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 Plasma Sci. Technol.
 Plasma Sci. Technol.--2018, 20 (7)   Published: 27 June 2018
The 1st Asia-Pacific Conference on Plasma Physics
Carlo POGGI, Théo GUILLAUME, Fabrice DOVEIL, et al
Plasma Sci. Technol. 2018, 20 (7): 074001 ;  doi: 10.1088/2058-6272/aabde3
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Show Abstract ( 48 )

The electric field induced Lyman-α emission diagnostic aims to provide a non intrusive and precise measurement of the electric field in plasma, using a beam of hydrogen atoms prepared in the metastable 2s state. The metastable particles are obtained by means of a proton beam extracted from a hydrogen plasma source, and neutralised by interaction with vaporised caesium. When a 2s atom enters a region where an electric field is present, it undergoes a transition to the 2p state (Stark mixing). It then quickly decays to the ground level, emitting Lyman-α radiation, which is collected by a photomultiplier. The 2s → 2p transition rate is proportional to the square of the magnitude of the electric field, and depends on the field oscillation frequency (with peaks around 1 GHz). By measuring the intensity of the Lyman-α radiation emitted by the beam it is possible to determine the magnitude of the field in a defined region. In this work, an analysis of the behaviour of the diagnostic under static or radiofrequency electric field is presented. Electric field simulations obtained with a finite element solver of Maxwell equations, combined with theoretical calculations of the Stark mixing transition rate, are used to develop a model for the interpretation of photomultiplier data. This method shows good agreement with experimental results for the static field case, and allows to measure the field magnitude for the oscillating case.

R L TANNA, J GHOSH, Harshita RAJ, et al
Plasma Sci. Technol. 2018, 20 (7): 074002 ;  doi: 10.1088/2058-6272/aabb4f
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Show Abstract ( 41 )

The Ohmically heated circular limiter tokamak ADITYA (R 0 = 75 cm, a = 25 cm) has been upgraded to a tokamak named the ADITYA Upgrade (ADITYA-U) with an open divertor configuration with divertor plates. The main goal of ADITYA-U is to carry out dedicated experiments relevant for bigger fusion machines including ITER, such as the generation and control of runaway electrons, disruption prediction, and mitigation studies, along with an improvement in confinement with shaped plasma. The ADITYA tokamak was dismantled and the assembly of ADITYA-U was completed in March 2016. Integration of subsystems like data acquisition and remote operation along with plasma production and preliminary plasma characterization of ADITYA-U plasmas are presented in this paper.

D B MELROSE
Plasma Sci. Technol. 2018, 20 (7): 074003 ;  doi: 10.1088/2058-6272/aab966
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Show Abstract ( 35 )

It is widely accepted that solar flares involve release of magnetic energy stored in the solar corona above an active region, but existing models do not include the explicitly time-dependent electrodynamics needed to describe such energy release. A flare paradigm is discussed that includes the electromotive force (EMF) as the driver of the flare, and the flare-associated current that links different regions where magnetic reconnection, electron acceleration, the acceleration of mass motions and current closure occur. The EMF becomes localized across regions where energy conversion occurs, and is involved in energy propagation between these regions.

Lu WANG (王璐), Shuitao PENG (彭水涛) and P H DIAMOND
Plasma Sci. Technol. 2018, 20 (7): 074004 ;  doi: 10.1088/2058-6272/aab5bc
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Show Abstract ( 46 )

Understanding the generation of intrinsic rotation in tokamak plasmas is crucial for future fusion reactors such as ITER. We proposed a new mechanism named turbulent acceleration for the origin of the intrinsic parallel rotation based on gyrokinetic theory. The turbulent acceleration acts as a local source or sink of parallel rotation, i.e., volume force, which is different from the divergence of residual stress, i.e., surface force. However, the order of magnitude of turbulent acceleration can be comparable to that of the divergence of residual stress for electrostatic ion temperature gradient (ITG) turbulence. A possible theoretical explanation for the experimental observation of electron cyclotron heating induced decrease of co-current rotation was also proposed via comparison between the turbulent acceleration driven by ITG turbulence and that driven by collisionless trapped electron mode turbulence. We also extended this theory to electromagnetic ITG turbulence and investigated the electromagnetic effects on intrinsic parallel rotation drive. Finally, we demonstrated that the presence of turbulent acceleration does not conflict with momentum conservation.

