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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2017, 19 (7)   Published: 06 June 2017
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Dynamics of oscillatory plasma flows prior to the H-mode in the HL-2A tokamak Hot!

Wulyu ZHONG (钟武律)1,4, Xiaolan ZOU (邹晓岚)2, Zhongbing SHI (石中兵)1, et al.
Plasma Sci. Technol. 2017, 19 (7): 070501 ;  doi: 10.1088/2058-6272/aa6538
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Show Abstract ( 193 )

This paper discusses edge oscillatory plasma flows, geodesic acoustic mode (GAM) and limit cycle oscillations (LCOs), which have been measured by Doppler reflectometry prior to the high confinement mode (H-mode) in the HL-2A tokamak. The complex relations between the flows and background turbulence have been analyzed. It was observed that the GAM and LCO coexist, and these two flows and turbulence have strong nonlinear interactions during the intermediate confinement phase (I-phase). Dynamics of the shear flows and turbulence prior to the H-mode shows that the oscillatory flows quench the turbulence along with the increase of the mean E × B flow at the early stage of the I-phase, then the oscillatory flows are damped and the further increased mean flow takes over the role in turbulence suppression. The reduced turbulent transport results in the formation of a steep edge transport barrier. It suggests that the oscillatory flows can initiate the L–H transition through providing a positive feedback for the increase of the mean E × B flow strength.

High-flux electron beams from laser wakefield accelerators driven by petawatt lasers Hot!

Ming ZENG (曾明) and Ovidiu TESILEANU
Plasma Sci. Technol. 2017, 19 (7): 070502 ;  doi: 10.1088/2058-6272/aa6437
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Show Abstract ( 149 )

Laser wakefield accelerators (LWFAs) are considered to be one of the most competitive nextg-eneration accelerator candidates. In this paper, we will study the potential high-flux electron beam production of an LWFA driven by petawatt-level laser pulses. In our three-dimensional particle-in-cell simulations, an optimal set of parameters gives ~40 nC of charge with 2 PW laser power, thus ~400 kA of instantaneous current if we assume the electron beam duration is 100 fs. This high flux and its secondary radiation are widely applicable in nuclear and QED physics, industrial imaging, medical and biological studies.

Basic Plasma Phenomena

The dynamics of an ion acting on two monochromatic obliquely propagating Alfvén waves

Limin YU (虞立敏)1, Zhengmao SHENG (盛正卯)2, Xianmei ZHANG (张先梅)1, et al.
Plasma Sci. Technol. 2017, 19 (7): 075001 ;  doi: 10.1088/2058-6272/aa6617
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Show Abstract ( 151 )
The interaction between a magnetized ion and two monochromatic shear Alfvén waves propagating obliquely to an ambient magnetic field is investigated both analytically and numerically. According to the Hamiltonian of this system, the invariant of motion at the lowest order and the half-island widths at the corresponding resonances are obtained analytically using the Lie transformation method. It is shown that these theoretical results agree with the numerical ones from the Poincaré surface of section. The regular motions from the invariant and the transition to stochasticity due to resonance overlapping are demonstrated. Compared to the case of a single wave, there may be a lower stochastic threshold in the multiple-wave problem.
Magnetically Confined Plasma

Development of a radiographic method for measuring the discrete spectrum of the electron beam from a plasma focus device

Plasma Sci. Technol. 2017, 19 (7): 075101 ;  doi: 10.1088/2058-6272/aa632e
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An indirect method is proposed for measuring the relative energy spectrum of the pulsed electron beam of a plasma focus device. The Bremsstrahlung x-ray, generated by the collision of electrons against the anode surface, was measured behind lead filters with various thicknesses using a radiographic film system. A matrix equation was considered in order to explain the relation between the x-ray dose and the spectral amplitudes of the electron beam. The electron spectrum of the device was measured at 0.6 mbar argon and 22 kV charging voltage, in four discrete energy intervals extending up to 500 keV. The results of the experiments show that most of the electrons are emitted in the 125–375 keV energy range and the spectral amplitude becomes negligible beyond 375 keV.

