mode competition is an essential first step to the implementation of gyrotron. In this paper, the nonlinear dynamics of the mode competition in the terahertz gyrotron operating at high-order mode are studied using a multifrequency time-dependent model. Based on the model, the simulations of mode competition in the designed gyrotron are presented by the corresponding code. The influence of the velocity spread on the mode competition is also studied. The simulations tracking five competing modes show that stable excitation of the second harmonic oscillation can be realized with a proper choice of the operation parameters. The results show that a single-mode second harmonic radiation with a power of over 120 kW at a frequency of 0.4 THz could be achieved.

Simulation of output coupler in a 170 GHz confocal gyro-TWT

The open sides of confocal waveguide diffract the spurious modes and operating mode at the same time. The operating mode must be converted to Gaussian mode to avoid transmission loss during a long transmission length. The transmission loss was analyzed and a simple output coupler was designed to convert the confocal TE06 mode to circular TE03 mode in this paper.

Mode competition between fundamental and second-harmonic modes in 0.4 THz gyrotron with gradually tapered complex cavity

The gyrotron has been demonstrated to be an efficient, high power source of terahertz radiation. However, one of the main obstacles in achieving stable, efficient operation at the cyclotron harmonics in a gyrotron is mode competition with lower harmonic modes. Therefore a theoretical investigation of mode competition is an essential first step to the implementation of gyrotron. In this article, the nonlinear dynamics of mode competition in 0.4 THz gyrotron with gradually tapered complex cavity are studied by using multifrequency, time-dependent theory. The results show that the operating modes TE17.3-TE17.4 can be stably excited and the output power can reach over 130 kW.

Design and simulation of a magnetron injected electronic gun for 0.4 THz gyrotron

A double anode magnetron injected gun (MIG) for a terahertz (THz) band cyclotron oscillation tube has been presented in this paper. Through design, simulation and optimization by the particle-in-cell (PIC) code, a double-anode electron gun with trans-velocity spread of 3.19% is obtained, and the ratio of the transverse velocity to the axial velocity is equal to 1.4. The designed electron gun well satisfies the requirement of the 0.4 THz gyrotron.

Linear analysis of a 170 GHz confocal gyrotron traveling wave tube

Confocal gyrotron traveling wave tube (gyro-TWT) is a novel gyrotron amplifier capable of operating in higher order modes while generating high power at the same time. In this paper, the linear theory is developed to analysis a 170-GHz confocal gyro-TWT based on previous work. The results fit well and seem to be promising.