Yashika GHAI, Nimardeep KAUR, Kuldeep SINGH, et al
Plasma Sci. Technol. 2018, 20 (7): 074005 ;  doi: 10.1088/2058-6272/aab491
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Show Abstract ( 38 )

We have presented a theoretical study of the dust acoustic (DA) shock structures in a magnetized, electron depleted dusty plasma in the presence of two temperature superthermal ions. By deriving a Korteweg–de Vries–Burgers equation and studying its shock solution, we aim to highlight the effects of magnetic field and obliqueness on various properties of the DA shock structures in the presence of kappa-distributed two temperature ion population. The present model is motivated by the observations of Geotail spacecraft in the Earth's magnetotail and it is seen that the different physical parameters such as superthermality of the cold and hot ions, the cold to hot ion temperature ratio, the magnetic field strength, obliqueness and the dust kinematic viscosity greatly influence the dynamics of the shock structures so formed. The results suggest that the variation of superthermalities of the cold and hot ions have contrasting effects on both positive and negative polarity shock structures. Moreover, it is noted that the presence of the ambient magnetic field affects the dispersive properties of the medium and tends to make the shock structures less wide and more abrupt. The findings of present investigation may be useful in understanding the dynamics of shock waves in dusty plasma environments containing two temperature ions where the electrons are significantly depleted.

Bo SHI (史博), Jinhong YANG (杨锦宏), Cheng YANG (杨程), et al
Plasma Sci. Technol. 2018, 20 (7): 074006 ;  doi: 10.1088/2058-6272/aab48e
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Show Abstract ( 50 )

The tokamak simulation code (TSC) is employed to simulate the complete evolution of a disruptive discharge in the experimental advanced superconducting tokamak. The multiplication factor of the anomalous transport coefficient was adjusted to model the major disruptive discharge with double-null divertor configuration based on shot 61 916. The real-time feed-back control system for the plasma displacement was employed. Modeling results of the evolution of the poloidal field coil currents, the plasma current, the major radius, the plasma configuration all show agreement with experimental measurements. Results from the simulation show that during disruption, heat flux about 8 MW m−2 flows to the upper divertor target plate and about 6 MW m−2 flows to the lower divertor target plate. Computations predict that different amounts of heat fluxes on the divertor target plate could result by adjusting the multiplication factor of the anomalous transport coefficient. This shows that TSC has high flexibility and predictability.

Jan WEILAND
Plasma Sci. Technol. 2018, 20 (7): 074007 ;  doi: 10.1088/2058-6272/aab20d
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Show Abstract ( 54 )

We will give an overview of results obtained by our reactive fluid model. It is characterised as a fluid model where all moments with sources in the experiment are kept. Furthermore, full account is taken for the highest moments appearing in unexpanded denominators also including full toroidicity. It has been demonstrated that the strength of zonal flows is dramatically larger in reactive fluid closures than in those which involve dissipation. This gives a direct connection between the fluid closure and the level of excitation of turbulence. This is because zonal flows are needed to absorb the inverse cascade in quasi 2D turbulence. This also explains the similarity in structure of the transport coefficients in our model with a reactive closure in the energy equation and models which have a reactive closure because of zero ion temperature such as the Hasegawa–Wakatani model. Our exact reactive closure unifies several well-known features of tokamak experiments such as the L–H transition, internal transport barriers and the nonlinear Dimits upshift of the critical gradient for onset of transport. It also gives transport of the same level as that in nonlinear gyrokinetic codes. Since these include the kinetic resonance this confirms the validity of the thermodynamic properties of our model. Furthermore, we can show that while a strongly nonlinear model is needed in kinetic theory a quasilinear model is sufficient in the fluid description. Thus our quasilinear fluid model will be adequate for treating all relevant problems in bulk transport. This is finally confirmed by the reproduction by the model of the experimental power scaling of the confinement time τ E ~ P -2/3. This confirms the validity of our reactive fluid model. This also gives credibility to our ITER simulations including the H-mode barrier. A new result is here, that alpha heating strongly reduces the slope of the H-mode barrier. This should significantly reduce the effects of ELM's.

