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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2018, 20 (8)   Published: 09 July 2018
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The 4th Workshop on Plasma Diagnostics

Design and development of a synchronized operation control system for Thomson scattering diagnostic on J-TEXT

Jingjun ZHOU (周靖钧), Li GAO (高丽), Yinan ZHOU (周乙楠), et al.
Plasma Sci. Technol. 2018, 20 (8): 084001 ;  doi: 10.1088/2058-6272/aabd73
Full Text: PDF (1467KB) ( 265 )
Show Abstract ( 77 )

A Thomson scattering diagnostic system is under construction at the Joint Texas Experimental Tokamak (J-TEXT). A 1064 nm Nd:YAG laser with 50 Hz repetition rate is used as the laser source. We have used a software for careful and precise control of the laser through serial communication. A time sequence operating system has been developed to synchronize the laser control and data acquisition system with the central control system (CSS). The system operates commands from the CSS of J-TEXT and generates triggers for the laser and data acquisition system in the proper sequence. It also measures an asynchronous time value that is needed for accurate time stamping. All functions are served by a field-programmable gate array development platform that is suitable for high-speed data and signal processing applications. Several embedded peripherals, including Ethernet and USB 2.0, provide communication with the CSS and the server.

Basic Plasma Phenomena

The effect of the direct current electric field on the dynamics of the ultracold plasma

Syed ZAHEERUDDIN, Yufan LI (李煜璠), Dongmei ZHAO (赵冬梅), et al.
Plasma Sci. Technol. 2018, 20 (8): 085001 ;  doi: 10.1088/2058-6272/aac166
Full Text: PDF (949KB) ( 273 )
Show Abstract ( 82 )

We created an ultracold plasma by photoionizing the laser-cooled and trapped rubidium atoms in a magneto-optical trap. In the externally applied direct current (DC) electric field environment, the electrons which escape from the potential well of the ultracold plasma were detected for different numbers of the ions and initial kinetic energies of the electrons. The results are in good agreement with the calculations, based on the Coulomb potential well model, indicating that the external DC field is an effective tool to adjust the depth of potential well of the plasma, and it is possible to create an ultracold plasma in a controlled manner.

Numerical studies on pair production in ultra-intense laser interaction with a thin solid-foil

Yunxian TIAN (田云先), Xiaolin JIN (金晓林), Xiaoliang GU (谷晓梁), et al.
Plasma Sci. Technol. 2018, 20 (8): 085002 ;  doi: 10.1088/2058-6272/aac42e
Full Text: PDF (1635KB) ( 281 )
Show Abstract ( 53 )

A theoretical and numerical model of photon and electron–positron pair production in strong-field quantum electrodynamics (QED) is described. Two processes are contained in our QED theoretical model, one is photon emission in the interaction of ultra-intense laser with relativistic electron (or positron), and the other is pair production by a gamma-ray photon interacting with the laser field. This model has been included in a PIC/MCC simulation code named BUMBLEBEE 1D, which is used to simulate the laser plasma interaction. Using this code, the evolutions of electron–positron pair and gamma-ray photon production in ultra-intense laser interaction with aluminum foil target are simulated and analyzed. The simulation results revealed that more positrons are moved in the opposite direction to the incident direction of the laser under the charge separation field.

Magnetically Confined Plasma

Geodesic acoustic modes in tokamak plasmas with anisotropic distribution and a radial equilibrium electric field

Yue MING (明玥), Deng ZHOU (周登) and Wenjia WANG (王文家)
Plasma Sci. Technol. 2018, 20 (8): 085101 ;  doi: 10.1088/2058-6272/aabc5c
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Show Abstract ( 66 )

The dispersion relation of standard geodesic acoustic modes in tokamak plasmas with anisotropic distribution and a radial equilibrium electric field is derived and analyzed. Both frequencies and damping rates increase with respect to the poloidal Mach number which indicates the strength of the radial electric field. The strength of anisotropy is denoted by the ratio of the parallel temperature  to the perpendicular temperature  It is shown that, when the parallel temperature is lower than the perpendicular temperature, the enhanced anisotropy tends to enlarge the real frequency but reduces the damping rate, and when the parallel temperature is higher than the perpendicular temperature, the effect is opposite. The radial equilibrium electric field has stronger effect on the frequency and damping rate for the case with higher parallel temperature than the case with higher perpendicular temperature.

