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Numerical simulation and experiment of error filed measurement using luminous trace of electron beam in SST-1
Plasma Sci. Technol.    2019, 21(10): 105101; doi: 10.1088/2058-6272/ab294
Abstract15)      PDF (3989KB)(24)      
In contrast to the earlier experiments conducted in other machines, here, in SST-1 the error field measurement experiment is performed with a filled gas pressure ∼8×10−4 mbar which helped to create a luminescent toroidal beam of electron path originated due to impact excitation and guided by the toroidal magnetic field. Beam path deviations are observed and recorded from radial and top ports using visible range cameras. Such creation and detection of the electron beam path differs from the earlier works where the gun emitted electron beam deviation in ultrahigh vacuum was detected on a collector-grid/fluorescent screen. In the present experiment, large beam deviations were observed. Later investigation of the experimental set-up reveals existence of a possible source of radial electric field in between the source and the vacuum vessel which are separately grounded. Thus, to understand the observed phenomena, experiments are numerically modeled with deviated TF coil set, PF coil set and the electron source location. A particle tracing code is used to follow the electron path in the magnetic field generated by the coil set of interest. Simulation results suggest that the large deviation corresponds to the E×B drifts and not due to the large field errors. Toroidally averaged field errors of the SST-1 TF coils at toroidal field of B0=15 kG are negligibly small ∼B0×10−6 or less, which should not adversely affect the plasma performance.
Spectroscopic studies of fuel recycling and impurity behaviors in the divertor region of Wendelstein 7-X
Yanling WEI (魏彦玲), Erhui WANG (王二辉), Yunfeng LIANG (梁云峰), et al.
Plasma Sci. Technol.    2019, 21(10): 105102; doi: 10.1088/2058-6272/ab273b
Abstract29)      PDF (3161KB)(17)      
The first divertor operation phase (OP1.2a) was carried out on Wendelstein 7-X in the second half of 2017. Fuel recycling and impurity behaviors in the divertor region were investigated by employing a newly built ultraviolet–visible–near infrared overview spectroscopy system. The characteristic spectral lines of the working gases (hydrogen and helium), intrinsic impurities (carbon, oxygen and iron), and seeded impurities (neon and nitrogen) were identified and analyzed. The divertor electron temperature and density were measured using He I (667.8, 706.5, and 728.1 nm) line intensity ratios. The Hα (656.3 nm), He I (587.6 nm), C II (514.5 nm), and O I (777.2 nm) emissions were investigated over a wide range of operating conditions. The results showed that fuel and impurity emissions in the divertor region exhibit a strong dependence on magnetic topology and plasma conditions. The levels of Hα, He I, C II, and O I emissions are all reduced moving from the standard configuration to the high mirror configuration, and even further reduced for the high iota configuration, which is associated with decreasing connection length in these island divertor configurations. The H/He influx ratio shows that the plasma is a mixture of helium and hydrogen. The neutral and impurity influxes from the divertor target tend to increase with increasing divertor electron temperature.
Fast estimation of ion temperature from EAST charge exchange recombination spectroscopy using neural network
Baoyue CHAI (柴宝玥), Yingying LI (李颖颖) , Ze CHEN (陈泽), et al.
Plasma Sci. Technol.    2019, 21(10): 105103; doi: 10.1088/2058-6272/ab2674
Abstract16)      PDF (999KB)(10)      
Ion temperature, as one of the most critical plasma parameters, can be diagnosed by charge exchange recombination spectroscopy (CXRS). Iterative least-squares fitting is conventionally used to analyze CXRS spectra to identify the active charge exchange component, which is the result of local interaction between impurity ions with a neutral beam. Due to the limit of the time consumption of the conventional approach (∼100 ms per frame), the Experimental Advanced Superconducting Tokamak CXRS data is now analyzed in-between shots. To explore the feasibility of real-time measurement, neural networks are introduced to perform fast estimation of ion temperature. Based on the same four-layer neural network architecture, two neural networks are trained for two central chords according to the ion temperature data acquired from the conventional method. Using the TensorFlow framework, the training procedures are performed by an error back-propagation algorithm with the regularization via the weight decay method. Good agreement in the deduced ion temperature is shown for the neural networks and the conventional approach, while the data processing time is reduced by 3 orders of magnitude (∼0.1 ms per frame) by using the neural networks.
Development of Zeff diagnostic system on the Globus-M (M2) tokamak and the first experimental results
Plasma Sci. Technol.    2019, 21(10): 105104; doi: 10.1088/2058-6272/ab305f
Abstract17)      PDF (1336KB)(17)      
Studying the behavior of effective ion charge Zeff, which indicates the degree of pollution of plasma and can provide valuable information about many processes taking place in plasma, mechanisms of the releasing impurities, transport, etc is of great interest. This paper describes the development of the Zeff diagnostic system for the Globus-M2 tokamak. The effective ion charge is determined on the basis of measurements of the bremsstrahlung intensity and Thomson scattering data—Te and ne profiles. The results of the first Zeff measurements obtained for several discharges on the Globus-M tokamak are presented in this paper. The results have been validated by simulation using the ASTRA transport code for the same discharges, and it has demonstrated a good agreement with the experimental results.
The application of limiter target electrostatic measurement system in J-TEXT tokamak