Inertially Confined Plasma

Enhanced photon emission and pair production in laser-irradiated plasmas Hot!

Feng WAN (弯峰)1, Chong LV (吕冲)1, Moran JIA (贾默然)1 and Baisong XIE (谢柏松)1,2
Plasma Sci. Technol. 2017, 19 (7): 075201 ;  doi: 10.1088/2058-6272/aa64ed
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Enhanced photon emission and pair production due to heavy ion mass in the interaction of an ultraintense laser with overdense plasmas is explored by particle-in-cell simulation. It is found that plasmas with heavier ion mass can excite a higher and broader electrostatic field, which causes the enhancement of backward photon emission. The pair yields are then enhanced due to the increase of backwards photons colliding with the incoming laser pulse. By examining the density evolution and angle distribution of each particle species, the origin of pair yield enhancement is clarified.

Low Temperature Plasma

Atmospheric-pressure dielectric barrier discharge generation by a full-bridge flying capacitor multilevel inverter

Plasma Sci. Technol. 2017, 19 (7): 075401 ;  doi: 10.1088/2058-6272/aa61f7
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A new configuration of a resonant full-bridge flying capacitor multicell inverter has been designed and constructed with the aim of achieving an extended output voltage frequency range with low harmonic distortion and reduced semiconductor commutation losses. This configuration was tested as a power supply for two different coaxial dielectric barrier discharge reactors, one of them employed for electric characterization and the other one for inorganic compound elimination in an aqueous solution. Two different gas mixtures, 90% Ar–10% O2 and 80% Ar–20% O2, were individually supplied during the experiments; the results showed a highefficiency removal of meta-cresol (m-cresol) to the order of 98%, which was obtained by adding more oxygen to the plasma gas mixture.

Numerical study on characteristics of radiofrequency discharge at atmospheric pressure in argon with small admixtures of oxygen

Yinan WANG (王一男)1,2,3 and Yue LIU (刘悦)2
Plasma Sci. Technol. 2017, 19 (7): 075402 ;  doi: 10.1088/2058-6272/aa6156
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In this paper, a 1D fluid model is developed to study the characteristics of a discharge in argon with small admixtures of oxygen at atmospheric pressure. This model consists of a series of equations, including continuity equations for electrons, positive ions, negative ions and neutral particles, the energy equation, and the Poisson equation for electric potential. Special attention has been paid to the electron energy dissipation and the mechanisms of electron heating, while the admixture of oxygen is in the range of 0.1%–0.6%. It is found that when the oxygen-to-argon ratio grows, the discharge is obviously divided into three stages: electron growth, electron reduction and the electron remaining unchanged. Furthermore, the cycle-averaged electric field, electron temperature, electron Ohmic heating, electron collisionless heating, electron energy dissipation and the net electron production are also studied in detail, and when the oxygen-toargon ratio is relatively larger (R=0.6%), double value peaks of electron Ohmic heating appear in the sheath. According to the results of the numerical simulation, various oxygen-to-argon ratios result in different amounts of electron energy dissipation and electron heating.

The effect of the configuration of a single electrode corona discharge on its acoustic characteristics

Xinlei ZHU (朱鑫磊)1, Liancheng ZHANG (张连成)1, Yifan HUANG (黄逸凡)1, et al.
Plasma Sci. Technol. 2017, 19 (7): 075403 ;  doi: 10.1088/2058-6272/aa6716
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Show Abstract ( 131 )

A new sparker system based on pulsed spark discharge with a single electrode has already been utilized for oceanic seismic exploration. However, the electro-acoustic energy efficiency of this system is lower than that of arc discharge based systems. A simple electrode structure was investigated in order to improve the electro-acoustic energy efficiency of the spark discharge. Experiments were carried out on an experimental setup with discharge in water driven by a pulsed power source. The voltage–current waveform, acoustic signal and bubble oscillation were recorded when the relative position of the electrode varied. The electro-acoustic energy efficiency was also calculated. The load voltage had a saltation for the invaginated electrode tip, namely an obvious voltage remnant. The more the electrode tip was invaginated, the larger the pressure peaks and first period became. The results show that electrode recessing into the insulating layer is a simple and effective way to improve the electro-acoustic energy efficiency from 2% to about 4%.

