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Plasma Sci. Technol.  
  Plasma Sci. Technol.--2018, 20 (11)   Published: 13 September 2018
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Structure-preserving geometric particle-in- cell methods for Vlasov-Maxwell systems Hot!

Jianyuan XIAO (肖建元) 1,Hong QIN (秦宏) 1,2 and Jian LIU (刘健) 1
Plasma Sci. Technol. 2018, 20 (11): 110501 ;  doi: 10.1088/2058-6272/aac3d1
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Recent development of structure-preserving geometric particle-in-cell (PIC) algorithms for Vlasov-Maxwell systems is summarized. With the arrival of 100 petaflop and exaflop computing power, it is now possible to carry out direct simulations of multi-scale plasma dynamics based on first-principles. However, standard algorithms currently adopted by the plasma physics community do not possess the long-term accuracy and fidelity required for these large-scale simulations. This is because conventional simulation algorithms are based on numerically solving the underpinning differential (or integro-differential) equations, and the algorithms used in general do not preserve the geometric and physical structures of the systems, such as the local energy-momentum conservation law, the symplectic structure, and the gauge symmetry. As a consequence, numerical errors accumulate coherently with time and long-term simulation results are not reliable. To overcome this difficulty and to harness the power of exascale computers, a new generation of structure-preserving geometric PIC algorithms have been developed. This new generation of algorithms utilizes modern mathematical techniques, such as discrete manifolds, interpolating differential forms, and non-canonical symplectic integrators, to ensure gauge symmetry, space-time symmetry and the conservation of charge, energy-momentum, and the symplectic structure. These highly desired properties are difficult to achieve using the conventional PIC algorithms. In addition to summarizing the recent development and demonstrating practical implementations, several new results are also presented, including a structure-preserving geometric relativistic PIC algorithm, the proof of the correspondence between discrete gauge symmetry and discrete charge conservation law, and a reformulation of the explicit non-canonical symplectic algorithm for the discrete Poisson bracket using the variational approach. Numerical examples are given to verify the advantages of the structure- preserving geometric PIC algorithms in comparison with the conventional PIC methods.

Basic Plasma Phenomena

THz plasma wave instability in field effect transistor with electron diffusion current density

Hongmei DU (杜洪梅), Liping ZHANG (张丽萍) and Dongao LI (李东澳)
Plasma Sci. Technol. 2018, 20 (11): 115001 ;  doi: 10.1088/2058-6272/aacaef
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The plasma wave instability in rectangle field effect transistors (FETs) is studied with electron diffusion current density by quantum hydrodynamic model in this paper. General dispersion relation including effects of electrical thermal motion, external friction associated with electron scattering effect, electron exchange-correlation contributions and quantum effects were obtained for rectangle FETs. The electron diffusion current density term is considered for further analysis in this paper. It is found that the quantum effects, the electron diffusion current density and electrical thermal motion enhance the radiation power and frequencies. But the electron exchange- correlation effects and the electron scattering effects reduce the radiation power and frequencies. Results showed that a transistor has advantages for the realization of practical terahertz sources.

Wakefield generation by chirped super- Gaussian laser pulse in inhomogeneous plasma

Zhengwei YAO (姚征伟), Lihong CHENG (成丽红), Rongan TANG (唐荣安), et al.
Plasma Sci. Technol. 2018, 20 (11): 115002 ;  doi: 10.1088/2058-6272/aacbbf
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We study the effect of nonlinearly chirped super-Gaussian (SG) laser pulse on wakefield generation in an inhomogeneous plasma. The different types of nonlinearly chirped pulse are employed, and different kinds of inhomogeneous plasma density are used. The maximum wakefield amplitude as the function of nonlinearly chirped laser pulse and inhomogeneous plasma density in parameter space are obtained. Moreover, the dependence of the maximum wakefield amplitude on the SG laser pulse index is discussed. This shows that a larger wakefield can be obtained when the chirped pulse and inhomogeneous density are in the critical regions. Wakefield generation can be controlled by adjusting the chirped SG pulse and inhomogeneous plasma density parameters. That is, we provide an efficient way for the controlled generation of the wakefield.
Magnetically Confined Plasma

Experimental observation of reverse- sheared Alfvén eigenmodes (RSAEs) in ELMy H-mode plasma on the EAST tokamak

Tao ZHANG (张涛) 1, Haiqing LIU (刘海庆) 1, Guoqiang LI (李国强) 1, et al.
Plasma Sci. Technol. 2018, 20 (11): 115101 ;  doi: 10.1088/2058-6272/aac9b5
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Reverse-sheared Alfvén eigenmodes (RSAEs) have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from ∼80 kHz to ∼110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about ρ=0.37–0.46, just around the position of qmin with ρ∼0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called ‘grand cascades’, was also clearly observed on the ECE signal at ρ=0.46. During the phase, a transient internal transport barrier (ITB) in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of qmin . A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease or qmin increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation.