Jie YANG (阳杰), Zhipeng CHEN (陈志鹏), Hai LIU (刘海), et al.
Plasma Sci. Technol.    2019, 21(10): 105105; doi: 10.1088/2058-6272/ab2e7f
Abstract16)      PDF (1574KB)(8)      
The limiter target electrostatic measurement system including limiter grounding current sensors and Langmuir probes have been newly developed for the measurement of the limiter target area on the Joint-Texas Experimental tokamak (J-TEXT). Current sensors fixed between graphite limiters and the vacuum vessel walls are used to measure the currents between limiters and vessel wall. Simultaneously, a rectangular poloidal array containing 54 Langmuir probes is embedded in the graphite tiles of limiters for a more localized measurement. Based on this system, the effect of both the plasma’s inherent behavior, including plasma motion and the 2/1 tearing mode, and the electrode biasing on probe and sensor signals have been observed and analyzed in the experiments.
Observation of filament-like structures in ELMy H-mode plasma with a VUV imaging system developed on the EAST tokamak
Fan ZHOU (周凡), Tingfeng MING (明廷凤), Yumin WANG (王嵎民), et al.
Plasma Sci. Technol.    2019, 21(9): 095101; doi: 10.1088/2058-6272/ab1b1b
Abstract106)      PDF (2547KB)(109)      
On the EAST tokamak, filament-like structures have been observed in ELMy H-mode discharges with a high-speed vacuum ultraviolet (VUV) imaging system. The topos, chronos and their weight can be obtained simultaneously by performing the so-called singular value decomposition (SVD) analysis of raw VUV imaging data. The fluctuation amplitude is observed to be suppressed and enhanced gradually in the edge localized mode (ELM) crash and pedestal recovery phase in the chronos, respectively, while filament-like structures can only be found in the pedestal recovery phase on the topos. The mode structure, i.e. m/n=36/9 (m and n are the poloidal and toroidal mode number, respectively) with ρ0=0.95, w0=0.07 (ρ0and w0 denote the mode location and mode width, respectively) is derived by a comparison of the synthetic images and the experimental imaging data.
Symmetry transformations for magnetohydrodynamics and Chew–Goldberger–Low equilibria revisited
Plasma Sci. Technol.    2019, 21(9): 095102; doi: 10.1088/2058-6272/ab2136
Abstract42)      PDF (1389KB)(75)      
We generalize the symmetry transformations for magnetohydrodynamic (MHD) equilibria with isotropic pressure and incompressible flow parallel to the magnetic field introduced by Bogoyavlenskij in the case of the respective Chew–Goldberger–Low (CGL) equilibria with anisotropic pressure. We find that the geometrical symmetry of the field-aligned equilibria can be broken by those transformations only when the magnetic field is purely poloidal. In this situation we derive three-dimensional CGL equilibria from given axisymmetric ones. Also, we examine the generic symmetry transformations for MHD and CGL equilibria with incompressible flow of an arbitrary direction, introduced in a number of papers, and find that they cannot break the geometrical symmetries of the original equilibria, unless the velocity and magnetic field are collinear and purely poloidal.
Spatial localization of ECE measurement in EAST LHW-heated plasmas
Yushu ZUO (左雨澍), Yong LIU (刘永), Tianfu ZHOU (周天富), et al.
Plasma Sci. Technol.    2019, 21(9): 095103; doi: 10.1088/2058-6272/ab247f
Abstract42)      PDF (891KB)(66)      
In this work, electron cyclotron emission (ECE) is simulated by using the code SPECE to study the spatial localization of ECE measurement in EAST plasmas heated by lower hybrid wave (LHW). The results indicate that generally there are two emission layers for an individual frequency in plasmas with non-thermal electrons, and they are separately attributed to the thermal electrons and non-thermal electrons. The emission layer due to the thermal electrons is nearly identical to that for the case with Maxwellian distribution. The emission layer due to non-thermal electrons is well localized in the location of the non-thermal electrons. Even though the non-thermal emission layer is broader, the emission intensity is smaller than that from the thermal emission layer for the cases studied in this work. Localized electron temperature fluctuations can still be distinguished by ECE measurement as long as it does not coexist with the non-thermal electrons. Sawtooth inversion radii and tearing mode island location determined respectively by the ECE measurement and the soft x-ray measurement for a LHW-heated plasma show a good agreement, and this indicates that the ECE measurement in the plasma core region is not seriously polluted.
Alfvén eigenmode stability analysis and energetic particle transport prediction for CFETR hybrid scenario
Yunpeng ZOU (邹云鹏) and Minyou YE (叶民友)
Plasma Sci. Technol.    2019, 21(9): 095104; doi: 10.1088/2058-6272/ab2110
Abstract41)      PDF (1078KB)(77)      
The hybrid scenario is a projection for CFETR operation with high plasma current and density. Therefore, the energetic particles (EPs) generated by fusion reactions can destabilize Alfvén eigenmodes (AEs), which could result in significant EPs loss and redistribution. Both the eigenvalue code NOVA-K and the wrapped local stability code TGLFEP are used to analyze AE stability. The simulation indicates the beta-induced Alfvén eigenmodes with n>5 in the core region are the most unstable. The NOVA-K code is used to benchmark the critical density gradient calculated by TGLFEP. In addition, the EPtran code is employed to predict EP transport induced by unstable AEs and turbulence, which reduce EP density in the core and drive approximately 30% EP transport from the core to the edge, thus the EP density profile flattens and EPs with lower energy deposit near the edge.
Investigation of ion fishbone stability on HL-2A using NIMROD
Yaorong YANG (杨耀荣), Yawei HOU (候雅巍), Wei CHEN (陈伟), et al.
Plasma Sci. Technol.    2019, 21(8): 085101; doi: 10.1088/2058-6272/ab1295
Abstract124)      PDF (2718KB)(102)      

