Overall design and cold test of a G-band gyrotron traveling-wave tube (gyro-TWT) amplifier are presented. This gyro-TWT aims at achieving a goal of 10-kW pulse output power in the range of 210–220 GHz. It is operated in a circular TE01 mode at the fundamental cyclotron harmonics, which is driven by a 50-kV, 3-A gyrating electron beam. Low velocity spread diode-type magnetron injection gun (MIG), multichannel TE01 mode input coupler, broadband metasurface output window, and lossy material loaded beam–wave interaction circuit are simulated and partially measured. Particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a saturated output power over 10 kW in the range of 210–228 GHz with a beam velocity spread of 2.29%.

Design and Cold Test of a G-Band 10-kW-Level Pulse TE01-Mode Gyrotron Traveling-Wave Tube

Recent hot test details and inspiring results of the W-band lossy-dielectric-loaded gyrotron traveling-wave tube (gyro-TWT) developed at the University Electronic Science and Technology of China (UESTC) are reported in this letter. Measurement of the gyro-TWT had achieved a pulse output power over 100 kW with a broad bandwidth of 8.5 GHz (92.0~100.5 GHz). For the first time as we know, the W-band gyro-TWT can stably operate with a 10% duty cycle and the average output power is over 10.3 kW in the frequency range. Excellent stability, pure spectrum and mode patterns are all verified in the hot test experiment. 

High Average Power Test of a W-Band Broadband Gyrotron Traveling Wave Tube

The dielectric load gyro-traveling wave tube (gyro-TWT) is a primary high-power millimeter source, and its power capacity is limited by the thermal capacity of the attenuated dielectric. The heavy outgassing was observed when the gyro-TWT operated in a long pulse duration in the experiment. The heat dissipated on the dielectric, and its distribution is analyzed. Then the thermal performance for different pulse duration is investigated. The dielectric temperature reaches 390 °C and 214 °C for the pulse duration of 2 ms and <inline-formula> <tex-math notation="LaTeX">$200 ~\mu \text{s}$ </tex-math></inline-formula> with a duty cycle of 10%, respectively. It indicates the thermal resistance of the dielectric limits the power capacity. The average power capacity in different pulse duration is evaluated. Then a high average power experiment is realized. A <inline-formula> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula>-band gyro-TWT is operated at 60.5 kV–9.4 A, with a pulse duration of <inline-formula> <tex-math notation="LaTeX">$200 ~\mu \text{s}$ </tex-math></inline-formula> and a duty cycle of 16%. A peak power of 126.1 kW and an average power of 20.2 kW are obtained at 94 GHz.

High Average Power Investigation of Dielectric Dissipation in the W-Band Gyro-TWT

The electron beam transportation profile is important for the gyro-traveling-wave tube (Gyro-TWT), and a transportation prototype of Gyro-TWT has been developed. The symmetric-rotating calibration (SRC) method for the superconducting magnet (SCM) alignment is proposed, and the tilt angle and transverse displacement of the magnetic field are reduced from 0.05° to 0.01° and 0.15 to 0.05 mm after alignment, respectively. Then, the beam transportation experiment is done, with a transportation rate of ~85%. The beam trajectory is analyzed and indicates that the stray electron is the main reason for transportation degeneration. Then, a new magnetron injection gun (MIG) with an optimized rear electrode, isolated thermal gap, and antiemission coating is designed and fabricated to enhance the transmission rate. The stability of the new gun is also analyzed, and the stray electron is reduced. Finally, the transportation rate improves to 97.1%, and the transported beam is with a pitch factor of 1.25 and a velocity ratio of 2.5%. The beam quality is good for the interaction circuit.