Inertially Confined Plasma

Simulation of electromagnetic pulses generated by escaped electrons in a high- power laser chamber

Hanbing JIN (金晗冰) 1,2, Cui MENG (孟萃) 1,2, Yunsheng JIANG (姜云升) 1,2, et al.
Plasma Sci. Technol. 2018, 20 (11): 115201 ;  doi: 10.1088/2058-6272/aac838
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Intensive electromagnetic pulses (EMPs) can be generated when a high-power laser strikes a target. The transient electromagnetic field can have an intensity of up to several hundred kV m -1 with a broad frequency of up to several gigahertz, which may affect diagnostics and interfere with, or even damage, electronic equipment. In this paper, the process in which hot electrons produced by the laser-target interaction radiate EMPs is studied and simulated. The physical process is divided into three stages which are: the production of hot electrons; the escape of hot electrons; and the generation of EMPs. Instead of using a general finite difference time domain (FDTD) method to solve the Maxwell equations, a particle-in-cell method together with a time- biased FDTD method is applied in EMP simulation to restrain high-frequency noise. The results show that EMPs are stronger with higher laser intensity and larger target size.

Astrophysics and Space Plasma

Threshold of parametric instability in the ionospheric heating experiments

Xiang WANG (王翔) 1,2, Chen ZHOU (周晨) 1, Moran LIU (刘默然) 1, et al.
Plasma Sci. Technol. 2018, 20 (11): 115301 ;  doi: 10.1088/2058-6272/aac71d
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Many observations in the ionospheric heating experiment, by a powerful high frequency electromagnetic wave with ordinary polarization launched from a ground-based facility, is attributed to parametric instability (PI). In this paper, the general dispersion relation and the threshold of the PI excitation in the heating experiment are derived by considering the inhomogeneous spatial distribution of pump wave field. It is shown that the threshold of PI is influenced by the effective electron and ion collision frequencies and the pump wave frequency. Both collision and Landau damping should be considered in the PI calculation. The derived threshold expression has been used to calculate the required threshold for excitation of PI for several ionospheric conditions during heating experiments conducted employing EISCAT high frequency transmitter in Troms?, Norway, on 2nd October 1998, 8th November 2001, 19th October 2012 and 7th July 2014. The results indicate that the calculated threshold is in good agreement with the experimental observations.

PMSE dependence on frequency observed simultaneously with VHF and UHF radars in the presence of precipitation

Safi ULLAH, Hailong LI (李海龙), Abdur RAUF, et al.
Plasma Sci. Technol. 2018, 20 (11): 115302 ;  doi: 10.1088/2058-6272/aac8d4
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Using PMSE (polar mesosphere summer echoes) observations in combination with particle flux measurements obtained with detectors onboard the Geostationary Operational Environmental Satellite (GOES) a special condition is shown for the occurrence of rare observed UHF PMSE. When electron flux observed from GOES satellites show a decrease, then after being in the presence of precipitation UHF PMSE occurs. The heating effect on PMSE is small when the UHF electron density is enhanced at 90 km due to particle precipitation. We analyzed and compared the frequency dependence of PMSE under the condition of high energy particle precipitation in July of 2004 and 2007 at well separated frequencies (224 and 930 MHz) at the same site, height, and time. The frequency index varies with height and time. At different heights, the maximum as well as the minimum value of volume reflectivity at VHF is greater than that at UHF with 2 to 3 orders of magnitude. A new qualitative method for the analysis of dust distribution is used by analyzing the relationship between volume reflectivity and frequency index. In agreement with the results of the model it is shown that dust particles of smaller size generally did not occur at the edges, instead they occurred in the middle PMSE regions.
Low Temperature Plasma

Identification of the normal and abnormal glow discharge modes in a neon-xenon gas mixture at low pressure

Plasma Sci. Technol. 2018, 20 (11): 115401 ;  doi: 10.1088/2058-6272/aac693
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In this paper, we focused on the identification of the normal and abnormal glow discharge modes in a neon-xenon gas mixture at low pressure. We considered four gas mixtures: 90%Ne-10%Xe, 80%Ne-20%Xe, 70%Ne-30%Xe and 50%Ne-50%Xe at 1.5 Torr. The range of the gap voltage is 150–500 V. A one-dimensional fluid model with multiple species was used in this work, and the metastable state of the atoms as well as the radiation effects were integrated into the model too. The input data changed for each percentage in the gas mixture, and was calculated by BOLSIG+ software. The parameters of particle transport and their rate coefficients strictly depend on the mean electron energy. The results show that the neon ion density is negligible compared to the xenon ion density, mostly in the case of 50%Ne-50%Xe.