Numerical calculations of resistive internal kink mode with effects of energetic particles (EPs) on HL-2A have been performed using the hybrid kinetic-MHD model inplemented in the NIMROD code. The m/n=1/1 resistive internal kink mode is unstable in MHD limit. However, with kinetic effects of energetic ions, a fishbone mode is excited with mode frequency around 10 kHz. We calculate the impact of resistivity on the growth rate and frequency of ion fishbone mode, and the results are in good agreement with the analytic solutions, which are obtained by solving the fishbone dispertion relation including resistivity effect. The effects of βfrac and cut off velocity of EP on fishbone mode are calculated in detail, where βfrac is the ratio of EP pressure to background plasma pressure. This work presents a clear explanation of the stabilizing effect of ECRH on ion fishbone, which is first observed on HL-2A.

Translation speed measurements of hydrogen, helium, and argon field-reversed configurations in the central cell of a KMAX mirror device
Hui LIAO (廖晖), Munan LIN (林木楠), Ming LIU(刘明), et al.
Plasma Sci. Technol.    2019, 21(8): 085102; doi: 10.1088/2058-6272/ab19e8
Abstract58)      PDF (2555KB)(47)      
A series of experimental results of field-reversed configurations (FRCs) on a KMAX (Keda Mirror with AXisymmetricity) tandem mirror machine are reported. Single-side FRC translation processes with three different gas species were measured by avalanche photodiodes. Consistent with the theoretical prediction, the measured FRC speeds were inversely proportional to the square root of the ion mass. However, the speeds of the hydrogen FRC increased even in a uniform magnetic field region while the speeds of the helium and argon FRCs decreased. Possible mechanisms are discussed. The speed of the second pass due to the reflection of the mirror fields was found to be ∼1/3 of the first pass speed. The internal magnetic fields were
measured for a colliding-merging argon FRC, and the results show that, even for very slowmoving FRCs, merging can occur.
Study of nonlinear mode–mode couplings between Alfvénic modes by the Fourier bicoherence and Lissajous-curve technique in HL-2A
Yumei HOU (侯玉梅), Wei CHEN (陈伟), Yi YU (余羿), et al.
Plasma Sci. Technol.    2019, 21(7): 075101; doi: 10.1088/2058-6272/ab08fe
Abstract29)      PDF (3382KB)(103)      

