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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2015, 17 (6)   Published: 18 May 2015
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Basic Plasma Phenomena

A New Insight into Energy Distribution of Electrons in Fuel-Rod Gap in VVER-1000 Nuclear Reactor

Plasma Sci. Technol. 2015, 17 (6): 441-447 ;  doi: 10.1088/1009-0630/17/6/01
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In order to calculate the electron energy distribution in the fuel rod gap of a VVER- 1000 nuclear reactor, the Fokker-Planck equation (FPE) governing the non-equilibrium behavior of electrons passing through the fuel-rod gap as an absorber has been solved in this paper. Besides, the Monte Carlo Geant4 code was employed to simulate the electron migration in the fuel-rod gap and the energy distribution of electrons was found. As for the results, the accuracy of the FPE was compared to the Geant4 code outcomes and a satisfactory agreement was found. Also, different percentage of the volatile and noble gas fission fragments produced in fission reactions in fuel rod, i.e. Krypton, Xenon, Iodine, Bromine, Rubidium and Cesium were employed so as to investigate their effects on the electrons’ energy distribution. The present results show that most of the electrons in the fuel rod’s gap were within the thermal energy limitation and the tail of the electron energy distribution was far from a Maxwellian distribution. The interesting outcome was that the electron energy distribution is slightly increased due to the accumulation of fission fragments in the gap. It should be noted that solving the FPE for the energy straggling electrons that are penetrating into the fuel-rod gap in the VVER-1000 nuclear reactor has been carried out for the first time using an analytical approach.

Magnetically Confined Plasma

Long Pulse H-Mode Scenarios Sustained by RF Heating on EAST

GAO Xiang (高翔)1, LI Guoqiang (李国强)1, ZHANG Tao (张涛)1, et al.
Plasma Sci. Technol. 2015, 17 (6): 448-453 ;  doi: 10.1088/1009-0630/17/6/02
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EAST has demonstrated its capability of long pulse operation using RF heating (LHCD and ICRF) in past experiments. One key issue to realize the long pulse H-mode experi- ments is to sustain the plasma current for steady state operation. Based on the calculations of the transport code ONETWO and its coupled RF code GENRAY, two scenarios have been proposed to achieve the 10 s H-mode plasma with Ip=400 kA, < ne >=4.5x1019 m-3, ßN=2, and the 100 s H-mode plasma with Ip=280 kA, < ne >=3.5£1019 m-3, ßN =1.8 recently. The current drive of lower hybrid wave is an important issue in the two scenarios. An experimental result on lower hybrid current drive in H-mode plasmas on EAST is also presented.

Optimization Study of ICRF Hydrogen Minority Heating in a Deuterium Plasma of EAST

ZHANG Xinjun (张新军), YAO Fangwei (姚方伟)
Plasma Sci. Technol. 2015, 17 (6): 454-457 ;  doi: 10.1088/1009-0630/17/6/03
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The full wave TORIC code and the Kinetic Fokker-Planck SSFPQL code are com- bined to perform self-consistent simulations of the ICRF heating in the EAST 2D magnetic config- uration. The combined package is applied to the ICRF hydrogen minority heating in a deuterium plasma with the hydrogen concentration up to 10%. The fast wave propagation and absorption properties, power partitions among the plasma species and the RF driven energetic tails have been analyzed. Meanwhile, in order to optimize the ICRF heating, changing the resonance locations has also been considered in EAST plasmas.

Inertially Confined Plasma

Effect of Foil Target Thickness in Proton Acceleration Driven by an Ultra-Short Laser

LU Jianxin (路建新), LAN Xiaofei (兰小飞), XI Xiaofeng (席晓峰), et al.
Plasma Sci. Technol. 2015, 17 (6): 458-460 ;  doi: 10.1088/1009-0630/17/6/04
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Proton acceleration experiments were carried out by a 1.2×10 18 W/cm 2 ultra-short laser interaction with solid foil targets. The peak proton energy observed from an optimum target thickness of 7 µm in our experiments was 2.1 MeV. Peak proton energy and proton yield were investigated for different foil target thicknesses. It was shown that proton energy and conversion efficiency increased as the target became thinner, on one condition that the preplasma generated by the laser prepulse did not have enough shock energy and time to influence or destroy the target rear-surface. The existence of optimum foil thickness is due to the effect of the prepulse and hot electron transportation behavior on the foil target.

