Ran Yan

Design and Experiment of a Q-band Gyro-TWT Loaded With Lossy Dielectric

Distributed lossy dielectric is of considerable interest for high-power milli- and submillimeter radiation sources, particularly for gyrotron traveling wave tubes (gyro-TWTs). The analytical design and experiment of a Q-band gyro-TWT loaded with periodic lossy dielectric rings operating in the circular TE01 mode at the fundamental cyclotron harmonic are presented. The gyro-TWT is driven by a 70-kV 10-A helical electron beam with a velocity ratio of 1.0. Hot test performs maximum peak output power of 152 kW, 41-dB saturated gain, and 21.7% efficiency at 47.6 GHz. The measured bandwidth over 100 kW is at the range of 47-49 GHz. PIC simulation and large-signal prediction show excellent agreements to the hot test results. The potential backward wave oscillation interactions involving the spurious modes TE11, TE21, and TE02 are effectively suppressed by loading lossy dielectric rings periodically. The Q-band gyro-TWT is zero-drive stable at the operating points, which demonstrates that loading lossy dielectric is excellent to stabilize the spurious oscillations. Moreover, the designs of magnetron injection gun, interaction circuits, and input and output structures are also achieved to satisfy the requirements.

Analytical theory of low-frequency space charge oscillations in gyrotrons

Low-frequency oscillations attributed to reflected electrons bouncing adiabatically between the electron gun and the interaction space have been observed in many gyrotrons. An analytical model is considered which allows one to apply space-charge wave theory to the analysis of these oscillations. In the framework of the small-signal theory, the regions of low-frequency oscillations, the oscillation frequency and the temporal and spatial growth rates of low-frequency oscillations are determined in the relevant parameter space. The mode frequency is determined not only by the particle travel time, but by the travel time of charge waves on the reflected electron beam. This explains the existence of modes with noncommensurate frequencies.

Design and measurement of a TE13 input converter for high order mode gyrotron travelling wave amplifiers

A technique to launch a circular TE13 mode to interact with the helical electron beam of a gyrotron travelling wave amplifier is proposed and verified by simulation and cold test in this paper. The high order (HOM) TE13 mode is excited by a broadband Y-type power divider with the aid of a cylindrical waveguide system. Using grooves and convex strips loaded at the lateral planes of the output cylindrical waveguide, the electric fields of the potential competing TE32 and TE71 modes are suppressed to allow the transmission of the dominant TE13 mode. The converter performance for different structural dimensions of grooves and convex strips is studied in detail and excellent results have been achieved. Simulation predicts that the average transmission is ∼−1.8 dB with a 3 dB bandwidth of 7.2 GHz (91.5–98.7 GHz) and port reflection is less than −15 dB. The conversion efficiency to the TE32 and TE71 modes are, respectively, under −15 dB and −24 dB in the operating frequency band. Such an HOM converter operating ...

Nonlinear Analysis of Low-Frequency Oscillations in Gyrotrons

Low-frequency oscillations (LFOs) had been observed in many gyrotron experiments. These oscillations are believed to be caused by electrons trapped in the region between the electron gun and the cavity. A 1-D numerical simulation method is developed to analyze the self-consistent accumulation and bunching caused by the space-charge field of these trapped electrons. Velocity spread and loss mechanisms for the trapped electrons are taken into account. The threshold conditions for excitation, the spatial character, and the nonlinear evolution of the LFOs are analyzed and interpreted. The numerical simulation results agree with some experimental data.

Genetic algorithm optimization of collector for W band Gyro-TWT

The collector is an important component in a Gyro-Travelling Wave Tube. The performance influences the capability of tube. The curved geometry is employed to average the dissipated power distribution. Compared to the previous cylindrical geometry, the maximum dissipated power is reduced from 0.83 kW/cm2 to 0.396 kW/cm2. The temperature on inner wall is reduced from 227 °C to 116 °C. The capability of collector is improved. Also the stability analysis is done, and it shows the curved geometry can operate stably.

Analytical theory of low frequency oscillations in gyrotrons

Low frequency oscillations attributed to reflected electrons bouncing adiabatically between the electron gun and the interaction space have been observed in many gyrotrons. An analytical model is considered which allows one to apply space-charge wave theory to the analysis of these oscillations. In the framework of the small signal theory, the regions of low frequency oscillations, the oscillation frequency and the temporal and spatial growth rates of low frequency oscillations are determined in the parameter relevant space. These results are consistent with some experimental data. They also allow recommending some means for suppressing low frequency oscillations.

Automatic Hot Test of Gyrotron-Traveling Wave Tubes by Adaptive PID Feedback Control

It is of great importance in many applications to provide a stable output power of a gyrotron-traveling wave tube (gyro-TWT) during a long operation time (over several hours). In order to achieve this goal, an automatic hot test platform of gyro-TWTs was set up and the power stabilization was obtained by an adaptive scheme of proportional integral derivative (PID) feedback by the beam current controlled by the filament heating power and cathode-anode beam voltage. With determined operating parameters, the PID controller can adaptively adjust the beam current and voltage, and then maintain the output power on a target. This PID controlling process can effectively eliminate the influence of the power supply voltage surge and cathode emission instability. Automatic hot test of the gyro-TWTs shows that the PID feedback controlling accuracy of the output power is better than 2%.

Wideband Circular TE 21 and TE 01 Mode Converters With Same Exciting Topologies

To accurately measure the performance of dielectric loaded circuits for gyrotron traveling wave tubes, a novel topological structure to generate the TE₂₁^O or TE₀₁^O (superscript represents circular waveguide) mode is proposed and verified in this paper. The topology is constituted by an overmoded TE₁₀^□ to TE₂₀^□ (superscript represents rectangular waveguide) mode converter with multiple matching steps and a taper from TE20^□ to TE₂₁^O or TE₀₁^O mode. This topology allows the TE₂₁^O or TE₀₁^O mode to be excited by changing the taper dimensions. Simulation results reveal that the TE₂₁^O and TE₀₁^O mode converters, respectively, have a 1-dB bandwidth of 12.8 and 9.3 GHz with an excellent performance. Converters for the back-to-back millimeter-wave measurements were fabricated with computerized numerical control and wire cutting techniques. Vector network analyzer measurements are in good agreement with the simulations.