Nonlinear couplings of various Alfvén modes driven by energetic particles in HL-2A are addressed by employing the Fourier bicoherence and Lissajous-curve technique. Long-lived modes and high-frequency coherent modes are presented. Then the squared bicoherence of three waves establishes the existence of three-wave coupling. Lissajous-curves of those waves manifest that their phases are locked, which again confirms that they are nonlinearly coupled to each other. Moreover, coupled modes triggered by supersonic molecular beam injection are investigated. The phase evolution of them is given by the Lissajous-curve. Further details of phase-flip and phase-slip are presented and discussed.

Evaluation of electron cyclotron current drive performance for CFETR
Wei WEI (韦维) 1, Xiaojie WANG (王晓洁) 2, Miaohui LI (李妙辉) 2, et al.
Plasma Sci. Technol.    2019, 21(6): 065101; doi: 10.1088/2058-6272/ab0841
Abstract20)      PDF (2174KB)(79)      

A number of simulations of electron cyclotron current drive (ECCD) have been carried out for the China Fusion Engineering Test Reactor (CFETR) using the C3PO/LUKE code to investigate the performance and optimize schemes of power injection for the design of the launcher. The operation ranges of the toroidal field, cutoff density, and resonance layer location are given at different source frequencies in CFETR phases I and II. A comparison of ECCD performance between the horizontal and top port launch is presented. ECCD efficiency (rEC) estimated for CFETR phase I is rEC=0.21 for top port launch and rEC=0.20 for horizontal port launch. The ECCD efficiency and second-harmonic absorption is calculated at different wave frequencies (from 170 to 230 GHz) in CFETR phase II. It is found that the highest driven efficiency is obtained at 210 GHz with the toroidal field of 6.5 T, and the second-harmonic absorption increases rapidly with the increase of frequency.