Low Temperature Plasma

Simulation Study on the Self-Sustained Oscillations in DC Driven Glow Discharges at Atmospheric Pressure Under Different Gas Gaps

WANG Xiaofei (王晓菲), HE Yafeng (贺亚峰), and LIU Fucheng (刘富成)
Plasma Sci. Technol. 2015, 17 (6): 461-468 ;  doi: 10.1088/1009-0630/17/6/05
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In this paper, a one-dimensional plasma fluid model is employed to study the self- sustained oscillations in DC-driven helium glow discharges at atmospheric pressure under different gas gaps. Our simulation results indicate that a harmonic current oscillation with tiny amplitude always occur at the onset of instability and transits into a relaxation one as the conductivity of the semiconductor is decreased. It is found that the dynamics of the oscillations are dependent on the gas gaps. The discharge can only exhibit a simple oscillation with unique amplitude and frequency at smaller gas gaps (<2 mm) while it can exhibit a more complex oscillation with several different amplitudes and frequencies at larger gas gaps (>2 mm). The discharge modes in these current oscillations have also been analyzed.

Numerical Modelling Point-to-Plane of Negative Corona Discharge in N2 Under Non-Uniform Electric Field Hot!

Plasma Sci. Technol. 2015, 17 (6): 469-474 ;  doi: 10.1088/1009-0630/17/6/06
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The paper presents a simulation model of the negative corona discharge in N 2 under various pressures. The simulated discharge is of a negative point-to-plane mass type, with an inter-electrode separation distance of 20 mm and a symmetry about the axis of discharge. This simulation investigates the behavior of the neutral density and temperature for different pressures in the range of 0.1-10.0 bar. The spatial and temporal evolution of the neutral gas is analyzed based upon the equations of continuity, momentum and energy in a two-dimensional cylindrical geometry model. For that geometry of the system, the FCT (Flux Corrected Transport) technique was adopted. The results show that the pressure plays a significant role of the neutrals dynamics.

Critical Length Criterion and the Arc Chain Model for Calculating the Arcing Time of the Secondary Arc Related to AC Transmission Lines

CONG Haoxi (丛浩熹) 1,LI Qingmin (李庆民) 2,XING Jinyuan (行晋源) 2,et al.
Plasma Sci. Technol. 2015, 17 (6): 475-480 ;  doi: 10.1088/1009-0630/17/6/07
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The prompt extinction of the secondary arc is critical to the single-phase reclosing of AC transmission lines, including half-wavelength power transmission lines. In this paper, a low- voltage physical experimental platform was established and the motion process of the secondary arc was recorded by a high-speed camera. It was found that the arcing time of the secondary arc rendered a close relationship with its arc length. Through the input and output power energy analysis of the secondary arc, a new critical length criterion for the arcing time was proposed. The arc chain model was then adopted to calculate the arcing time with both the traditional and the proposed critical length criteria, and the simulation results were compared with the experimental data. The study showed that the arcing time calculated from the new critical length criterion gave more accurate results, which can provide a reliable criterion in term of arcing time for modeling and simulation of the secondary arc related with power transmission lines.