Effects of HF frequency on plasma characteristics in dual-frequency helium discharge at atmospheric pressure by fluid modeling

Yinan WANG (王一男) 1, Shuaixing LI (李帅星) 1, Li WANG (王莉) 1, et al.
Plasma Sci. Technol. 2018, 20 (11): 115402 ;  doi: 10.1088/2058-6272/aac71e
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On the basis of the fluid theory and the drift–diffusion approximation, a numerical model for dual-frequency atmospheric pressure helium discharge is established, in order to investigate the effects of the high frequency source (HF) on the characteristics of dual-frequency atmospheric pressure helium discharge. The numerical results showed that the electron heating rate increases with enhancing HF frequency, as well as the particles densities, electron dissipation rate, current density, net electron generation and bulk plasma region. Moreover, it is also observed that the efficient electron heating region moves when the HF frequency has been changed. The plasma parameters are not linear change with the HF frequency linearly increasing.

Volume added surface barrier discharge plasma excited by bipolar nanosecond pulse power in atmospheric air: optical emission spectra influenced by gap distance

Zilu ZHAO (赵紫璐), Dezheng YANG (杨德正), Wenchun WANG (王文春), et al.
Plasma Sci. Technol. 2018, 20 (11): 115403 ;  doi: 10.1088/2058-6272/aac881
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In this paper, volume barrier discharge with different gap distances is added on the discharge border of high-voltage electrode of annular surface barrier discharge for generating volume added surface barrier discharge (V-SBD) excited by bipolar nanosecond high-voltage pulse power in atmospheric air. The excited V-SBDs consist of surface barrier discharge (d=0 mm) and volume added surface barrier discharges (d=2 mm and 3 mm). The optical emission spectra are recorded for calculating emission intensities of N2 (C 3u →B3Πg ) and N2+ (B 2 Σu+ → X 2Σg+ ), and simulating rotational and vibrational temperatures. The influences of gap distance of V-SBD on emission intensity and plasma temperature are also investigated and analyzed. The results show that d=0 mm structure can excite the largest emission intensity of N 2 (C 3 Πu →B 3Πg ), while the existence of volume barrier discharge can delay the occurrence of the peak value of the emission intensity ratio of N2+ (B 2 Σu+ → X 2 Σg+ )/N 2(C3Πu →B3Πg ) during the rising period of the applied voltage pulse and weaken it during the end period. The increasing factor of emission intensity is effected by the pulse repetition rate. The d=3 mm structure has the highest threshold voltage while it can maintain more emission intensity of N2(C3 Π u →B 3Πg ) than that of d=2 mm structure. The structure of d=2 mm can maintain more increasing factor than that of the d=3 mm structure with varying pulse repetition rate. Besides, the rotational temperatures of three V-SBD structures are slightly affected when the gap distance and pulse repetition rate vary. The vibrational temperatures have decaying tendencies of all three structures with the increasing pulse repetition rate.

Plasma Technology

Atmospheric pressure gliding arc discharge plasma treatments for improving germination, growth and yield of wheat

N C ROY 1, M M HASAN 2, A H KABIR 3, et al.
Plasma Sci. Technol. 2018, 20 (11): 115501 ;  doi: 10.1088/2058-6272/aac647
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 Wheat (Triticum aestivum) seeds were treated with atmospheric pressure gliding arc discharge plasmas to investigate the effects on water absorption, seed germination rate, seedling growth and yield in wheat. The surface architectures and functionalities of the seeds were found to modify due to plasma treatments. 6min treatment was provided 95%–100% germination rate. For the treatment duration of 3 and 9 min the growth activity, dry matter accumulation, leaves chlorophyll contents, longest spikes, number of spikes/spikelet and total soluble protein content in shoots were improved. The grain yield of wheat was increased ∼20% by 6min treatment with H 2 O/O 2 plasma with respect to control.