Lei YE (叶磊), Xiaotao XIAO (肖小涛), Yingfeng XU (徐颖峰), et al
Plasma Sci. Technol. 2018, 20 (7): 074008 ;  doi: /10.1088/2058-6272/aac013
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Show Abstract ( 46 )

Field-aligned coordinates have been implemented in the gyrokinetic semi-Lagrangian code NLT, Ye et al (2016 J. Comput. Phys. 316 180), to improve the computational efficiency for the numerical simulations of tokamak turbulence and transport. 4D B-spline interpolation in field-aligned coordinates is applied to solve the gyrokinetic Vlasov equation. A fast iterative algorithm is proposed for efficiently solving the quasi-neutrality equation. A pseudo transform method is used for the numerical integration of the gyro-average operator for perturbations with a high toroidal mode number. The new method is shown to result in an improved code performance for reaching a given accuracy. Some numerical tests are presented to illustrate the new methods.

Nimardeep KAUR, Kuldeep SINGH, Yashika GHAI, et al
Plasma Sci. Technol. 2018, 20 (7): 074009 ;  doi: 10.1088/2058-6272/aac37a
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The propagation characteristics of dust acoustic solitary and rogue waves are investigated in an unmagnetized ion beam plasma with electrons and ions following kappa-type distribution in nonplanar geometry. The reductive perturbation method (RPM) is employed to derive the cylindrical/spherical Korteweg–de Vries (KdV) equation, which is further transformed into standard KdV equation by neglecting the geometrical effects. Using new stretching coordinates, nonlinear Schr?dinger equation (NLSE) has been derived from the standard KdV equation to study the different order rational solutions of dust acoustic rogue waves (DARWs). The impact of various physical parameters on the characteristics of dust acoustic solitary waves (DASWs) is elaborated specifically in nonplanar geometry. Further, the effects of ion beam and superthermality of electrons/ions on the characteristics of DARWs are studied. The results obtained in the present investigation may be useful in comprehending a variety of phenomena in Earth's magnetosphere polar cap region where the presence of positive ion beam has been detected and also in other regions of space/astrophysical environments where dust along with superthermal electrons and ions exists.

Tianheng XU (徐天衡), Gang XIONG (熊刚), Jun XIAO (肖君), et al
Plasma Sci. Technol. 2018, 20 (7): 074010 ;  doi: 10.1088/2058-6272/aac573
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Dielectronic recombination is an important process in high temperature plasmas. In the present work, the KLn (n = L, M, N and O) DR resonance strengths of He-like to O-like xenon ions are measured at the Shanghai electron beam ion trap using a fast electron beam energy scanning method. The experiment uncertainty reaches about 6% with significant improvement of statistics. A relativistic configuration interaction calculation is also made. Theoretical results agree with the experiment results within 15% in most cases.

Magnetically Confined Plasma
Wei WANG (王玮), Zhengxiong WANG (王正汹), Jiquan LI (李继全), et al
Plasma Sci. Technol. 2018, 20 (7): 075101 ;  doi: 10.1088/2058-6272/aab48f
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Characteristics of the magnetic-island-induced ion temperature gradient (MITG) mode are studied through gyrofluid simulations in the slab geometry, focusing on the effects of Landau damping, equilibrium magnetic shear (EMS), and pressure flattening. It is shown that the magnetic island may enhance the Landau damping of the system by inducing the radial magnetic field. Moreover, the radial eigenmode numbers of most MITG poloidal harmonics are increased by the magnetic island so that the MITG mode is destabilized in the low EMS regime. In addition, the pressure profile flattening effect inside a magnetic island hardly affects the growth of the whole MITG mode, while it has different local effects near the O-point and the X-point regions. In comparison with the non-zero-order perturbations, only the quasi-linear flattening effect due to the zonal pressure is the effective component to impact the growth rate of the mode.

