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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2017, 19 (10)   Published: 30 August 2017
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The 3rd Chinese laser and microwave aided plasma diagnostics worshop

Development of signal analysis method for the motional Stark effect diagnostic on EAST

Jia FU (符佳)1, Bo LYU (吕波)1, Haiqing LIU (刘海庆)1, et al.
Plasma Sci. Technol.. 2017, 19 (10): 104001 ;  doi: 10.1088/2058-6272/aa7941
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Show Abstract ( 21 )

A pilot single-channel Motional Stark Effect (MSE) diagnostic has been developed on EAST since 2015. The dual photo-elastic modulators (PEM) were employed to encode the polarization angle into a time-varying signal. The pitch angle was related to the ratio of modulation amplitude at the second harmonic frequency. A digital harmonic analyzer (DHA) technique was developed for extracting the second harmonic amplitude. The results were validated with a hardware phase lock-in amplfier, and is also consistent with the software dual phase-locking algorithm.

Basic Plasma Phenomena

Generation of Alfvén wave energy during magnetic reconnection in Hall MHD Hot!

Lingjie LI (李灵杰), Zhiwei MA (马志为) and Licheng WANG (王理程)
Plasma Sci. Technol.. 2017, 19 (10): 105001 ;  doi: 10.1088/2058-6272/aa7c17
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Show Abstract ( 26 )

The effect of the reconnection rate on the generation of Alfvén wave energy is systematically investigated using Hall magnetohydrodynamics (MHD). It is well known that a decrease in magnetic energy is proportional to the reconnection rate. It is found that an instantaneous increase in Alfvén wave energy in unit Alfvén time is the square dependence on the reconnection rate. The converted Alfvén wave energy is strongly enhanced due to the large increase in the reconnection rate in Hall MHD. For solar-terrestrial plasmas, the maximum converted Alfvén wave energy in unit Alfvén time with the Hall effect can be over 50 times higher than that without the Hall effect during magnetic reconnection.

Magnetically Confined Plasma

Observation and characterization of the effect of electron cyclotron waves on toroidal rotation in EAST L-mode discharges

Jun CHEN (陈俊)1,2, Ruiji HU (胡睿佶)1,2, Bo LYU (吕波)2, et al.
Plasma Sci. Technol.. 2017, 19 (10): 105101 ;  doi: 10.1088/2058-6272/aa7cec
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The change in the toroidal rotation of plasma caused by electron cyclotron wave (ECW) injection has been observed in EAST. It is found that the response of the rotation is similar for all possible ECW toroidal injection angles. The core toroidal rotation velocity increases in the co-current direction along with a rise in the plasma temperature and stored energy. The profile of the electron temperature, ion temperature and toroidal rotation velocity gradually become peaked. The change in toroidal rotation in the core increases with the ECW injection power. Different behavior is observed when the ECWs are injected into low hybrid current drive (LHCD) target plasmas, where the electron temperature and rotation profile become peaked, while the ion temperature profile flattens after ECW injection, suggesting different transport characteristics in energy and momentum.

Low Temperature Plasma

Characteristics of long-gap AC streamer discharges under low pressure conditions

Yaqi YANG (杨亚奇), Weiguo LI (李卫国), Yu XIA (夏喻), et al.
Plasma Sci. Technol.. 2017, 19 (10): 105401 ;  doi: 10.1088/2058-6272/aa79fe
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The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current (AC) voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.

Characteristics of temporal evolution of particle density and electron temperature in helicon discharge

Xiong YANG (杨雄), Mousen CHENG (程谋森), Dawei GUO (郭大伟), et al.
Plasma Sci. Technol.. 2017, 19 (10): 105402 ;  doi: 10.1088/2058-6272/aa808a
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On the basis of considering electrochemical reactions and collision relations in detail, a direct numerical simulation model of a helicon plasma discharge with three-dimensional two-fluid equations was employed to study the characteristics of the temporal evolution of particle density and electron temperature. With the assumption of weak ionization, the Maxwell equations coupled with the plasma parameters were directly solved in the whole computational domain. All of the partial differential equations were solved by the finite element solver in COMSOL MultiphysicsTM with a fully coupled method. In this work, the numerical cases were calculated with an Ar working medium and a Shoji-type antenna. The numerical results indicate that there exist two distinct modes of temporal evolution of the electron and ground atom density, which can be explained by the ion pumping effect. The evolution of the electron temperature is controlled by two schemes: electromagnetic wave heating and particle collision cooling. The high RF power results in a high peak electron temperature while the high gas pressure leads to a low steady temperature. In addition, an OES experiment using nine Ar I lines was conducted using a modified CR model to verify the validity of the results by simulation, showing that the trends of temporal evolution of electron density and temperature are well consistent with the numerically simulated ones.