Investigation of biomass surface modification using non-thermal plasma treatment

Mooktzeng LIM (林木森) and Ahmad Zulazlan Shah ZULKIFLI
Plasma Sci. Technol. 2018, 20 (11): 115502 ;  doi: 10.1088/2058-6272/aac819
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The effects of non-thermal plasma (NTP) treatment on biomass in the form of pulverized palm- based empty fruit bunches (EFB) are investigated. Specifically, this study investigates the effects of NTP treatment on the surface reactivity, morphology, oxygen-to-carbon (O/C) ratio of the EFB at varying treatment times. The surface reactivity is determined by the reaction of antioxidant functional groups or reactive species with 2,2-diphenyl-1-picrylhydrazyl (DPPH). By measuring the concentration of the DPPH with a spectrophotometer, the change in the amount of antioxidant functional groups can be measured to determine the surface reactivity. The reactions of the various lignin components in the EFB with respect to the NTP treatment are discussed by qualitatively assessing the changes in the Fourier transform infrared (FTIR) spectra. The surface morphology is examined by a scanning electron microscope. To determine the amount of oxygen deposited on the EFB by the air-based NTP treatment, the oxygen and carbon contents are measured by an energy dispersive x-ray detector to determine the O/C ratio. The results show that the NTP reactor produced reactive species such as atomic oxygen and ozone, increasing the surface reactivity and chemical scavenging rate of the EFB. Consequently, the surface morphology changed, with an observed rougher surface from the images of the EFB samples. The change in the appearance of the surface is accompanied by a high O/C ratio, and is caused by reactions of certain components of lignin due to the NTP treatment. The lignin component that was modified is believed to be syringyl, as the syringyl portion in the lignin of EFBs is higher compared to the other components. Syringyl components are detected in the range of FTIR wavenumbers of 1109–1363cm −1 . With increasing NTP treatment times, the absorbance (of the peaks in the FTIR spectra) for syringyl related C−H and lignin associated C=C bonds decreases as the syringyl decomposes. The resulting release of carboxyl compounds increases the absorbance for the carbonyl C=O group. The results show that NTP treatment is able to modify the surface properties of EFB, and that the surface reactivity can be increased to improve their conversion and processing efficiencies

Experimental study on bacteria disinfection using a pulsed cold plasma jet with helium/ oxygen mixed gas

Guanlei DENG (邓官垒) 1, Qikang JIN (金杞糠) 1, Shengyong YIN (殷胜勇) 2, et al.
Plasma Sci. Technol. 2018, 20 (11): 115503 ;  doi: 10.1088/2058-6272/aacaee
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Cold atmospheric plasma jet is widely used in many fields due to the reactive oxygen species and low temperature for heat-sensitive products. This paper presents the inactivation of bacteria via a pulsed plasma jet with He/O2 mixed gas. To evaluate the disinfection performance, Staphylococcus aureus was used as an indicator bacteria for experiments. When the plasma jet dealt with agar plates spraying bacteria, it was found that mixed gas has a better performance than pure inert gas, indicated by the disinfection area. The increment of oxygen gas addition was beneficial to the disinfection ability of the plasma jet, while the gas had an opposite effect on the length of jet production. The experiments showed the efficacy of Staphylococcus aureus disinfection could reach up to 99.47% via a helium/oxygen (2%) plasma jet.

Analysis of flow separation control using nanosecond-pulse discharge plasma actuators on a flying wing

Zheng LI (李铮) 1, Zhiwei SHI (史志伟) 1, Hai DU (杜海) 2, et al.
Plasma Sci. Technol. 2018, 20 (11): 115504 ;  doi: 10.1088/2058-6272/aacaf0
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Abstract Dielectric barrier discharge (DBD) plasma is one of most promising flow control method for its several advantages. The present work investigates the control authority of nanosecond pulse DBD plasma actuators on a flying wing model’s aerodynamic characteristics. The aerodynamic forces and moments are studied by means of experiment and numerical simulation. The numerical simulation results are in good agreement with experiment results. Both results indicate that the NS-DBD plasma actuators have negligible effect on aerodynamic forces and moment at the angles of attack smaller than 16°. However, significant changes can be achieved with actuation when the model’s angle of attack is larger than 16° where the flow separation occurs. The spatial flow field structure results from numerical simulation suggest that the volumetric heat produced by NS-DBD plasma actuator changes the local temperature and density and induces several vortex structures, which strengthen the mixing of the shear layer with the main flow and delay separation or even reattach the separated flow.

Fusion Engineering

Design of the poloidal field system for KTX

Wei YOU (尤玮) 1, Hong LI (李弘) 1, Wenzhe MAO (毛文哲) 1, et al.
Plasma Sci. Technol. 2018, 20 (11): 115601 ;  doi: 10.1088/2058-6272/aac8d5
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The design of the poloidal field (PF) system includes the ohmic heating field system and the equilibrium (EQ) field system, and is the basis for the design of a magnetic confinement fusion device. A coupling between the poloidal and plasma currents, especially the eddy current in the stabilizing shell, yields design difficulties. The effects of the eddy current in the stabilizing shell on the poloidal magnetic field also cannot be ignored. A new PF system design is thus proposed. By using a low-μ material (μ=0.001, ε=1) instead of a conductive shell, an electromagnetic model is established that can provide a continuous eddy current distribution on the conductive shell. In this model, a 3D time-domain problem with shells translates into a 2D magnetostatic problem, and the accuracy of the calculation is improved. Based on these current distributions, we design the PF system and analyze how the EQ coils and conductive shell affect the plasma EQ when the plasma ramps up. To meet the mainframe design requirements and achieve an efficient power-supply design, the position and connection of the poloidal coils are optimized further.

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