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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2018, 20 (2)   Published: 11 January 2018
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THE 18th NATIONAL CONFERENCE ON PLASMA SCIENCE AND TECHNOLOGY

Investigation of flame structure in plasma-assisted turbulent premixed methane-air flame

Hualei ZHANG (张华磊), Liming HE (何立明), Jinlu YU (于锦禄), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024001 ;  doi: 10.1088/2058-6272/aa9850
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The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes (pretreatment of reactants and direct in situ discharge). OH-planar laser-induced fluorescence (PLIF) technique is used to diagnose the turbulent structure methane-air flame, and the experimental apparatus consists of dump burner, plasma-generating system, gas supply system and OH-PLIF system. Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes: regime I for voltage lower than 6.6 kV; regime II for voltage between 6.6 and 11.1 kV; and regime III for voltage between 11.1 and 12.5 kV. In regime I, aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role, while in regime III, the temperature rising effect will probably superimpose on the chemical effect and amplify it. For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field, the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape. With regard to in situ plasma discharge in flames, the discharge pattern changes from streamer type to glow type. Compared with the case of reactants pretreatment, the flame propagates further in the upstream direction. In the discharge region, the OH intensity is highest for in situ plasma assisted combustion, indicating that the plasma energy is coupled into flame reaction zone.

Automatic emissive probe apparatus for accurate plasma and vacuum space potential measurements

Jianquan LI (李建泉)1,2, Wenqi LU (陆文琪)1,2,Jun XU (徐军)1,2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024002 ;  doi: 10.1088/2058-6272/aa97cd
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We have developed an automatic emissive probe apparatus based on the improved inflection point method of the emissive probe for accurate measurements of both plasma potential and vacuum space potential. The apparatus consists of a computer controlled data acquisition card, a working circuit composed by a biasing unit and a heating unit, as well as an emissive probe. With the set parameters of the probe scanning bias, the probe heating current and the fitting range, the apparatus can automatically execute the improved inflection point method and give the measured result. The validity of the automatic emissive probe apparatus is demonstrated in a test measurement of vacuum potential distribution between two parallel plates, showing an excellent accuracy of 0.1 V. Plasma potential was also measured, exhibiting high efficiency and convenient use of the apparatus for space potential measurements.

Study on the plasma generation characteristics of an induction-triggered coaxial pulsed plasma thruster

Weisheng CUI (崔伟胜)1, Wenzheng LIU (刘文正)1, Jia TIAN (田甲)1, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024003 ;  doi: 10.1088/2058-6272/aa8a5e
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At present, spark plugs are used to trigger discharge in pulsed plasma thrusters (PPT), which are known to be life-limiting components due to plasma corrosion and carbon deposition. A strong electric field could be formed in a cathode triple junction (CTJ) to achieve a trigger function under vacuum conditions. We propose an induction-triggered electrode structure on the basis of the CTJ trigger principle. The induction-triggered electrode structure could increase the electric field strength of the CTJ without changing the voltage between electrodes, contributing to a reduction in the electrode breakdown voltage. Additionally, it can maintain the plasma generation effect when the breakdown voltage is reduced in the discharge experiments. The induction-triggered electrode structure could ensure an effective trigger when the ablation distance of Teflon increases, and the magnetic field produced by the discharge current could further improve the plasma density and propagation velocity. The induction-triggered coaxial PPT we propose has a simplified trigger structure, and it is an effective attempt to optimize the micro-satellite thruster.

Design of the high voltage isolation transmission module with low delay for ECRH system on J-TEXT

Haiyan MA (马海燕), Donghui XIA (夏冬辉), Zhijiang WANG (王之江), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024004 ;  doi: 10.1088/2058-6272/aa99d3
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As a flexible auxiliary heating method, the electron cyclotron resonance heating (ECRH) has been widely used in many tokamaks and also will be applied for the J-TEXT tokamak. To meet requirements of protection and fault analysis for the ECRH system on J-TEXT, signals of gyrotrons such as the cathode voltage and current, the anode voltage and current, etc should be transmitted to the control and data acquisition system. Considering the high voltage environment of gyrotrons, isolation transmission module based on FPGA and optical fiber communication has been designed and tested. The test results indicate that the designed module has strong anti-noise ability, low error rate and high transmission speed. The delay of the module is no more than 5 μs which can fulfill the requirements.