Low Temperature Plasma

Numerical study on an atmospheric pressure helium discharge propagating in a dielectric tube: influence of tube diameter

Zhoutao SUN (孙洲涛), Wen YAN (晏雯), Longfei JI (季龙飞), et al.
Plasma Sci. Technol. 2018, 20 (8): 085401 ;  doi: 10.1088/2058-6272/aab3d2
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Show Abstract ( 69 )

In this work, a two-dimensional numerical simulation of the discharge characteristics of helium plasma propagating inside a dielectric tube was performed. A trapezoidal +9 kV pulse lasting 400 ns was applied on a needle electrode set inside the dielectric tube to ignite the discharge. The discharges generated in the tubes with a variable or a constant inner diameter were investigated. The focus of this study was on clarifying the effect of the tube diameter on the discharge structure and dynamics. The comparison of the discharge characteristics generated in dielectric tubes with different diameters was carried out. It was shown that the tube diameter plays a significant role in discharge behavior of plasma propagating in the dielectric tube.

Bended probe diagnostics of the plasma characteristics within an ECR ion source with a rectangular waveguide

Juan YANG (杨涓), Yuliang FU (付瑜亮), Xianchuang LIU (刘宪闯), et al.
Plasma Sci. Technol. 2018, 20 (8): 085402 ;  doi: 10.1088/2058-6272/aabb9f
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Show Abstract ( 38 )

To reveal the argon plasma characteristics within the entire region of an electron cyclotron resonance (ECR) ion source, the plasma parameters were diagnosed using a bended Langmuir probe with the filament axis perpendicular to the diagnosing plane. Experiments indicate that, with a gas volume flow rate and incident microwave power of 4 sccm and 8.8 W, respectively, the gas was ionized to form plasma with a luminous ring. When the incident microwave power was above 27 W, the luminous ring was converted to a bright column, the dark area near its axis was narrowed, and the microwave power absorbing efficiency was increased. This indicates that there was a mode transition phenomenon in this ECR ion source when the microwave power increased. The diagnosis shows that, at an incident microwave power of 17.4 W, the diagnosed electron temperature and ion density were below 8 eV and 3 × 1017 m−3, respectively, while at incident microwave power levels of 30 W and 40 W, the maximum electron temperature and ion density were above 11 eV and 6.8 × 1017 m−3, respectively. Confined by magnetic mirrors, the higher density plasma region had a bow shape, which coincided with the magnetic field lines but deviated from the ECR layer.

Generation characteristics of a metal ion plasma jet in vacuum discharge

Jia TIAN (田甲), Wenzheng LIU (刘文正), Weisheng CUI (崔伟胜), et al.
Plasma Sci. Technol. 2018, 20 (8): 085403 ;  doi: 10.1088/2058-6272/aabedf
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Show Abstract ( 55 )

To improve the performance of a metal ion plasma jet in vacuum discharge, an anode-insulated cone-cylinder electrode with insulating sleeve is proposed in this paper. Discharge characteristics and generation characteristics of plasma of the electrode are investigated, effects of diameter of insulating sleeve, variety of cathode material and length of the insulating sleeve on characteristics of metal ion plasma jet are discussed. Results indicate that a directional and steady plasma jet is formed by using the novel electrode with insulating sleeve under high vacuum conditions. Moreover, the properties of metal ion plasma jet are improved by using the aluminum cathode and thin and long insulating sleeve. The study provides strong support for research of vacuum metal ion plasma thruster and ion implantation technology.

Characteristic analysis of CO2 switching arcs under a DC current

Xiaoling ZHAO (赵小令), J D YAN and Dengming XIAO (肖登明)
Plasma Sci. Technol. 2018, 20 (8): 085404 ;  doi: 10.1088/2058-6272/aabf34
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Show Abstract ( 40 )

The current interruption capability of a gas, when used in high voltage gas-blast circuit breakers, depends not only on its material properties but also the flow field since turbulence plays a dominant role in arc cooling during the interruption process. Based on available experimental results, a study of CO2 switching arcs under a DC (direct current) current in the model circuit breaker has been conducted to calibrate CO2 arc model and to analyse its electric and thermal property. Through detailed analysis of the results mechanisms responsible for the temperature distribution are identified and the domain energy transportation process of different region discussed. The present work provides significant coefficients for CO2 switching arc simulation and gives a better understanding of CO2 arc burning mechanisms.