Effect of the Mesh Transparency on the Electrical Characteristics of DC Pseudo Discharge

Plasma Sci. Technol. 2015, 17 (6): 481-487 ;  doi: 10.1088/1009-0630/17/6/08
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A DC pseudo discharge for air has been studied. Air pressure is used in the range between 0.7 Torr and 12 Torr. The breakdown occurs between a plane cathode and a mesh anode at transparencies of 19%, 46%, and 65%. The current-voltage characteristic curves of the discharge, which are measured at different pressures, distances, and mesh transparences, take effect in the region of abnormal glow. The discharge voltage decreases as the air pressure increases, while more voltage is needed to maintain the discharge when either the mesh transparency or the inter- electrode distance is increased. An increment of mesh transparency causes high negative potential behind the mesh due to the high concentration of electrons, which accumulate and collide with neutral atoms. Paschen curves deviate from the expected regular one. The left side of Paschen curves appears at inter-electrode distance of 1 mm, whereas the right side appears at inter-electrode distance of 5 mm. The intermediate region is observed only at 3 mm distance between the two electrodes. For the transparency range used in this work, it is found that the decrement of the breakdown voltage, on the right side, depends on the mesh transparency. For different electrode separations, the measured Paschen curves are coincident and deviate from the standard ones of Paschen’s law.

Plasma Technology

Optical Characterization of Amorphous Hydrogenated Carbon (a-C:H) Thin Films Prepared by Single RF Plasma Method

Plasma Sci. Technol. 2015, 17 (6): 488-495 ;  doi: 10.1088/1009-0630/17/6/09
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Methane (CH 4 ) plasma was used to produce amorphous hydrogenated carbon (a- C:H) films by a single capacitively coupled radio frequency (RF) powered plasma system. The system consists of two parallel electrodes: the upper electrode is connected to 13.56 MHz RF power and the lower one is connected to the ground. Thin films were deposited on glass slides with different sizes and on silicon wafers. The influence of the plasma species on film characteristics was studied by changing the plasma parameters. The changes of plasma species during the deposition were investigated by optical emission spectroscopy (OES). The structural and optical properties were analyzed via Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and UV-visible spectroscopy, and the thicknesses of the samples were measured by a profilometer. The sp 3 /sp 2 ratio and the existing H atoms play a significant role in the determination of the chemical properties of thin films in the plasma. The film quality and deposition rate were both increased by raising the power and the flow rate.

Effects of Surface Pretreatment on Nucleation and Growth of Ultra-Nanocrystalline Diamond Films

LIU Cong (刘聪), WANG Jianhua (汪建华), LIU Sijia(刘斯佳), et al.
Plasma Sci. Technol. 2015, 17 (6): 496-501 ;  doi: 10.1088/1009-0630/17/6/10
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The effects of different surface pretreatment methods on the nucleation and growth of ultra-nanocrystalline diamond (UNCD) films grown from focused microwave Ar/CH4/H2 (argon-rich) plasma were systematically studied. The surface roughness, nucleation density, microstructure, and crystallinity of the obtained UNCD films were characterized by atomic force microscope (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The results indicate that the nucleation enhancement was found to be sensitive to the different surface pretreatment methods, and a higher initial nucleation density leads to highly smooth UNCD films. When the silicon substrate was pretreated by a two-step method, i.e., plasma treatment followed by ultrasonic vibration with diamond nanopowder, the grain size of the UNCD films was greatly decreased: about 7.5 nm can be achieved. In addition, the grain size of UNCD films depends on the substrate pretreatment methods and roughness, which indicates that the surface of substrate profile has a "genetic characteristic".

Experimental Investigation on Aerodynamic Control of a Wing with Distributed Plasma Actuators

HAN Menghu (韩孟虎)1, LI Jun (李军)1, LIANG Hua (梁华), et al.
Plasma Sci. Technol. 2015, 17 (6): 502-509 ;  doi: 10.1088/1009-0630/17/6/11
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Experimental investigation of active flow control on the aerodynamic performance of a flying wing is conducted. Subsonic wind tunnel tests are performed using a model of a 35o swept flying wing with an nanosecond dielectric barrier discharge (NS-DBD) plasma actuator, which is installed symmetrically on the wing leading edge. The lift and drag coefficient, lift-todrag ratio and pitching moment coefficient are tested by a six-component force balance for a range of angles of attack. The results indicate that a 44.5% increase in the lift coefficient, a 34.2% decrease in the drag coefficient and a 22.4% increase in the maximum lift-to-drag ratio can be achieved as compared with the baseline case. The effects of several actuation parameters are also investigated, and the results show that control efficiency demonstrates a strong dependence on actuation location and frequency. Furthermore, we highlight the use of distributed plasma actuators at the leading edge to enhance the aerodynamic performance, giving insight into the different mechanism of separation control and vortex control, which shows tremendous potential in practical flow control for a broad range of angles of attack.