Astrophysics and Space Plasma
Fusheng WANG (王富生), Xiangteng MA (马襄腾), Han CHEN (陈汉), et al
Plasma Sci. Technol. 2018, 20 (7): 075301 ;  doi: 10.1088/2058-6272/aab841
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Show Abstract ( 31 )

In order to solve the load problem for aircraft lightning strikes, lightning channel evolution is simulated under the key physical parameters for aircraft lightning current component C. A numerical model of the discharge channel is established, based on magnetohydrodynamics (MHD) and performed by FLUENT software. With the aid of user-defined functions and a user-defined scalar, the Lorentz force, Joule heating and material parameters of an air thermal plasma are added. A three-dimensional lightning arc channel is simulated and the arc evolution in space is obtained. The results show that the temperature distribution of the lightning channel is symmetrical and that the hottest region occurs at the center of the lightning channel. The distributions of potential and current density are obtained, showing that the difference in electric potential or energy between two points tends to make the arc channel develop downwards. The arc channel comes into expansion on the anode surface due to stagnation of the thermal plasma and there exists impingement on the copper plate when the arc channel comes into contact with the anode plate.

Low Temperature Plasma
J R LEGORRETA, J L PATINO and F B YOUSIF
Plasma Sci. Technol. 2018, 20 (7): 075401 ;  doi: 10.1088/2058-6272/aab48d
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Determination of the negative ion number density of  $\fn_jvn \100dpi \inline {{\rm{O}}}_{{\rm{2}}}^{-}$ and  $\fn_jvn \100dpi \inline {{\rm{O}}}^{-}$ in a DC discharge of oxygen plasma was made employing Langmuir probe in conjunction with eclipse laser photo-detachment technique. The temporal evolution of the extra electrons resulting from the photo-detachment of $\fn_jvn \100dpi \inline {{\rm{O}}}_{{\rm{2}}}^{-}$ and $\fn_jvn \100dpi \inline {{\rm{O}}}^{-}$ were used to evaluate the negative ion number density. The ratio of $\fn_jvn \100dpi \inline {{\rm{O}}}_{{\rm{2}}}^{-}$ number density to    $\fn_jvn \100dpi \inline {{\rm{O}}}^{-}$ varied from 0.03 to 0.22. Number density of both $\fn_jvn \100dpi \inline {{\rm{O}}}_{{\rm{2}}}^{-}$ and $\fn_jvn \100dpi \inline {{\rm{O}}}^{-}$ increased with increasing power and decreased as the pressure was increased. Electron number density was evaluated from the electron energy distribution function (EEDF) using the I–V recorded characteristic curves. Electron temperature between 2 and 2.7 eV were obtained. Influence of the $\fn_jvn \100dpi \inline {{\rm{O}}}_{{\rm{2}}}({a}^{{\rm{1}}}{{\rm{\Delta }}}_{{\rm{g}}})$ metastable state is discussed.

Gao ZHAO (赵高), Wanying ZHU (朱婉莹), Huihui WANG (王慧慧), et al
Plasma Sci. Technol. 2018, 20 (7): 075402 ;  doi: 10.1088/2058-6272/aab4f1
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In this work we used a passive measurement method based on a high-impedance electrostatic probe and an optical emission spectroscope (OES) to investigate the characteristics of the double layer (DL) in an argon helicon plasma. The DL can be confirmed by a rapid change in the plasma potential along the axis. The axial potential variation of the passive measurement shows that the DL forms near a region of strong magnetic field gradient when the plasma is operated in wave-coupled mode, and the DL strength increases at higher powers in this experiment. The emission intensity of the argon atom line, which is strongly dependent on the metastable atom concentration, shows a similar spatial distribution to the plasma potential along the axis. The emission intensity of the argon atom line and the argon ion line in the DL suggests the existence of an energetic electron population upstream of the DL. The electron density upstream is much higher than that downstream, which is mainly caused by these energetic electrons.

