Gaofeng Liu

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 and Experiment of a W-Band Tunable Gyrotron Oscillator

A gyrotron capable of both frequency and power tuning is a promising coherent millimeter-THz wave source. A self-consistent nonlinear theory is applied to investigate the electron cyclotron interaction between electron beam and wave modes of axial nonfixed profiles in an extended W-band TE01 mode cylindrical cavity. It is revealed that tuning the magnetic field strength can excite electron cyclotron resonances on forward wave, backward wave, and even simultaneous on both waves, which makes the system operate under distinctive states, namely the gyrotron backward wave oscillation state and the gyromonotron state. In this paper, a W-band prototype gyrotron oscillator based on an extended cylindrical waveguide cavity is built, and the experiment test indicates that the system starts oscillation in a relative wide range of the operation parameters. The measured frequency spectrum reveals the system iteratively switches between the lower order instability axial modes, and it operates under nonstationary oscillation states. The experimental measurement of highest output power ~8 kW is consistent with the theoretical predictions. An optimized gyrotron circuit with efficiency exceeding 20% and tunable bandwidth over 10 GHz is also presented. The free oscillation behaviors revealed in this paper provide interesting guidance for developing tunable gyrotrons in millimeter-THz wave range.

Design of a High-Harmonic Gyrotron With a Permanent Magnet System

This paper presents the conceptual design of a W-band high-harmonic gyrotron oscillator with a permanent magnet system. Mode selection, cavity design, beam-wave interaction calculations, electron-optical system, and quasi-optical mode converter output system are described. The gyrotron operates at the third harmonic of the electron cyclotron frequency with the beam interacting with a TE02 circular electric mode. The permanent magnet system provided the maximum axial magnetic field of ~ 1.19 T in the cavity region of the gyrotron. In the simulations, an output power of 14 kW has been obtained at a beam voltage of 40 kV with beam current of 3 A, corresponding to an efficiency of 11.7% through optimizing system parameters.

Simulation of transverse field sweeping system and thermal analysis of an undepreesed collector for a gyrotron

Abstract In this paper, the simulation of transverse sweeping system (TFSS) and thermal analysis of undepressed collector for a 94 GHz, 30 kW CW gyrotron are presented. A smooth power-density distribution profile on the collector is obtained through repeated comparison of the TFSS simulation results, which indicates the range of spent beam spread and the peak power density are about 370 mm and 130 W/cm2, respectively. The sensitivities of the TFSS’s parameters to the power distribution profile are also studied and analyzed. To avoid the melting of the collector, a cooling channel groove is designed and the thermal analysis with finite element software ANSYS is performed under the non-uniform heat flux. The maximum outer and inner surface temperatures are 94.3 and 102.4 °C with water temperature at 20 °C, respectively.

Design and simulation of the interaction circuit for a W-band quasi-optical gyrotron oscillator

In this paper, the design and simulation of a high-frequency structure for a W-band quasi-optical gyrotron oscillator, which operates at the TE62 mode, has been presented. According to the self-consistent nonlinear theory, the parameters of the high-frequency resonator has been designed and optimized. The effects of electron beam voltage, current, velocity spread, magnetic field and electron beam guiding center on the output power and efficiency have been analyzed. The maximum output power and efficiency of the designed gyrotron oscillator is about 39.7kW and 44.1%, respectively.

Input coupler design for Ka band gyrotron TWT

In this paper, an input coupler for Ka band gyrotron TWT (Traveling Wave Tube) has been designed. This input coupler can convert a TE10 rectangular waveguide mode into a TE01 cylindrical waveguide mode. The measured results agree well with the simulated results. With the measured -3dB bandwidth of more than 5GHz, the input coupler can be well used in the experiment of Ka band gyrotron traveling wave amplifier.

PIC simulation of a W-band gyroklystron amplifier

A W-band four-cavity gyroklystron amplifier has been studied using the particle-in-cell (PIC) code. The amplifier operates in the fundamental harmonic TE01 circular electric mode. The simulation predicts 143kw output power at W-band with 33.8% efficiency, 37dB gain and 1GHz bandwidth.