Electron Transportation Experiment and Analysis for a G-Band Magnetron Injection Gun

In this article, the design and experiment of a high continuous-wave (CW) power Ku-band gyrotron traveling-wave tube (gyro-TWT) are described. The hot experiment results indicate that the TE11 mode gyro-TWT, driven by a 50 kV–4.5 A electron beam, can produce a maximum CW output power of 49 kW, with an efficiency of 21.8% at 16 GHz. The saturated bandwidth is 2 GHz, and more than 30-kW CW output power is achieved from 15 to 17 GHz. However, the outgassing is observed in the low-frequency range, which limits the enhancement of CW output power. Then, the simulation is performed and indicates that the attenuated power distribution and heat dissipation on the dielectric-loaded circuit (DLC) are dependent on the frequency, attenuation of lossy ceramic, and input power. Also, the maximum temperature of lossy ceramic at 15 and 15.5 GHz is too high for stable operation. A nonuniform DLC is utilized to dissipate power dispersedly. The simulation results show that the maximum power loss on a single lossy ceramic is reduced by 77.9% and 48.9% at 15 and 15.5 GHz, respectively. It indicates that the nonuniform DLC could adjust the dissipation power on the lossy ceramics via the attenuation profile varying with frequency and effectively enhance the power capacity of dielectric-loaded gyro-TWT.

Experiment and Power Capacity Investigation for a Ku-Band Continuous-Wave Gyro-TWT

A diode magnetron injection gun (MIG) has been designed and analyzed for G-band gyro-traveling wave tube (Gyro-TWT). A novel curved surface emitter is proposed to enhance the beam quality. An electron beam with velocity ratio of 1.20 and transverse velocity spread of 2.11% is obtained. The beam transportation stability with geometry misalignment is also analyzed, and the assembly accuracy should be controlled under 0.1 mm. The stray electron is intercepted by the capture structure to avoid the bombardment to the interaction circuit. Then the cathode is fabricated, and the temperature distribution is measured. The experimental result matches well with the simulation. The electron beam quality considering expansion, surface roughness, and thermal emission is estimated, and the total transverse velocity spread is about 2.63%.

Design and Analysis of a Diode Magnetron Injection Gun for a G-Band Gyro-TWT

Effects of the output window reflection instability on the performance of our designed ${W}$ -band broadband gyrotron traveling wave tube (gyro-TWT) were studied by the simulation and hot test experiments. Strong oscillations and frequency self-modulation (FSM) in the after-cavity interaction (ACI) region were observed in the zero-driven state with a two-layer window and are verified with the beam-wave particle-in-cell simulation. Oscillations and FSM of 80–81.8 GHz and 91.03–94.68 GHz are, respectively, generated from the TE22 and TE02 modes in the ACI region due to the high window reflection level. By using the “sandwich” and meta-surface windows with low reflection and broad bandwidth, the parasitic ACI oscillations can be effectively suppressed. The maximum saturated output power is improved to 156.8 kW at 94 GHz and is zero-driven stable in our recent hot test experiments.

Influence of the Output Window Reflection on the Performance of W-Band Gyrotron Traveling Wave Tubes

Axis coincidence consideration is necessary in the design of gyrotron solenoids; besides, radial field component strongly affects the performance of gyrodevices owing to the transverse particle–wave interaction mechanism in it, especially for high-frequency operation. In this article, the theory of multiple eccentric coils is derived. It is capable of calculating the off-axis field inside the solenoid with specified displacement and tilt. Numerical calculations are validated by 3-D electromagnetic simulation methods. Based on the measured magnet field, the displacement and tilt of a three-coil magnet system are successfully predicted with a global optimization algorithm, and then, the magnet system is well calibrated using a pair of flanges.

Wei Jiang

Papers

Design and Cold Test of a G-Band 10-kW-Level Pulse TE01-Mode Gyrotron Traveling-Wave Tube

Design and Analysis of a Diode Magnetron Injection Gun for a G-Band Gyro-TWT

High Average Power Investigation of Dielectric Dissipation in the W-Band Gyro-TWT

Influence of the Output Window Reflection on the Performance of W-Band Gyrotron Traveling Wave Tubes

Theoretical and Experimental Study of Gyro-TWT Solenoids With Displacement and Tilt