Frequency dependence of electron temperature in hollow cathode-type discharge as measured by several different floating probe methods

Shuichi SATO, Hiromu KAWANA, Tatsushi FUJIMINE and Mikio OHUCHI
Plasma Sci. Technol. 2018, 20 (8): 085405 ;  doi: 10.1088/2058-6272/aabfcd
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Show Abstract ( 46 )
We measured electron temperatures through a hollow cathode-type discharge tube using several different floating probe methods. This method detected a shift in the floating potential when an AC voltage was applied to a probe through an intermediary blocking capacitor. The shift in the floating potential is described as a function of the electron temperature and the applied AC voltage. In this study, the effects of the frequency and waveform on the electron temperatures were systematically investigated. The electron temperature measured when using the floating probe method with applied sinusoidal and triangular voltages was lower than that measured with an applied rectangular voltage. The value in the high frequency range was close to that of the tail electron temperature.
Plasma Technology

Numerical simulation of laser ablation of molybdenum target for laser-induced breakdown spectroscopic application Hot!

Cailong FU (付彩龙), Qi WANG (王奇) and Hongbin DING (丁洪斌)
Plasma Sci. Technol. 2018, 20 (8): 085501 ;  doi: 10.1088/2058-6272/aab661
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Show Abstract ( 83 )

Laser-induced breakdown spectroscopy has been recognized as a significant tool for element diagnostics in plasma–wall interaction. In this work, a one-dimensional numerical model is developed to simulate the laser ablation processes of a molybdenum (Mo) target in vacuum conditions. The thermal process of the interaction between the ns-pulse laser with wavelength of 1064 nm and the Mo target is described by the heat conduction equation. The plasma plume generation and expansion are described by Euler equations, in which the conservation of mass density, momentum and energy are included. Saha equations are used to describe the local thermal equilibrium of electrons, Mo atoms, and  Plasma shielding and emission are all considered in this model. The mainly numerical results are divided into three parts, as listed below. Firstly, the rule of the plasma shielding effect varying with laser intensity is demonstrated quantitatively and fitted with the Nelder function. Secondly, the key parameters of plasma plume, such as the number density of species, the propagation velocity and the temperature, are all calculated in this model. The results indicate that the propagation velocity of the plume center increased with time in a general trend, however, one valley value appeared at about 20 ns due to the pressure gradient near the target surface leading to negative plasma velocity. Thirdly, the persistent lines of a Mo atom in the wavelength range from 300 nm to 600 nm are selected and the spectrum is calculated. Moreover, the temporal evolutions of Mo's spectral lines at wavelength of 550.6494 nm, 553.3031 nm and 557.0444 nm are given and the results are compared with experimental data in this work.

Investigation of cathode spot characteristics in vacuum under transverse magnetic field (TMF) contacts

Xianhai PANG (庞先海), Ting WANG (王婷), Shixin XIU (修士新), et al.
Plasma Sci. Technol. 2018, 20 (8): 085502 ;  doi: 10.1088/2058-6272/aab782
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Show Abstract ( 54 )

With the continuous improvement of current levels in power systems, the demands on the breaking capacity requirements of vacuum circuit breakers are getting higher and higher. The breaking capacity of vacuum breakers is determined by cathode spots, which provide electrons and metal vapor to maintain the arc. In this paper, experiments were carried out on two kinds of transverse magnetic field (TMF) contacts in a demountable vacuum chamber, the behavior of the cathode spots was recorded by a high-speed charge-coupled device (CCD) video camera, and the characteristics of the cathode spots were analyzed through the image processing method. The phenomenon of cathode spot groups and the star-shaped pattern of the spots were both discovered in the experiment. The experimental results show that with the condition of TMF contacts the initial expansion speed of cathode spots is influenced by some parameters, such as the tested current, contact gap, the structure of the contact, the contact diameter, the number of slots, etc. In addition, the influence of the magnetic field on the formation of the cathode spot groups, the distribution, and the dynamic characteristics of the cathode spots were analyzed. It is concluded that the characteristics of the cathode spots are due to the effect of the magnetic field on the near-cathode plasma. The study of the characteristics of cathode spots in this paper would be helpful in the exploration of the physical process of vacuum arcs, and would be of guiding significance in optimizing the design of vacuum circuit breakers.