Hafiz Imran Ahmad QAZI, Yiying XIN (辛怡颖), Muhammad Ajmal KHAN, et al
Plasma Sci. Technol. 2018, 20 (7): 075403 ;  doi: 10.1088/2058-6272/aab4f2
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In this paper, the AC-excited helium discharges generated between the powered needle electrode enclosed in a conical quartz tube and the grounded de-ionized water electrode are investigated. The current and voltage waveforms exhibit a transition from the glow-like to streamer-like mode discharges, which forms a stable cone-shaped structure at the gas–liquid interface. In this region, the air and water vapor diffusion initiate various physical–chemical processes leading to substantial changes of the primary species emission intensities (e.g., OH, N2, NO, and O) and the rotational temperatures. The experimentally measured rotational temperature at the gas–liquid interface is 870 K from the N2(C–B) band with a power input of 26 W. With the prolongation of the discharge time, significant changes in the discharge voltage and current, discharge emission patterns, instantaneous concentrations of the secondary species (e.g., H2O2$\fn_jvn \100dpi \inline {{\rm{NO}}}_{2}^{-},$ and $\fn_jvn \100dpi \inline {{\rm{NO}}}_{3}^{-}$) in the liquid phase, pH values and electrical conductivities of the liquids are observed experimentally. The present study is helpful for deepening the understandings to the basic physical–chemical processes in the discharges in contact with liquids, especially to those occurring in the vicinity of the gas–liquid interface, and also for promoting existing and potential applications of such type of discharges in the fields of environmental protection, biomedicine, agriculture, and so on.

Sen WANG (王森), Dezheng YANG (杨德正), Feng LIU (刘峰), et al
Plasma Sci. Technol. 2018, 20 (7): 075404 ;  doi: 10.1088/2058-6272/aabac8
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Show Abstract ( 33 )

Atmospheric gas-liquid discharge with argon as a working gas is presented by employed nanosecond pulse power. The discharge is presented in a glow-like mode. The discharge powers are determined to be less than 1 W, and remains almost constant when the discharge duration time increases. Bountiful active species are determined by capturing optical emission spectra, and their main generation processes are also discussed. The plasma gas temperature is calculated as 350 K by comparing the experimental spectra and the simulated ones of $\fn_jvn \100dpi \inline {{\rm{N}}}_{2}({\rm{C}}{}^{3}{\rm{\Pi }}_{{\rm{g}}}\to {\rm{B}}{}^{3}{\rm{\Pi }}_{{\rm{g}}},{\rm{\Delta }}{\rm{\nu }}=-2).$ The time resolved vibrational and rotational temperature is researched to present the stability of discharge when pulse voltage and discharge duration vary. The electron density is determined to be 1016 cm−3 according to the Stark broadening effect of the Hα line.

Plasma Technology
Junying WU (伍俊英), Long WANG (汪龙), Yase LI (李雅瑟), et al
Plasma Sci. Technol. 2018, 20 (7): 075501 ;  doi: 10.1088/2058-6272/aab783
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To improve the energy utilization efficiency of metal bridge foil explosion, and increase the function range of plasmas, array bridge foil explosion experiments with different structures were performed. A Schlieren photographic measurement system with a double-pulse laser source was used to observe the flow field of a bridge foil explosion. The evolution laws of plasmas and shock waves generated by array bridge foil explosions of different structures were analyzed and compared. A multi-phase flow calculation model was established to simulate the electrical exploding process of a metal bridge foil. The plasma equation of state was determined by considering the effect of the changing number of particles and Coulomb interaction on the pressure and internal energy. The ionization degree of the plasma was calculated via the Saha–Eggert equation assuming conditions of local thermal equilibrium. The exploding process of array bridge foils was simulated, and the superposition processes of plasma beams were analyzed. The variation and distribution laws of the density, temperature, pressure, and other important parameters were obtained. The results show that the array bridge foil has a larger plasma jet diameter than the single bridge foil for an equal total area of the bridge foil. We also found that the temperature, pressure, and density of the plasma jet's center region sharply increase because of the superposition of plasma beams.

