Yongqian Xiong

Terahertz radiation sources based on free electron lasers and their applications

The tunability, high power and flexible picoseconds-pulse time structure of terahertz (THz) radiation make the THz free-electron laser (FEL) a very attractive THz source of coherent radiation. This paper focuses on the development and perspectives of THz radiation sources based on the FEL. The principles of the low gain THz FEL oscillator, the SASE THz FEL amplifier and the supperradiant THz FEL are reviewed briefly, and the key technologies of THz FEL, such as injector, accelerator, undulator and optical cavity, are discussed, respectively. The current status of and future prospects for THz radiation sources based on the FEL are emphasized in this paper. Recent research and development have shown bright future in free-electron laser (FEL) THz radiation sources. The potential applications can be carried out in the field of imaging, material research, biology medicine, communication, diagnostics and many others.

Analysis and modeling of the active compulsator

This paper presents a mathematical model that considers the variation of speed and excitation current of the active compulsator. The effect of excitation current on pulse current waveform is analyzed by simulating the discharge of the active compulsator based on this model. This paper also shows how the effects of saturation of the main flux path can be incorporated in the generalized state equations of the active compulsator in which the linear magnetic conditions are assumed. To consider saturation of the main flux path, the modification of the main inductances is described and a dynamic saturation model is established. The analysis shows that the peak current is reduced and the pulsewidth is increased due to the effects of saturation. Comparisons with practical results from a small prototype show that this model gives good predictions of performance.

Terahertz magnetoplasmon-polaritons with nonlocal corrections for lossy two dimensional electron gas in GaN-based heterostructures

The high electron concentration in GaN-based heterostructures necessitate the consideration of nonlocal corrections in the magnetoconductivity for the study of magnetoplasmon-polaritons (MPPs) in the lossy two dimensional electron gas system with grating couplers. With the complete set of Maxwell equations, the complex dispersion frequency and absorption spectrum are calculated numerically at different magnetic field (B max  =  10 T) and plasmon wavevectors (k max  =  5  ×  107 m-1). Taking the AlGaN/GaN structure as a representative case, we observe the cyclotron resonance and its high order harmonics, as well as the MPP modes. The effective aspect ratio of the grating is determined according to the absorption spectrum. Effects of the nonlocal corrections on the dispersion frequency and absorption spectrum are analyzed in detail. The contributions of LO phonons, electron collisions, THz wave polarizations and retardations are also discussed.

A Novel Design of Insulated Core Transformer High Voltage Power Supply

Insulated core transformer (ICT) high voltage power supply is an ideal model for industrial radiation accelerator at energy below 1MeV. Compared to the traditional scheme, a novel ICT high voltage power supply was put forward. Conventional silicon steel sheets were replaced with manganese zinc ferrites, raising working frequency from 50Hz to thousand hertz. Magnetic structure was changed from three-phase structure to four-phase structure. Accordingly, excitation voltage was changed from three-phase sinusoidal wave to square wave. Polyimide was chosen as insulation material instead of teflon or mica. A prototype of 400kV/50mA was designed, simulated and verified with the aid of finite element analysis software. To optimize the voltage distribution, corresponding flux compensation methods were raised to solve the problem of flux leakages.

Magnetic field calculation for a 10 MeV positron emission tomography cyclotron

The magnetic field calculation and correction for a 10 MeV positron emission tomography cyclotron is presented. 3D TOSCA analysis results are compared with the measured data, and the calculation error is used to calibrate the B-H curve to obtain a very precise finite element method estimator, which is used to predict the correction of the magnet pole for achieving the isochronous field. The isochronous field error is approximated with the effects of a set of standard patches. On the assumption that the effect of each small patch is proportional to its surface, the correction of the magnet pole is found by solving a system of equations using the least square scheme. The magnet shimming is performed and the measured magnetic field is found in good agreement with the prediction, with an error less than 2 G.