Fusion Engineering

Analysis of an Extreme Scenario in the Vacuum Vessel in KTX

LUO Zhiren (罗志仁), YANG Qingxi (杨庆喜), SONG Yuntao (宋云涛), et al.
Plasma Sci. Technol. 2015, 17 (6): 510-516 ;  doi: 10. 1088/1009- 0630/17/6/12
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The Vacuum Vessel (VV) system is a vital component of Keda Torus for eXperiment (KTX). Various accidental scenarios might occur on the VV. In this report, an extreme scenario is assumed and studied: plasma accidental termination during the flat-top stage. Numerical simulations based on finite element are performed as the major tool for analyses. The detailed distributions of eddy and the reaction forces on VV are extracted, and the total eddy current and the maximum reaction force due to electromagnetic load are figured out. In addition, according to the results, the VV can be approximately regarded as a centrally symmetric structure, even though its ports distribution is asymmetric.

Study on the Dynamic Performance of the Helium Turboexpander for EAST Subsystems

CHEN Shuangtao (陈双涛) 1,YANG Shanju (杨山举) 1,FU Bao (付豹) 2,et al.
Plasma Sci. Technol. 2015, 17 (6): 517-523 ;  doi: 10.1088/1009-0630/17/6/13
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An increase of the cooling capacities in the liquid helium temperature area is re- quired by Experimental Advanced Superconducting Tokamak (EAST) due to the extension of its subsystems in the near future. Limited by the heat exchangers, cryogenic pipes, and cryogenic valves, it is difficult to enlarge the present EAST helium system. 10 2 W@4.5 K level helium cryogenic systems are needed in view of feasibility and economy. A turboexpander is the key com- ponent of a helium cryogenic system. In this article, a hydrostatic gas lubricated cryogenic helium turboexpander for a 900 W@4.5 K cryogenic helium system was developed for the EAST updated subsystem by the Institute of Plasma Physics, Chinese Academy of Sciences and the Institute of Cryogenic and Refrigeration of Xi’an Jiaotong University. The main components, such as gas bearings, expansion wheel, shaft, and brake wheel, were briefly presented. The dynamic perfor- mance of the journal and thrust gas bearings was investigated numerically. The rotordynamic performance of the developed turboexpander was studied experimentally. The results show that the axial and radial load capacities supplied by the journal gas bearing and thrust gas bearing are enough to balance the axial force and radial force of the rotor. A 43% overspeed operation was achieved, which validated the reasonable design of the turboexpander.

Molecular Statics Simulation of Hydrogen Defect Interaction in Tungsten

LI Xiaochun (李小椿) 1,LIU Yinan (刘亦男) 2,YU Yi (余一) 2,et al.
Plasma Sci. Technol. 2015, 17 (6): 524-528 ;  doi: 10.1088/1009-0630/17/6/14
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Hydrogen (H) defect interactions have been investigated by molecular statics sim- ulations in tungsten (W), including H-H interactions and interactions between H and W self- interstitial atoms. The interactions between H and small H-vacancy clusters are also demonstrated; the binding energies of an H, a vacancy and a self-interstitial W to an H-vacancy cluster depend on the H-to-vacancy ratio. We conclude that H bubble formation needs a high concentration of H in W for the H bubble nucleation and growth, which are also governed by the H-to-vacancy ratio of the cluster. The vacancy first combines with H atoms and a cluster forms, then the H-vacancy cluster goes through the whole process of vacancy capture, H capture, and vacancy capture again, and as a result the H-vacancy cluster grows larger and larger. Finally, the H bubble forms.

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