Directional power absorption in helicon plasma sources excited by a half-helix antenna

Plasma Sci. Technol.. 2017, 19 (10): 105403 ;  doi: 10.1088/2058-6272/aa8030
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This paper deals with the investigation of the power absorption in helicon plasma excited through a half-helix antenna driven at 13.56 MHz. The simulations were carried out by means of a code, HELIC. They were carried out by taking into account different inhomogeneous radial density  profiles and for a wide range of plasma densities, from 1011 cm-3 to 1013 cm-3. The magnetic field was 200, 400, 600 and 1000 G. A three-parameter function was used for generating various density profiles with different volume gradients, edge gradients and density widths. The density profile had a large effect on the efficient Trivelpiece–Gould (TG) and helicon mode excitation and antenna coupling to the plasma. The fraction of power deposition via the TG mode was extremely dependent on the plasma density near the plasma boundary. Interestingly, the obtained efficient parallel helicon wavelength was close to the anticipated value for Gaussian radial density profile. Power deposition was considerably asymmetric when the n/B0ratio was more than a specific value  for a determined density width. The longitudinal power absorption was symmetric at approximately n0 =1011cm-3 , irrespective of the magnetic field supposed. The asymmetry  became more pronounced when the plasma density was 10 12cm-3. The ratio of density width to the magnetic field was an important parameter in the power coupling. At high magnetic fields, the maximum of the power absorption was reached at higher plasma density widths. There was at least one combination of the plasma density, magnetic field and density width for which the RF power deposition at both side of the tube reached its maximum value.

Plasma Technology

Hybrid–PIC simulation of sputtering product distribution in a Hall thruster

Xifeng CAO (曹希峰)1, Guanrong HANG (杭观荣)2, Hui LIU (刘辉)1, et al.
Plasma Sci. Technol.. 2017, 19 (10): 105501 ;  doi: 10.1088/2058-6272/aa7940
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Hall thrusters have been widely used in orbit correction and the station-keeping of geostationary satellites due to their high specific impulse, long life, and high reliability. During the operating life of a Hall thruster, high-energy ions will bombard the discharge channel and cause serious erosion. As time passes, this sputtering process will change the macroscopic surface morphology of the discharge channel, especially near the exit, thus affecting the performance of the thruster. Therefore, it is necessary to carry out research on the motion of the sputtering products and erosion process of the discharge wall. To better understand the moving characteristics of sputtering products, based on the hybrid particle-in-cell (PIC) numerical method, this paper simulates the different erosion states of the thruster discharge channel in different moments and analyzes the moving process of different particles, such as B atoms and B+ ions. In this paper, the main conclusion is that B atoms are mainly produced on both sides of the channel exit, and B+ ions are mainly produced in the middle of the channel exit. The ionization rate of B atoms is approximately 1%.

Experiment and analysis of the neutralization of the electron cyclotron resonance ion thruster

Yizhou JIN (金逸舟)1, Juan YANG (杨涓)1, Jun SUN (孙俊)2,3, et al.
Plasma Sci. Technol.. 2017, 19 (10): 105502 ;  doi: 10.1088/2058-6272/aa76d9
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An electron cyclotron resonance ion thruster must emit an electron current equivalent to its ion beam current to prevent the thruster system from being electrically charged. This operation is defined as neutralization. The factors which influence neutralization are categorized into the ion beam current parameters, the neutralizer input parameters, and the neutralizer position. To understand the mechanism of neutralization, an experiment and a calculation study on how these factors influence thruster neutralization are presented. The experiment results show that the minimum bias voltage of the neutralizer was -60 V at the ion beam current of 80 mA for the argon propellant, and a critical gas flow rate existed, below which the coupling voltage increased sharply. Based on the experiment, the neutralization was analyzed by means of a one-dimensional calculation model. The computation results show that the coupling voltage was influenced by the beam divergence and the negative potential zone near the grids.

Environmental and economic vision of plasma treatment of waste in Makkah

Ahmed RIDA GALALY1,2 and Guido VAN OOST3,4
Plasma Sci. Technol.. 2017, 19 (10): 105503 ;  doi: 10.1088/2058-6272/aa77ef
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An environmental and economic assessment of the development of a plasma-chemical reactor equipped with plasma torches for the environmentally friendly treatment of waste streams by plasma is outlined with a view to the chemical and energetic valorization of the sustainability in the Kingdom of Saudi Arabia (KSA). This is especially applicable in the pilgrimage season in the city of Makkah, which is a major challenge since the amount of waste was estimated at about 750 thousand tons through Arabic Year 1435H (2015), and is growing at a rate of 3%–5% annually. According to statistics, the value of waste in Saudi Arabia ranges between 8 and 9 billion EUR. The Plasma-Treatment Project (PTP) encompasses the direct plasma treatment of all types of waste (from source and landfill), as well as an environmental vision and economic evaluation of the use of the gas produced for fuel and electricity production in KSA, especially in the pilgrimage season in the holy city Makkah. The electrical power required for the plasma-treatment process is estimated at 5000 kW (2000 kW used for the operation of the system and 3000 kW sold), taking into account the fact that: (1) the processing capacity of solid waste is 100 tons per day (2) and the sale of electricity amounts to 23.8 MW at 0.18 EUR per kWh. (3) The profit from the sale of electricity per year is estimated at 3.27 million EUR and the estimated profit of solid-waste treatment amounts to 6 million EUR per year and (4) the gross profit per ton of solid waste totals 8 million EUR per year. The present article introduces the first stage of the PTP, in Makkah in the pilgrimage season, which consists of five stages: (1) study and treatment of waste streams, (2) slaughterhouse waste treatment, (3) treatment of refuse-derived fuel, (4) treatment of car tires and (5) treatment of slag (the fifth stage associated with each stage from the four previous stages).