Study on the ignition process of a segmented plasma torch

Xiuquan CAO (曹修全), Deping YU (余德平)1, Yong XIANG (向勇), et al.
Plasma Sci. Technol. 2017, 19 (7): 075404 ;  doi: 10.1088/2058-6272/aa62f9
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Show Abstract ( 138 )

Direct current plasma torches have been applied to generate unique sources of thermal energy in many industrial applications. Nevertheless, the successful ignition of a plasma torch is the key process to generate the unique source (plasma jet). However, there has been little study on the underlying mechanism of this key process. A thorough understanding of the ignition process of a plasma torch will be helpful for optimizing the design of the plasma torch structure and selection of the ignition parameters to prolong the service life of the ignition module. Thus, in this paper, the ignition process of a segmented plasma torch (SPT) is theoretically and experimentally modeled and analyzed. Corresponding electrical models of different stages of the ignition process are set up and used to derive the electrical parameters, e.g. the variations of the arc voltage and arc current between the cathode and anode. In addition, the experiments with different ignition parameters on a home-made SPT have been conducted. At the same time, the variations of the arc voltage and arc current have been measured, and used to verify the ones derived in theory and to determine the optimal ignition parameters for a particular SPT.

Plasma Technology

Novel electrode structure in a DBD reactor applied to the degradation of phenol in aqueous solution

Plasma Sci. Technol. 2017, 19 (7): 075501 ;  doi: 10.1088/2058-6272/aa6715
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Phenol degradation experimental results are presented in a similar wastewater aqueous solution using a non-thermal plasma reactor in a coaxial dielectric barrier discharge. The novelty of the work is that one of the electrodes of the reactor has the shape of a hollow screw which shows an enhanced efficiency compared with a traditional smooth structure. The experimentation was carried out with gas mixtures of 90% Ar–10% O2, 80% Ar–20% O2 and 0% Ar–100% O2. After one hour of treatment the removal efficiency was 76%, 92%, and 97%, respectively, assessed with a gas chromatographic mass spectrometry technique. For both reactors used, the ozone concentration was measured. The screw electrode required less energy, for all gas mixtures, than the smooth electrode, to maintain the same ozone concentration. On the other hand, it was also observed that in both electrodes the electrical conductivity of the solution changed slightly from ~0.0115 S m-1 up to ~0.0430 S m-1 after one hour of treatment. The advantages of using the hollow screw electrode structure compared with the smooth electrode were: (1) lower typical power consumption, (2) the generation of a uniform plasma throughout the reactor benefiting the phenol degradation, (3) a relatively lower temperature of the aqueous solution during the process, and (4) the plasma generation length is larger.

Growth and structural properties of silicon on Ag films prepared by 40.68 MHz veryhigh-frequency magnetron sputtering

Jiamin GUO (郭佳敏), Chao YE (叶超), Xiangying WANG (王响英), et al.
Plasma Sci. Technol. 2017, 19 (7): 075502 ;  doi: 10.1088/2058-6272/aa6395
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The growth of silicon on Ag films via 40.68 MHz very-high-frequency (VHF) magnetron sputtering was investigated. The energy distribution and flux density of the ions on the substrate were also measured. The results showed that 40.68 MHz magnetron sputtering can produce ions with higher energy and lower flux density. The impact of these ions onto the grown surface promotes the growth of silicon, which is related to the crystalline nature and microstructure of the underlayer of the Ag films, and there is large particle growth of silicon on Ag films with a preferred orientation of (111), and two-dimensional growth of silicon on Ag films with a better face-centered cubic structure.