Diagnostic of capacitively coupled radio frequency plasma from electrical discharge characteristics: comparison with optical emission spectroscopy and fluid model simulation Hot!

Xiang HE (何湘)1, Chong LIU (刘冲)1, Yachun ZHANG (张亚春)2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024005 ;  doi: 10.1088/2058-6272/aa9a31
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The capacitively coupled radio frequency (CCRF) plasma has been widely used in various fields. In some cases, it requires us to estimate the range of key plasma parameters simpler and quicker in order to understand the behavior in plasma. In this paper, a glass vacuum chamber and a pair of plate electrodes were designed and fabricated, using 13.56 MHz radio frequency (RF) discharge technology to ionize the working gas of Ar. This discharge was mathematically described with equivalent circuit model. The discharge voltage and current of the plasma were measured at different pressures and different powers. Based on the capacitively coupled homogeneous discharge model, the equivalent circuit and the analytical formula were established. The plasma density and temperature were calculated by using the equivalent impedance principle and energy balance equation. The experimental results show that when RF discharge power is 50–300 W and pressure is 25–250 Pa, the average electron temperature is about 1.7–2.1 eV and the average electron density is about 0.5×1017 – 3.6×1017 m-3. Agreement was found when the results were compared to those given by optical emission spectroscopy and COMSOL simulation.

Soft switching circuit to improve efficiency of all solid-state Marx modulator for DBDs

Liqing TONG (童立青)1,2, Kefu LIU (刘克富)1,2 and Yonggang WANG (王永刚)1,
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024006 ;  doi: 10.1088/2058-6272/aa9b28
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For an all solid-state Marx modulator applied in dielectric barrier discharges (DBDs), hard switching results in a very low efficiency. In this paper, a series resonant soft switching circuit, which series an inductance with DBD capacitor, is proposed to reduce the power loss. The power loss of the all circuit status with hard switching was analyzed, and the maximum power loss occurred during discharging at the rising and falling edges. The power loss of the series resonant soft switching circuit was also presented. A comparative analysis of the two circuits determined that the soft switching circuit greatly reduced power loss. The experimental results also demonstrated that the soft switching circuit improved the power transmission efficiency of an all solid-state Marx modulator for DBDs by up to 3 times.

The observation of small ELM post-cursor mode in EAST

Nan ZHAO (赵楠)1,2,Ning YAN (颜宁)2, Guosheng XU (徐国盛)2,et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024007 ;  doi: 10.1088/2058-6272/aa9bd8
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A ‘post-cursor’ quasi-coherent mode with frequency ~50 kHz has been observed following the crash of small edge localized modes (ELMs) in the Experimental Advanced Superconducting Tokamak by using a reciprocating Langmuir probe system inserted at the outboard midplane. This mode with strong potential and magnetic perturbations propagates in the electron-diamagnetic drift direction in the laboratory frame. In addition, these quasi-coherent fluctuations appear to be modulated by a MHD mode with lower frequency (~5kHz).The bi-coherence analysis shows that the post-cursor mode and the MHD mode have nonlinear interaction through three-wave coupling. The understanding on post-cursor mode can enhance our knowledge of ELMs and pedestal physics, and give new insight into the ELM process itself.

Study on the influences of ionization region material arrangement on Hall thruster channel discharge characteristics

Xiang HU (胡翔)1, Ping DUAN (段萍)1, Jilei SONG (宋继磊)1, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024008 ;  doi: 10.1088/2058-6272/aa9b87
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There exists strong interaction between the plasma and channel wall in the Hall thruster, which greatly affects the discharge performance of the thruster. In this paper, a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel. The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel. The influences of segmented electrode placed at the ionization region on electric potential, ion number density, electron temperature, ionization rate, discharge current and specific impulse are discussed. The results show that, when segmented electrode is placed at the ionization region, the axial length of the acceleration region is shortened, the equipotential lines tend to be vertical with wall at the acceleration region, thus radial velocity of ions is reduced along with the wall corrosion. The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region. Furthermore, the electron-wall collision frequency and ionization rate also increase, the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.