Mode filtering based on ponderomotive force nonlinearity in a plasma filled rectangular waveguide

Plasma Sci. Technol.. 2017, 19 (10): 105504 ;  doi: 10.1088/2058-6272/aa8089
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Here a new scheme for mode filtering is proposed. Based on the ponderomotive force effect, propagation of the microwave dual-mode through a plasma-filled metallic rectangular waveguide is investigated. To excite the TE20 mode in a rectangular waveguide, the existence of fundamental modes is unavoidable. To filter the destructive mode (TE10), the waveguide is filled with a collisional plasma. Based on the coupling effect, the energy of this destructive TE10 mode is transferred to the TE20 mode. The proposed structure acts like a mode convertor. The TE10 mode become more attenuated and instead the TE20 mode is amplified. The plasma filled rectangular waveguide acts as a mode filtering tool.

Plasma jet array treatment to improve the hydrophobicity of contaminated HTV silicone rubber

Ruobing ZHANG (张若兵)1, Qianting HAN (韩倩婷), Yan XIA (夏衍), et al.
Plasma Sci. Technol.. 2017, 19 (10): 105505 ;  doi: 10.1088/2058-6272/aa7c16
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An atmospheric-pressure plasma jet array specially designed for HTV silicone rubber treatment is reported in this paper. Stable plasma containing highly energetic active particles was uniformly generated in the plasma jet array. The discharge pattern was affected by the applied voltage. The divergence phenomenon was observed at low gas flow rate and abated when the flow rate increased. Temperature of the plasma plume is close to room temperature which makes it feasible for temperature-sensitive material treatment. Hydrophobicity of contaminated HTV silicone rubber was significantly improved after quick exposure of the plasma jet array, and the effective treatment area reached 120 mm×50 mm (length×width). Reactive particles in the plasma accelerate accumulation of the hydrophobic molecules, namely low molecular weight silicone chains, on the contaminated surface, which result in a hydrophobicity improvement of the HTV silicone rubber.

Fusion Engineering

Repetitive cleaning of a stainless steel first mirror using radio frequency plasma

Jiao PENG (彭姣)1,2, Rong YAN (鄢容)1, Rui DING (丁锐)1, et al.
Plasma Sci. Technol.. 2017, 19 (10): 105601 ;  doi: 10.1088/2058-6272/aa7629
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First mirrors (FMs) are crucial components of optical diagnostic systems in present-day tokamaks and future fusion reactors. Their lifetimes should be extremely limited due to their proximity to burning plasma, greatly influencing the safe operation of corresponding diagnostics. Repetitive cleaning is expected to provide a solution to the frequent replacement of contaminated FMs, thus prolonging their lifetimes. Three repetitive cleaning cycles using radio frequency plasma were applied to stainless steel (SS) FM samples, to evaluate the change of the mirrors’ optical properties and morphology during each cycle. Amorphous carbon films were deposited on mirror surfaces under identical conditions in three cycles. In three cycles with identical cleaning parameters, the total reflectivity was restored at up to 95%. Nevertheless, with successive cleaning cycles, the FM surfaces gradually appeared to roughen due to damage to the grain boundaries. Correspondingly, the diffuse reflectivity increased from a few percent to 20% and 27% after the second and third cycles. After optimizing the cleaning parameters of the second and third cycles, the roughness showed a significant decrease, and simultaneously the increase of diffuse reflectivity was remarkably improved.

Calorimetric power measurements in the EAST ECRH system

Weiye XU (徐伟业), Handong XU (徐旵东), Fukun LIU (刘甫坤), et al.
Plasma Sci. Technol.. 2017, 19 (10): 105602 ;  doi: 10.1088/2058-6272/aa7ec9
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In this paper, the measurement method of calorimetric power for an electron cyclotron resonance heating (ECRH) system for EAST is presented. This method requires measurements of the water flow through the cooling circuits and the input and output water temperatures in each cooling circuit. Usually, the inlet water temperature stability is controlled to obtain more accurate results. The influence of the inlet water temperature change on the measurement results is analyzed for the first time in this paper. Also, a novel temperature calibration method is proposed. This kind of calibration method is accurate and effective, and can be easily implemented.

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