Effect of distances between lens and sample surface on laser-induced breakdown spectroscopy with spatial confinement

Zhenhua JIANG (姜振华), Junfeng SHAO (邵俊峰), Tingfeng WANG (王挺峰), et al.
Plasma Sci. Technol. 2018, 20 (8): 085503 ;  doi: 10.1088/2058-6272/aabc5e
Full Text: PDF (1079KB) ( 189 )
Show Abstract ( 52 )

Spatial confinement can significantly enhance the spectral intensity of laser-induced plasma in air. It is attributed to the compression of plasma plume by the reflected shockwave. In addition, optical emission spectroscopy of laser-induced plasma can also be affected by the distance between lens and sample surface. In order to obtain the optimized spectral intensity, the distance must be considered. In this work, spatially confined laser-induced silicon plasma by using a Nd: YAG nanosecond laser at different distances between lens and sample surface was investigated. The laser energies were 12 mJ, 16 mJ, 20 mJ, and 24 mJ. All experiments were carried out in an atmospheric environment. The results indicated that the intensity of Si (I) 390.55 nm line firstly rose and then dropped with the increase of lens-to-sample distance. Moreover, the spectral peak intensity with spatial confinement was higher than that without spatial confinement. The enhancement ratio was approximately 2 when laser energy was 24 mJ.

Fast inactivation of microbes and degradation of organic compounds dissolved in water by thermal plasma

Ahmed Rida GALALY and Guido VAN OOST
Plasma Sci. Technol. 2018, 20 (8): 085504 ;  doi: 10.1088/2058-6272/aac1b7
Full Text: PDF (1847KB) ( 191 )
Show Abstract ( 48 )

The multifunctionality and the advantages of thermal plasma for the fast inactivation of viable cells and degradation of organic compounds dissolved in waste water are presented. A complete bacterial inactivation process was observed and studied using a thermal plasma treatment source with very short application times, in particular for Staphylococcus aureus bundle spore survival. The survival curves and analyses of the experimental data of the initial and final densities of S. aureus bacteria show a dramatic inhibitory effect of the plasma discharge on the residual bacteria survival ratio. As the exposure time increased, the inactivation process rate increased for direct exposure more than it did for indirect exposure. The evaluation of direct and indirect exposure was based on the analysis of the ultraviolet spectrum from the absorbance spectra of the organic compound dye called benzene sulfonate (C16H11N2NaO4S) and of viable cells called S. aureus. Organic compounds were degraded and viable cells were killed in a short time by thermal plasma. Moreover, analyses of total carbon, total organic carbon, and total inorganic carbon showed a fast decrease in organically bound carbon, however, this was not as fast as the absorbance spectra revealed by the exposure time increasing more for direct exposure than indirect exposure. After 100 s of exposure to the organic compound dye the removal had a maximun of 40% for samples with indirect exposure to the plasma and a maximum of 90% for samples with the direct exposure. For both samples, where some organic contaminants still remained in treated water, four electrolytes (KCl, NaCl, Na2SO4, and CH3COONa) were added to be effective for complete sterilization, reaching a purity of 100%. A proposal is made for an optimized thermal plasma water purification system (TPWPS) to improve fast inactivation of microbes and the degradation of organic compounds dissolved in water (especially for direct exposure rather than indirect exposure) using a hybrid plasma torch with an electrical power of 125 kW (500 V–250 A) producing a high-temperature (10 000 K–19 000 K) plasma jet with a maximum gas consumption of 28 mg s−1.

The impact of dielectrics on the electrical capacity, concentration, efficiency ozone generation for the plasma reactor with mesh electrodes

Ernest GNAPOWSKI, Sebastian GNAPOWSKI and Jaroslaw PYTKA
Plasma Sci. Technol. 2018, 20 (8): 085505 ;  doi: 10.1088/2058-6272/aac1b6
Full Text: PDF (1117KB) ( 188 )
Show Abstract ( 44 )

This paper presents experimental results concerning the effect of dielectric type on ozone concentration and the efficiency of its generation in plasma reactor with two mesh electrodes. Three types of dielectric solid were used in the study; glass, micanite and Kapton insulating foil. The experiments were conducted for voltage ranges from 2.3 to 13 kV. A plasma reactor equipped with two 0.3×0.3 mm2 mesh electrodes made of acid resistant AISI 304 mesh was used in the experiments. The influence of the dielectric type on the concentration and efficiency of ozone generation was described. The resulting maximum concentration of the ozone was about 2.70–9.30 g O3 m−3, depending on the dielectrics used. The difference between the maximum and the minimum ozone concentration depends on the dielectric used, this accounts for 70% at the variance. The reactor capacity has also been described in the paper; total Ct and dielectric capacitance Cd depending on the dielectric used and its thickness.