Regulation characteristics of oxide generation and formaldehyde removal by using volume DBD reactor

Bingyan CHEN (陈秉岩)1,2,3, Xiangxiang GAO (高香香)1,3,4, Ke CHEN (陈可)1,3, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024009 ;  doi: 10.1088/2058-6272/aa9b7a
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Discharge plasmas in air can be accompanied by ultraviolet (UV) radiation and electron impact, which can produce large numbers of reactive species such as hydroxyl radical (OH·), oxygen radical (O·),ozone (O3), and nitrogen oxides (NOx), etc. The composition and dosage of reactive species usually play an important role in the case of volatile organic compounds (VOCs) treatment with the discharge plasmas. In this paper, we propose a volume discharge setup used to purify formaldehyde in air, which is configured by a plate-to-plate dielectric barrier discharge (DBD) channel and excited by an AC high voltage source. The results show that the relative spectral-intensity from DBD cell without formaldehyde is stronger than the case with formaldehyde. The energy efficiency ratios (EERs) of both oxides yield and formaldehyde removal can be regulated by the gas flow velocity in DBD channel, and the most desirable processing effect is the gas flow velocity within the range from 2.50 to 3.33 m s-1. Moreover, the EERs of both the generated dosages of oxides (O3 and NO 2) and the amount of removed formaldehyde can also be regulated by both of the applied voltage and power density loaded on the DBD cell. Additionally, the EERs of both oxides generation and formaldehyde removal present as a function of normal distribution with increasing the applied power density, and the peak of the function is appeared in the range from 273.5 to 400.0 W l -1. This work clearly demonstrates the regulation characteristic of both the formaldehyde removal and oxides yield by using volume DBD, and it is helpful in the applications of VOCs removal by using discharge plasma.

Steady and oscillatory plasma properties in the near-field plume of a hollow cathode

Zun ZHANG (张尊)1, Kan XIE (谢侃)2, Jiting OUYANG (欧阳吉庭)1, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-024010 ;  doi: 10.1088/2058-6272/aa9d7d
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Hollow cathodes serve as electron sources in Hall thrusters, ion thrusters and other electric propulsion systems. One of the vital problems in their application is the cathode erosion. However, the basic erosion mechanism and the source of high-energy ions cause of erosion are not fully understood. In this paper, both potential measurements and simulation analyses were performed to explain the formation of high-energy ions. A high-speed camera, a single Langmuir probe and a floating emissive probe were used to determine the steady and oscillatory plasma properties in the near-field plume of a hollow cathode. The temporal structure, electron temperature, electron density, and both static and oscillation of plasma potentials of the plume have been obtained by the diagnostics mentioned above. The experimental results show that there exists a potential hill (about 30 V) and also severe potential oscillations in the near-plume region. Moreover, a simple 2D particle-in-cell model was used to analyze the energy transition between the potential hill and/or its oscillations and the ions. The simulation results show that the energy of ions gained from the static potential background is about 20 eV, but it could reach to 60 eV when the plasma oscillates.

Basic Plasma Phenomena

Surface plasmon oscillations in a semi-bounded semiconductor plasma

M SHAHMANSOURI1 and A P MISRA2
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025001 ;  doi: 10.1088/2058-6272/aa9213
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We study the dispersion properties of surface plasmon (SP) oscillations in a semi-bounded semiconductor plasma with the effects of the Coulomb exchange (CE) force associated with the spin polarization of electrons and holes as well as the effects of the Fermi degenerate pressure and the quantum Bohm potential. Starting from a quantum hydrodynamic model coupled to the Poisson equation, we derive the general dispersion relation for surface plasma waves. Previous results in this context are recovered. The dispersion properties of the surface waves are analyzed in some particular cases of interest and the relative influence of the quantum forces on these waves are also studied for a nano-sized GaAs semiconductor plasma. It is found that the CE effects significantly modify the behaviors of the SP waves. The present results are applicable to understand the propagation characteristics of surface waves in solid density plasmas.

Magnetically Confined Plasma

Soliton formation in electron-temperature-gradient-driven magnetoplasma

M Yaqub KHAN and Javed IQBAL
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025101 ;  doi: 10.1088/2058-6272/aa8f3b
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Electron-temperature-gradient (ETG)-driven solitons are studied in a plasma. We derive the linear dispersion relation and an admitted solitary wave solution Korteweg–de Vries-type equation (KdV) for the ETG mode in the nonlinear regime by using the Braginskii model and a transformation. It is found that the ETG mode supports only rarefactive solitons. It is also observed that the ratio of electron-to-ion temperature   τ= Te/Ti   the ratio of gradient scale lengths ηe = L n/LT   and the magnetic field B affect both the amplitude and width of a soliton. It  is found that the soliton profile changes with variation in these parameters. We apply the homotopy perturbation method to the derived KdV equation. It is found this method is computationally attractive and the results are very impressive. This work may be useful to study the low electrostatic modes in inhomogeneous electron–ion plasma with density and ETG gradients. For illustration, the model has been applied to tokamak plasma.