Decay characters of charges on an insulator surface after different types of discharge

Cheng PAN (潘成)1,4, Ju TANG (唐炬)1,2, Dibo WANG (王邸博)3, et al.
Plasma Sci. Technol. 2017, 19 (7): 075503 ;  doi: 10.1088/2058-6272/aa6436
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In an insulating system including solid and gas dielectrics, discharge type has a strong impact on charge accumulation at the interface between two dielectrics, and hence charge decay. In order to clarify the influence, a surface charge measurement system was constructed, and three types of discharge, i.e. surface discharge, and low intensity and high intensity coronas, were introduced to cause surface charge accumulation. The decay behavior of surface charges after different types of discharge was obtained at various temperatures. It was found that total surface charges monotonically decreased with time, and the decay rate became larger as temperature increased. However, after a surface discharge or a high intensity corona, surface charge density in the local area appeared to fluctuate during the decay process. Compared with this, the fluctuation of surface charge density was not observed after a low intensity corona. The mechanisms of surface charge accumulation and decay were analysed. Moreover, a microscopic physical model involving charge production, accumulation, and decay was proposed so that the experimental results could be explained.

Plasma effects on the bacteria Escherichia coli via two evaluation methods

Danijela VUJOSEVIC, Uros CVELBAR, Urska REPNIK, et al.
Plasma Sci. Technol. 2017, 19 (7): 075504 ;  doi: 10.1088/2058-6272/aa656b
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The degradation of Escherichia coli bacteria by treatment with cold, weakly ionised, highly dissociated oxygen plasma, with an electron temperature of 3 eV, a plasma density of 8×1015 m−3 and a neutral oxygen atom density of 3.5×1021 m−3 was studied. To determine the ‘real’ plasma effects, two methods were used for evaluation and determination, as well as a comparison of the number of bacteria that had survived: the standard plate count technique (PCT) and advanced fluorescence-activated cell sorting (FACS). Bacteria were deposited onto glass substrates and kept below 50 °C during the experiments with oxygen plasma. The results showed that the bacteria had fully degraded after about 2 min of plasma treatment, depending slightly on the amount of bacteria that had been?deposited on the substrates. The very precise determination of the O flux on the substrates and the two-method comparison allowed for the determination of the critical dose of oxygen atoms required for the destruction of a bacterial cell wall—about 6×1024 m−2—as well as deactivation of the substrates—about 8×1025 m−2. These results were taken in order to discuss other results obtained by comparable studies and scientific method?evaluations in the determination of plasma effects on bacteria.

Investigation of working pressure on the surface roughness controlling technology of glow discharge polymer films based on the diagnosed plasma

Ling ZHANG (张玲), Guo CHEN (陈果), Zhibing HE (何智兵), et al.
Plasma Sci. Technol. 2017, 19 (7): 075505 ;  doi: 10.1088/2058-6272/aa6618
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The effects of working pressure on the component, surface morphology, surface roughness, and deposition rate of glow discharge polymer (GDP) films by a trans-2-butene/hydrogen gas mixture were investigated based on plasma characteristics diagnosis. The composition and ion energy distributions of a multi-carbon (C4H8/H2) plasma mixture at different working pressures were diagnosed by an energy-resolved mass spectrometer (MS) during the GDP film deposition process. The Fourier transform infrared spectroscopy (FT–IR), field emission scanning electron microscope (SEM) and white-light interferometer (WLI) results were obtained to investigate the structure, morphology and roughness characterization of the deposited films, respectively. It was found that the degree of ionization of the C4H8/H2 plasma reduces with an increase in the working pressure. At a low working pressure, the C–H fragments exhibited small-mass and high ion energy in plasma. In this case, the film had a low CH3/CH2 ratio, and displayed a smooth surface without any holes, cracks or asperities. While the working pressure increased to 15 Pa, the largest number of large-mass fragments led to the deposition rate reaching a maximum of 2.11 μm h−1, and to hole defects on the film surface. However, continuing to increase the working pressure, the film surface became smooth again, and the interface between clusters became inconspicuous without etching pits.

