Liansheng Huang

Orbital symmetry induced conductance switching in a graphene nanoribbon heterojunction with different edge hydrogenations

First principles calculations are performed to investigate the electron transport through a zigzag-edged graphene nanoribbon (ZGNR) heterojunction constructed by connecting a monohydrogenated ZGNR and a dihydrogenated ZGNR and its response to external magnetic fields. It is found that the heterojunction can be switched between a conducting state and an insulating state by tuning the magnetic fields. It arises from the matching or mismatching between the π or π* states of the two ribbons under different magnetic fields. This mechanism of conductance switching by tuning the orbital symmetry can be considered in the future design of graphene based electronic devices.

On the Sequential Control of ITER Poloidal Field Converters for Reactive Power Reduction

Sequential control applied to the International Thermonuclear Experimental Re- actor (ITER) poloidal fleld converter system for the purpose of reactive power reduction is the subject of this investigation. Due to the inherent characteristics of thyristor-based phase-controlled converter, the poloidal fleld converter system consumes a huge amount of reactive power from the grid, which subsequently results in a voltage drop at the 66 kV busbar if no measure is taken. The installation of a static var compensator rated for 750 MVar at the 66 kV busbar is an essential way to compensate reactive power to the grid, which is the most efiective measure to solve the problem. However, sequential control of the multi-series converters provides an additional method to improve the natural power factor and thus alleviate the pressure of reactive power demand of the converter system without any additional cost. In the present paper, by comparing with the symmetrical control technique, the advantage of sequential control in reactive power consumption is highlighted. Simulation results based on SIMULINK are found in agreement with the theoretical analysis.

The Design on the Real-Time Wavelet Filter for ITER PF AC/DC Converter Control System

Since noise degrades the control precision of International Thermonuclear Experimental Reactor (ITER) poloidal field (PF) ac/dc converter control system, a real-time wavelet filter algorithm for feedback signal denoising is studied. According to the requirements of ITER PF ac/dc converter controller, a real-time digital filter based on a wavelet transform is designed in this paper. The Daubechies wavelet is used as the wavelet basis and the mandatory denoising method is taken to remove the noise. In order to improve the boundary effects of wavelet transform when the width of sliding window and decomposition scale is quite small, the left and right symmetric boundary extension method is proposed. Three different boundary extension methods are compared, and the effectiveness of the proposed boundary extension method for denoising are described. The experiments are carried out on ITER PF ac/dc converter sequential control test platform to prove that the designed real-time wavelet filter eliminates the noise effectively and guarantees the real-time processing of feedback signal.

Wide bandgap switching devices for fusion reactor power supply systems

High voltage, high current power supplies are required to generate plasma in a tokamak. This equipment converts high voltage AC from the grid into both AC and DC energy sources. Recently, new switching devices based on wide bandgap semiconductors have emerged that could significantly improve the efficiency of these systems. This paper explores the benefits of using silicon carbide (SiC) MOSFET power modules in the power supplies that operate the Experimental Advanced Superconducting Tokamak (EAST). The case study for this work is the vertical stability (VS) coil and the neutral beam injection (NBI) power supplies. The analysis predicts that the efficiency of the VS and the NBI power supplies can be improved by up to 36 % and 66 %, respectively, by using these new components.