Tomography of a simply magnetized toroidal plasma

Ruggero BARNI1, Stefano CALDIROLA1, Luca FATTORINI1,2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025102 ;  doi: 10.1088/2058-6272/aa9028
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Optical emission spectroscopy is a passive diagnostic technique, which does not perturb the plasma state. In particular, in a hydrogen plasma, Balmer-alpha (Hα) emission can be easily measured in the visible range along a line of sight from outside the plasma vessel. Other emission lines in the visible spectral range from hydrogen atoms and molecules can be exploited too, in order to gather complementary pieces of information on the plasma state. Tomography allows us to capture bi-dimensional structures. We propose to adopt an emission spectroscopy tomography for studying the transverse profiles of magnetized plasmas when Abel inversion is not exploitable. An experimental campaign was carried out at the Thorello device, a simple magnetized torus. The characteristics of the profile extraction method, which we implemented for this purpose are discussed, together with a few results concerning the plasma profiles in a simply magnetized torus configuration.

Two-dimensional vacuum ultraviolet images in different MHD events on the EAST tokamak

Zhijun WANG (王智君)1,2, Xiang GAO (高翔)1,2, Tingfeng MING (明廷凤)1, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025103 ;  doi: 10.1088/2058-6272/aa9477
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A high-speed vacuum ultraviolet (VUV) imaging telescope system has been developed to measure the edge plasma emission (including the pedestal region) in the Experimental Advanced Superconducting Tokamak (EAST). The key optics of the high-speed VUV imaging system consists of three parts: an inverse Schwarzschild-type telescope, a micro-channel plate (MCP) and a visible imaging high-speed camera. The VUV imaging system has been operated routinely in the 2016 EAST experiment campaign. The dynamics of the two-dimensional (2D) images of magnetohydrodynamic (MHD) instabilities, such as edge localized modes (ELMs), tearing-like modes and disruptions, have been observed using this system. The related VUV images are presented in this paper, and it indicates the VUV imaging system is a potential tool which can be applied successfully in various plasma conditions.

Characterization of high flux magnetized helium plasma in SCU-PSI linear device

Xiaochun MA (马小春)1, Xiaogang CAO (曹小岗)1, Lei HAN (韩磊)2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025104 ;  doi: 10.1088/2058-6272/aa936e
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A high-flux linear plasma device in Sichuan University plasma-surface interaction (SCU-PSI) based on a cascaded arc source has been established to simulate the interactions between helium and hydrogen plasma with the plasma-facing components in fusion reactors. In this paper, the helium plasma has been characterized by a double-pin Langmuir probe. The results show that the stable helium plasma beam with a diameter of 26mm was constrained very well at a magnetic field strength of 0.3 T. The core density and ion flux of helium plasma have a strong dependence on the applied current, magnetic field strength and gas flow rate. It could reach an electron density of 1.2×10 19 m -3 and helium ion flux of 3.2×10 22 m -2 s -1 , with a gas flow rate of 4 standard liter per minute, magnetic field strength of 0.2 T and input power of 11kW. With the addition of -80 V applied to the target to increase the helium ion energy and the exposure time of 2h, the flat top temperature reached about 530°C. The different sizes of nanostructured fuzz on irradiated tungsten and molybdenum samples surfaces under the bombardment of helium ions were observed by scanning electron microscopy. These results measured in the SCU-PSI linear device provide a reference for International Thermonuclear Experimental Reactor related PSI research.

Low Temperature Plasma

Simulation of propagation of the HPM in the low-pressure argon plasma

Zhigang LI (李志刚), Zhongcai YUAN (袁忠才), Jiachun WANG (汪家春), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025401 ;  doi: 10.1088/2058-6272/aa93f8
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The propagation of the high-power microwave (HPM) with a frequency of 6 GHz in the low-pressure argon plasma was studied by the method of fluid approximation. The two-dimensional transmission model was built based on the wave equation, the electron drift-diffusion equations and the heavy species transport equations, which were solved by means of COMSOL Multiphysics software. The simulation results showed that the propagation characteristic of the HPM was closely related to the average electron density of the plasma. The attenuation of the transmitted wave increased nonlinearly with the electron density. Specifically, the growth of the attenuation slowed down as the electron density increased uniformly. In addition, the concrete transmission process of the HPM wave in the low-pressure argon plasma was given.

