Weiye Xu

Absorption of surface acoustic waves by graphene

We present a theoretical study on interactions of electrons in graphene with surface acoustic waves (SAWs). We find that owing to momentum and energy conservation laws, the electronic transition accompanied by the SAW absorption cannot be achieved via inter-band transition channels in graphene. For graphene, strong absorption of SAWs can be observed in a wide frequency range up to terahertz at room temperature. The intensity of SAW absorption by graphene depends strongly on temperature and can be adjusted by changing the carrier density. This study is relevant to the exploration of the acoustic properties of graphene and to the application of graphene as frequency-tunable SAW devices.

Status of ECRH project on EAST Tokamak

A 140GHz electron cyclotron resonance heating and current drive (EC H&CD) project for EAST Tokamak is launched in 2011 with a total power of 4MW and pulse length of 100 s. The main objectives of the system are to provide central H&CD, assist start-up and control of MHD activities. The system comprises four gyrotrons each with nominal output power of 1MW at 140GHz. The RF power, transmitted through four evacuated corrugated waveguides will be injected into plasma from the low field side (radial port). The front steering equatorial launcher directs the RF beam over ±25° toroidally and scans over 38° poloidally. At present, the construction of the first 1MW system is undergoing for the expected campaign in the end of 2013. In this paper, the current status of the development and the design of the 140-GHz ECRH system are presented.

Overcurrent protection for gyrotrons on the experimental advanced superconducting tokamak

In this paper, an overcurrent protection system is designed to ensure the safety of the gyrotrons in the EAST ECRH system. Two overcurrent protection systems were established, a fast one and a slow one. The fast one uses the current transformers as the current sensors. The models of the current transformers and the superconducting magnet were built to analyze the effect of the environmental magnetic field on the current transformers using FI method. The analysis results show that the magnetic induction at the position near the current transformers must less than 0.002 T, i.e., the current transformers should be placed at a distance greater than 2.2 meters from the magnet center to ensure its normal work. The slow one uses the shunt to monitor the currents. An anti-fuse FPGA and a timer is used to realize the signal processing in the fast protection circuit and the slow protection circuit respectively. The response time of the fast protection circuit is less than 100 ns, and the response time of the slow protection circuit is less than 31 μs.

The Design of Directional Coupler for ECRH System

A study of coupling characteristic between circular corrugated waveguide and rectangular smooth waveguide has been presented in this paper, which aims to develop a directional coupler integrated into a miter bend for an electron cyclotron resonance heating system on EAST. The performances of the several aperture array distributions are analyzed based on the aperture diffraction theory in detail. The broadband, good in-band coupling flatness, and excellent directivity are achieved by using the proposed novel aperture array on the mirror surface. For verification purpose, a prototype of the directional coupler has been designed and fabricated.

Millimeter Wave Power Monitoring in EAST ECRH System

This paper presents a stable millimeter-wave power monitoring system for ECRH on EAST tokamak. The real-time power monitoring was realized based on power monitor miter bends. The “Exchange”method was proposed to monitor the reflection power for RF protection. A cRIO-based signal process unit was designed, it is robust and has the response time of 1 μs. The power monitoring system was tested and the experimental results were discussed.

A novel ultra-wideband waveguide-to-microstrip transition for millimeter-wave applications

In this paper, a novel waveguide-to-microstrip transition is developed based on probe coupling. It is designed for millimeter wave applications. The proposed structure is simple and easy to fabricate with good performance. Simulation results show that the return loss is less than -18.9 dB in the entire D-band.

Research on long pulse ECRH system of EAST in support of ITER

Experimental Advanced Superconducting Tokamak (EAST), as a fully superconducting tokamak in China, aims to achieve high performance plasma under steady-state operation. To fulfill the physical objectives of EAST, a program of 4-MW long pulse electron cyclotron resonance heating and current drive (EC H&CD) system, which would offer greater flexibility for plasma shape and plasma stabilization has been launched on EAST since 2011. The system, composed of 4 gyrotrons with nominal 1MW output power and 1000s pulse length each, is designed with the feature of steerable power handling capabilities at 140 GHz, using second harmonic of the extraordinary mode(X2). The missions of the ECRH system are to provide plasma heating, current drive, plasma profile tailoring and control of magneto-hydrodynamic (MHD) instabilities. Presently, the first two 140-GHz 1-MW gyrotrons, provided by GYCOM and CPI, respectively, have been tested at long pulse operation. The tubes, the associated power supplies, cooling system, cryogen...