Liqiu WEI (魏立秋), Wenbo LI (李文博), Yongjie DING (丁永杰), et al
Plasma Sci. Technol. 2018, 20 (7): 075502 ;  doi: 10.1088/2058-6272/aabae0
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In this paper, a direct connection between the discharge current amplitude and the thruster performance is established by varying solely the capacitance of the filter unit of the Hall thrusters. To be precise, the variation characteristics of ion current, propellant utilization efficiency, and divergence angle of plume at different low-frequency oscillation amplitudes are measured. The findings demonstrate that in the case of the propellant in the discharge channel just meets or falls below the full ionization condition, the increase of low-frequency oscillation amplitude can significantly enhance the ionization degree of the neutral gas in the channel and increase the thrust and anode efficiency of thruster. On the contrary, the increase in the amplitude of low-frequency oscillation will lead to increase the loss of plume divergence, therefore the thrust and anode efficiency of thruster decrease.

Le YANG (杨乐), Tianping ZHANG (张天平), Juanjuan CHEN (陈娟娟), et al
Plasma Sci. Technol. 2018, 20 (7): 075503 ;  doi: 10.1088/2058-6272/aac012
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A two-dimensional particle-in-cell plasma model is built in the R–Z plane to investigate the low-frequency plasma oscillations in the discharge channel of a 5 kW LHT-140 Hall thruster. In addition to the elastic, excitation, and ionization collisions between neutral atoms and electrons, the Coulomb collisions between electrons and electrons and between electrons and ions are analyzed. The sheath characteristic distortion is also corrected. Simulation results indicate the capability of the built model to reproduce the low-frequency oscillation with high accuracy. The oscillations of the discharge current and ion density produced by the model are consistent with the existing conclusions. The model predicts a frequency that is consistent with that calculated by the zero-dimensional theoretical model.

Dan LUO (罗丹), Ying LIU (刘英), Xiangyu LI (李香宇), et al
Plasma Sci. Technol. 2018, 20 (7): 075504 ;  doi: 10.1088/2058-6272/aabc5d
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A diode-pumped solid-state laser (DPSSL) with a high energetic stability and long service life is applied to ablate the steel samples instead of traditional Nd:YAG laser pumped by a xenon lamp, and several factors, such as laser pulse energy, repetition rate and argon flow rate, that influence laser-induced breakdown spectroscopy (LIBS) analytical performance are investigated in detail. Under the optimal experiment conditions, the relative standard deviations for C, Si, Mn, Ni, Cr and Cu are 3.3%–8.9%, 0.9%–2.8%, 1.2%–4.1%, 1.7%–3.0%, 1.1%–3.4% and 2.5%–8.5%, respectively, with the corresponding relative errors of 1.1%–7.9%, 1.0%–6.3%, 0.4%–3.9%, 1.5%–6.3%, 1.2%–4.0% and 1.2%–6.4%. Compared with the results of the traditional spark discharge optical emission spectrometry technique, the analytical performance of LIBS is just a little inferior due to the less stable laser-induced plasma and smaller amount of ablated sample by the laser. However, the precision, detection limits and accuracy of LIBS obtained in our present work were sufficient to meet the requirements for process analysis. These technical performances of higher stability of output energy and longer service life for DPSSL, in comparison to the Q-switch laser pumped by xeon lamp, qualify it well for the real time online analysis for different industrial applications.

Fusion Engineering