Control of vacuum arc source cathode spots contraction motion by changing electromagnetic field

Xin SONG (宋鑫), Qing WANG (王庆)1, Zeng LIN (蔺增), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025402 ;  doi: 10.1088/2058-6272/aa8a30
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This paper investigates the magnetic field component impact on cathode spots motion trajectory and the mechanism of periodic contraction. Electromagnetic coils and permanent magnets were installed at the different sides of cathode surface, the photographs of cathode spots motion trajectory were captured by a camera. Increasing the number of magnets and decreasing the distance between magnets and cathode both lead to enhancing cathode spots motion velocity. Radii of cathode spots trajectory decrease gradually with the increasing of electromagnetic coil’s current, from 40 mm at 0 A to 10 mm at 2.7 A. Parallel magnetic field component intensity influence the speed of cathode spots rotate motion, and perpendicular magnetic field component drives spots drift in the radial direction. Cathode spot’s radial drift is controlled by changing the location of the ‘zero line’ where perpendicular magnetic component shifts direction and the radius of cathode spots trajectory almost equal to ‘zero line’.

Study of electron-extraction characteristics of an inductively coupled radio-frequency plasma neutralizer

Jianwu HE (贺建武)1,2, Longfei MA (马隆飞)1,2, Senwen XUE (薛森文)1,2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025403 ;  doi: 10.1088/2058-6272/aa89e1
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Inductively coupled radio-frequency (RF) plasma neutralizer (RPN) is an insert-free device that can be employed as an electron source in electric propulsion applications. Electron-extraction characteristics of the RPN are related to the bulk plasma parameters and the device’s geometry. Therefore, the effects of different electron-extraction apertures and operational parameters upon the electron-extraction characteristics are investigated according to the global nonambipolar flow andsheathmodel. Moreover,these models canalsobeusedtoexplain why the electron-extraction characteristics of the RPN strongly depend upon the formation of the anode spot. During the experimental study, two types of anode spots are observed. Each of them has unique characteristics of electron extraction. Moreover, the hysteresis of an anode spot is observed by changing the xenon volume-flow rates or the bias voltages. In addition, the rapid ignited method, gas-utilization factor, electron-extraction cost and other factors that need to be considered in the design of the RPN are also discussed.

Feedback model of secondary electron emission in DC gas discharge plasmas

Saravanan ARUMUGAM1, Prince ALEX and Suraj Kumar SINHA
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025404 ;  doi: 10.1088/2058-6272/aa8e3f
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Feedback is said to exist in any amplifier when the fraction of output power in fed back as an input. Similarly, in gaseous discharge ions that incident on the cathode act as a natural feedback element to stabilize and self sustain the discharge. The present investigation is intended to emphasize the feedback nature of ions that emits secondary electrons (SEs) from the cathode surface in DC gas discharges. The average number of SEs emitted per incident ion and non ionic species (energetic neutrals, metastables and photons) which results from ion is defined as effective secondary electron emission coefficient (ESEEC,gE). In this study, we derive an analytic expression that corroborates the relation between gE and power influx by ion to the cathode based on the feedback theory of an amplifier. In addition, experimentally, we confirmed the typical positive feedback nature of SEE from the cathode in argon DC glow discharges. The experiment is done for three different cathode material of same dimension (tungsten (W),copper (Cu) and brass) under identical discharge conditions (pressure: 0.45 mbar, cathode bias: -600 V, discharge gab: 15 cm and operating gas: argon). Further, we found that the ?E value of these cathode material controls the amount of feedback power given by ions. The difference in feedback leads different final output i.e the power carried by ion at cathode (P'i∣C). The experimentally obtained value of P'i∣C is 4.28 W, 6.87 W and 9.26 W respectively for W, Cu and brass. In addition, the present investigation reveals that the amount of feedback power in a DC gas discharges not only affect the fraction of power fed back to the cathode but also the entire characteristics of the discharge.