Fusion Engineering

Power supply for generating frequency-variable resonant magnetic perturbations on the J-TEXT tokamak

Guo XU (徐国), Bo RAO (饶波), Yonghua DING (丁永华), et al.
Plasma Sci. Technol. 2018, 20 (8): 085601 ;  doi: 10.1088/2058-6272/aabd2f
Full Text: PDF (1669KB) ( 165 )
Show Abstract ( 56 )

To further research the response of the tearing mode (TM) to dynamic resonant magnetic perturbation (DRMP) on the J-TEXT tokamak, a modified series resonant inverter power supply (MSRIPS) with a function of discrete variable frequency is designed for DRMP coils in this study. The MSRIPS is an AC–DC–AC converter, including a phase-controlled rectifier, an LC filter, an insulated gate bipolar transistor (IGBT) full bridge, a matching transformer, three resonant capacitors with different capacitance values, and three corresponding silicon controlled rectifier (SCR) switches. The function of discrete variable frequency is realized by switching over different resonant capacitors with corresponding SCR switches while matching the corresponding driving frequency of the IGBT full bridge. A detailed switching strategy of the SCR switch is put forward to obtain sinusoidal current waveform and realize current waveform smooth transition during frequency conversion. In addition, a resistor and thyristor bleeder is designed to protect the SCR switch from overvoltage. Manufacturing of the MSRIPS is completed, and the MSRIPS equipment can output current with an amplitude of 1.5 kA when its working frequency jumps among different frequencies. Moreover, the current waveform is sinusoidal and can smoothly transition during frequency conversion. Furthermore, the transition time when the current amplitude rises from zero to a steady state is less than 2 ms during frequency conversion. By using the MSRIPS, the expected discrete variable frequency DRMP is generated, and the phenomenon of the TM being locked to the discrete variable frequency DRMP is observed on the J-TEXT tokamak.

The development of data acquisition and control system for extraction power supply of prototype RF ion source

Meichu HUANG (黄梅初), Chundong HU (胡纯栋), Yuanzhe ZHAO (赵远哲), et al.
Plasma Sci. Technol. 2018, 20 (8): 085602 ;  doi: 10.1088/2058-6272/aabde5
Full Text: PDF (1192KB) ( 180 )
Show Abstract ( 59 )

A 16 kV/20 A power supply was developed for the extraction grid of prototype radio frequency (RF) ion source of neutral beam injector. To acquire the state signals of extraction grid power supply (EGPS) and control the operation of the EGPS, a data acquisition and control system has been developed. This system mainly consists of interlock protection circuit board, photoelectric conversion circuit, optical fibers, industrial compact peripheral component interconnect (CPCI) computer and host computer. The human machine interface of host computer delivers commands and data to program of the CPCI computer, as well as offers a convenient client for setting parameters and displaying EGPS status. The CPCI computer acquires the status of the power supply. The system can turn-off the EGPS quickly when the faults of EGPS occur. The system has been applied to the EGPS of prototype RF ion source. Test results show that the data acquisition and control system for the EGPS can meet the requirements of the operation of prototype RF ion source.

Development of a helicon-wave excited plasma facility with high magnetic field for plasma–wall interactions studies

Guilu ZHANG (张桂炉), Tianyuan HUANG (黄天源), Chenggang JIN (金成刚), et al.
Plasma Sci. Technol. 2018, 20 (8): 085603 ;  doi: 10.1088/2058-6272/aac014
Full Text: PDF (1763KB) ( 195 )
Show Abstract ( 56 )

The high magnetic field helicon experiment system is a helicon wave plasma (HWP) source device in a high axial magnetic field ( B0 ) developed for plasma–wall interactions studies for fusion reactors. This HWP was realized at low pressure (5 × 10−3 − 10 Pa) and a RF (radio frequency, 13.56 MHz) power (maximum power of 2 kW) using an internal right helical antenna (5 cm in diameter by 18 cm long) with a maximum B 0 of 6300 G. Ar HWP with electron density ~1018–1020 m−3 and electron temperature ~4–7 eV was produced at high B0 of 5100 G, with an RF power of 1500 W. Maximum Ar+ ion flux of 7.8 × 1023 m−2 s−1 with a bright blue core plasma was obtained at a high B0 of 2700 G and an RF power of 1500 W without bias. Plasma energy and mass spectrometer studies indicate that Ar+ ion-beams of 40.1 eV are formed, which are supersonic (~3.1c s). The effect of Ar HWP discharge cleaning on the wall conditioning are investigated by using the mass spectrometry. And the consequent plasma parameters will result in favorable wall conditioning with a removal rate of 1.1 × 1024 N2/m2h.

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