Fusion Engineering

Extended Bonner sphere spectrometer for dynamic neutron spectrum on HL-2A

Chunyu JIANG (蒋春雨)1,2,4, Jing CAO (曹靖)1,2, Zihao LIU (刘子豪)1,2, et al.
Plasma Sci. Technol. 2017, 19 (7): 075601 ;  doi: 10.1088/2058-6272/aa667a
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The extended Bonner sphere spectrometer (EBSS) at the HL-2A tokamak for the neutron spectrum is described. This device was developed on the basis of previous Bonner sphere spectrometry (BSS), aiming to obtain a more accurate neutron spectrum in the HL-2A tokamak hall. The previous BSS contained eight Bonner spheres (BS). This EBSS contains 13 3He-filled detectors embedded in polyethylene spheres (PS), pre-amplifiers, and a parallel processing data acquisition system (DAQ). A response matrix is simulated in Geant4 taking the effect of the environment into account.

The development of data acquisition and processing application system for RF ion source

Xiaodan ZHANG (张小丹)1, Xiaoying WANG (王晓英)1, Chundong HU (胡纯栋)2, et al.
Plasma Sci. Technol. 2017, 19 (7): 075602 ;  doi: 10.1088/2058-6272/aa61f5
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As the key ion source component of nuclear fusion auxiliary heating devices, the radio frequency (RF) ion source is developed and applied gradually to offer a source plasma with the advantages of ease of control and high reliability. In addition, it easily achieves long-pulse steady-state operation. During the process of the development and testing of the RF ion source, a lot of original experimental data will be generated. Therefore, it is necessary to develop a stable and reliable computer data acquisition and processing application system for realizing the functions of data acquisition, storage, access, and real-time monitoring. In this paper, the development of a data acquisition and processing application system for the RF ion source is presented. The hardware platform is based on the PXI system and the software is programmed on the LabVIEW development environment. The key technologies that are used for the implementation of this software programming mainly include the long-pulse data acquisition technology, multithreading processing technology, transmission control communication protocol, and the Lempel–Ziv–Oberhumer data compression algorithm. Now, this design has been tested and applied on the RF ion source. The test results show that it can work reliably and steadily. With the help of this design, the stable plasma discharge data of the RF ion source are collected, stored, accessed, and monitored in real-time. It is shown that it has a very practical application significance for the RF experiments.

A precision control method for plasma electron density and Faraday rotation angle measurement on HL-2A

Wei ZHANG (张伟)1,2, Tongyu WU (吴彤宇)1,2, Baogang DING (丁宝钢)3, et al.
Plasma Sci. Technol. 2017, 19 (7): 075603 ;  doi: 10.1088/2058-6272/aa64cd
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The precision of plasma electron density and Faraday rotation angle measurement is a key indicator for far-infrared laser interferometer/polarimeter plasma diagnosis. To improve the precision, a new multi-channel high signal-to-noise ratio HCOOH interferometer/polarimeter has been developed on the HL-2A tokamak. It has a higher level requirement for phase demodulation precision. This paper introduces an improved real-time fast Fourier transform algorithm based on the field programmable gate array, which significantly improves the precision. We also apply a real-time error monitoring module (REMM) and a stable error
inhibiting module (SEIM) for precision control to deal with the weak signal. We test the interferometer/polarimeter system with this improved precision control method in plasma discharge experiments and simulation experiments. The experimental results confirm that the plasma electron density precision is better than 1/3600 fringe and the Faraday rotation angle measurement precision is better than 1/900 fringe, while the temporal resolution is 80 ns. This performance can fully meet the requirements of HL-2A.

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