Plasma Technology

Effects of treatment time and temperature on the DC corona pretreatment performance of waste activated sludge

Yu GAO (高宇), Ning ZHAO (赵宁), Yongdi DENG (邓永迪), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025501 ;  doi: 10.1088/2058-6272/aa8bfd
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In order to improve the anaerobic digestion efficiency of waste activated sludge (WAS),a pretreatment procedure should be carried out so as to disrupt the microbial cell structure, thus releasing intracellular organic matters. In this paper, a corona discharge triggered by a DC voltage was employed to pre-treat WAS for various time periods under different temperatures. The magnitude of the DC voltage was 4 kV at both negative and positive polarities. The changes in the soluble chemical oxygen demand, phosphorus and nitrogen content, and pH value within the WAS were utilized to estimate the pretreatment performance of the DC corona. It was found that with increasing treatment time, the pretreatment efficiency tends to be reduced. With increased temperature, the pretreatment efficiency appears to be better. It is suggested that the oxidative species and the active particles generated in the corona discharge play an important role in disrupting the microbial cell structure, which is dependent upon the treatment time and the temperature.

Visualization of particulates distribution from electrode erosion

Wei ZHONG (钟伟),AoXU (徐翱), Yunlong LIU (刘云龙), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025502 ;  doi: 10.1088/2058-6272/aa9327
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Particulates generated from electrode erosion in gas spark gap is inevitable and may initiate self-breakdown behavior with high risk. Traditionally, this problem is addressed by empirical method qualitatively. To push this old problem forward, this paper conducts laser confocal microscopy measurement of eroded surface and a statistical method is introduced to obtain visualization of particulates distribution from electrode erosion after different shots. This method allows dense particulates to be classified with their heights in z direction and scattered figures of particulates within certain height range are obtained. Results indicate that the higher-than-10 μm particulates start to emerge after 200 discharge shots and particulates number has a waved radial distribution with a 0.5 mm wide deposition zone. Based on these quantitative results, the risk of reignition and field-distortion failure that are triggered by particulates can be assessed.

Plasma characteristics in the discharge region of a 20A emission current hollow cathode

Mingming SUN (孙明明)1, Tianping ZHANG (张天平)1, Xiaodong WEN (温晓东)1, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025503 ;  doi: 10.1088/2058-6272/aa8edb
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Numerical calculation and fluid simulation methods were used to obtain the plasma characteristics in the discharge region of the LIPS-300 ion thruster’s 20 A emission current hollow cathode and to verify the structural design of the emitter. The results of the two methods indicated that the highest plasma density and electron temperature, which improved significantly in the orifice region, were located in the discharge region of the hollow cathode. The magnitude of plasma density was about 1021 m-3 in the emitter and orifice regions, as obtained by numerical calculations, but decreased exponentially in the plume region with the distance from the orifice exit. Meanwhile, compared to the emitter region, the electron temperature and current improved by about 36% in the orifice region. The hollow cathode performance test results were in good agreement with the numerical calculation results, which proved that that the structural design of the emitter and the orifice met the requirements of a 20 A emission current. The numerical calculation method can be used to estimate plasma characteristics in the preliminary design stage of hollow cathodes.

Deposition of SiCxHyOz thin film on epoxy resin by nanosecond pulsed APPJ for improving the surface insulating performance

Qing XIE (谢庆)1, Haofan LIN (林浩凡)1,2, Shuai ZHANG (张帅)2,3, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025504 ;  doi: 10.1088/2058-6272/aa97d0
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Non-thermal plasma surface modification for epoxy resin (EP) to improve the insulation properties has wide application prospects in gas insulated switchgear and gas insulated transmission line. In this paper, a pulsed Ar dual dielectrics atmospheric-pressure plasma jet (APPJ) was used for SiCxHyOz thin film deposition on EP samples. The film deposition was optimized by varying the treatment time while other parameters were kept at constants (treatment distance: 10 mm, precursor flow rate: 0.6 l min-1, maximum instantaneous power: 3.08 kW and single pulse energy: 0.18 mJ). It was found that the maximum value of flashover voltages for negative and positive voltage were improved by 18% and 13% when the deposition time was 3 min, respectively. The flashover voltage reduced as treatment time increased. Moreover, all the surface conductivity, surface charge dissipation rate and surface trap level distribution reached an optimal value when thin film deposition time was 3 min. Other measurements, such as atomic force microscopy and scanning electron microscope for EP surface morphology, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy for EP surface compositions, optical emission spectra for APPJ deposition process were carried out to better understand the deposition processes and mechanisms. The results indicated that the original organic groups (C–H, C–C, C=O, C=C) were gradually replaced by the Si containing inorganic groups (Si–O–Si and Si–OH). The reduction of C=O in ester group and C=C in p-substituted benzene of the EP samples might be responsible for shallowing the trap level and then enhancing the flashover voltage. However, when the plasma treatment time was longer than 3 min, the significant increase of the surface roughness might increase the trap level depth and then deteriorate the flashover performance.

Mechanism of high growth rate for diamond-like carbon films synthesized by helicon wave plasma chemical vapor deposition

Peiyu JI (季佩宇)1,2, Jun YU (於俊)1,2, Tianyuan HUANG (黄天源)1,2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025505 ;  doi: 10.1088/2058-6272/aa94bd
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A high growth rate fabrication of diamond-like carbon (DLC) films at room temperature was achieved by helicon wave plasma chemical vapor deposition (HWP-CVD) using Ar/CH4 gas mixtures. The microstructure and morphology of the films were characterized by Raman spectroscopy and scanning electron microscopy. The diagnosis of plasma excited by a helicon wave was measured by optical emission spectroscopy and a Langmuir probe. The mechanism of high growth rate fabrication for DLC films by HWP-CVD has been discussed. The growth rate of the DLC films reaches a maximum value of 54 μmh-1 at the CH4 flow rate of 85 sccm, which is attributed to the higher plasma density during the helicon wave plasma discharge. The CH and Hα radicals play an important role in the growth of DLC films. The results show that the Hα radicals are beneficial to the formation and stabilization of C=C bond from sp2 to sp3.

Fusion Engineering

Acceleration optimization of real-time equilibrium reconstruction for HL-2A tokamak discharge control

Rui MA (马瑞), Fan XIA (夏凡), Fei LING (凌飞), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025601 ;  doi: 10.1088/2058-6272/aa9432
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Real-time equilibrium reconstruction is crucially important for plasma shape control in the process of tokamak plasma discharge. However, as the reconstruction algorithm is computationally intensive, it is very difficult to improve its accuracy and reduce the computation time, and some optimizations need to be done. This article describes the three most important aspects of this optimization: (1) compiler optimization; (2) some optimization for middle-scale matrix multiplication on the graphic processing unit and an algorithm which can solve the block tri-diagonal linear system efficiently in parallel; (3) a new algorithm to locate the X&O point on the central processing unit. A static test proves the correctness and a dynamic test proves the feasibility of using the new code for real-time reconstruction with 129×129 grids; it can complete one iteration around 575 μs for each equilibrium reconstruction. The plasma displacements from real-time equilibrium reconstruction are compared with the experimental measurements, and the calculated results are consistent with the measured ones, which can be used as a reference for the real-time control of HL-2A discharge.

Recent developments in the structural design and optimization of ITER neutral beam manifold

Chengzhi CAO (曹诚志), Yudong PAN (潘宇东), Zhiwei XIA (夏志伟), et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025602 ;  doi: 10.1088/2058-6272/aa9562
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This paper describes a new design of the neutral beam manifold based on a more optimized support system. A proposed alternative scheme has presented to replace the former complex manifold supports and internal pipe supports in the final design phase. Both the structural reliability and feasibility were confirmed with detailed analyses. Comparative analyses between two typical types of manifold support scheme were performed. All relevant results of mechanical analyses for typical operation scenarios and fault conditions are presented. Future optimization activities are described, which will give useful information for a refined setting of components in the next phase.

Thermal analysis and optimization of the EAST ICRH antenna

Qingxi YANG (杨庆喜)1, Wei SONG (宋伟)1,2, Qunshan DU (杜群山)1,2, et al.
Plasma Sci. Technol.. 2018, 20 (2): 2018-2-025603 ;  doi: 10.1088/2058-6272/aa9609
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The ion cyclotron resonance of frequency heating (ICRH) plays an important role in plasma heating. Two ICRH antennas were designed and applied on the EAST tokamak. In order to meet the requirement imposed by high-power and long-pulse operation of EAST in the future, an active cooling system is mandatory to be designed to remove the heat load deposited on the components. Thermal analyses for high heat-load components have been carried out, which presented clear temperature distribution on each component and provided the reference data to do the optimization. Meanwhile, heat pipes were designed to satisfy the high requirement imposed by a Faraday shield and lateral limiter.

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