Soft x-ray tomographic reconstruction of Heliotron J plasma for the study of magnetohydrodynamic equilibrium and stability
S PUROHIT 1, Y SUZUKI 1,2, S OHDACHI 1,2, et al.
Plasma Sci. Technol.    2019, 21(6): 065102; doi: 10.1088/2058-6272/ab0846
Abstract22)      PDF (2253KB)(70)      

The preparation for an experimental soft x-ray tomography study on the Heliotron J (H-J) machine is carried out, with the objectives of evaluating the capability of the current soft x-ray tomographic system in terms of the identification of different mode structures and their poloidal rotation, and the axis shift with different plasma and machine parameters, and fixing the physics goals for the experimental study. These preparations were carried out via a simulated soft x-ray data set arising from different plasma conditions, such as magnetic islands, low beta and high beta. Soft x-ray tomography (SXT) is performed by the discrete pixel method including singular value decomposition and Phillips–Tikhonov regularization, to obtain clear and smooth images. The H-J soft x-ray tomography results from simulated soft x-rays for the equilibrium H-J plasma sensed the magnetic axis shift clearly and an estimate was also achieved. Successful reconstruction for mode structure m=1/n=1 was obtained along with the realization of the poloidal rotation of the structure. The reconstruction for the m=2/n=1 mode was not very clear for the current soft x-ray diagnostic design. Effective mode identification was not possible due to the lack of measurements. The SXT from the current soft x-ray diagnostic on H-J, the magnetic axis shift can be estimated and the m=1/n=1 mode can be studied. Study of higher poloidal modes is difficult with the current design.

Effects of resonant magnetic perturbation on locked mode of neoclassical tearing modes
Weikang TANG (汤炜康), Lai WEI (魏来), Zhengxiong WANG (王正汹), et al.
Plasma Sci. Technol.    2019, 21(6): 065103; doi: 10.1088/2058-6272/ab0a18
Abstract32)      PDF (1195KB)(40)      

The effects of externally applied resonant magnetic perturbation (RMP) on the locked mode of the neoclassical tearing mode (NTM) are numerically investigated by means of a set of reduced magnetohydrodynamic equations. It is found that, for a small bootstrap current fraction, three regimes, namely the slight suppression regime, the small locked island (SLI) regime and the big locked island (BLI) regime, are discovered with the increase of RMP strength. For a much higher bootstrap current fraction, however, a new oscillation regime appears instead of the SLI regime, although the other regimes still remain. The physical process in each regime is analyzed in detail based on the phase difference between the NTM and the RMP. Moreover, the critical values of the RMP in both SLI and BLI regimes are obtained, and their dependence on key plasma parameters is discussed as well.

Development of a fast electron bremsstrahlung diagnostic system based on LYSO and silicon photomultipliers during lower hybrid current drive for tokamak
Weiwei FAN (范伟伟) 1,2, Bowen ZHENG (郑博文) 1,2, Jing CAO (曹靖) 1,2, et al.
Plasma Sci. Technol.    2019, 21(6): 065104; doi: 10.1088/2058-6272/ab0a77
Abstract22)      PDF (2569KB)(77)      

A novel real time fast electron bremsstrahlung (FEB) diagnostic system based on the lutetium yttrium oxyorthosilicate scintillators (LYSO) and silicon photomultipliers (SiPM) has been developed for tokamak. The diagnostic system is dedicated to study the FEB emission in the hard x-ray (HXR) energy range between 10 and 200 keV during the lower hybrid current drive. The system consists of a detection module and three data acquisition and processing (DAP) boards. The detection module consists of annulus LYSO-SiPM detector array and a 12-channel preamplifier module. The DAP boards upload the data to the host computer for displaying and storing through PXI bus. The time and space resolutions of the system are 10 ms and 4cm, respectively. The experimental results can show the evolution over time and the spatial distribution of FEB. This paper presents the system performance and typical discharge results.

Magnetic field topology modeling under resonant magnetic perturbations on EAST
Jie HUANG (黄杰) , Yasuhiro SUZUKI (铃木康浩) , Yunfeng LIANG (梁云峰) , et al.
Plasma Sci. Technol.    2019, 21(6): 065105; doi: 10.1088/2058-6272/ab0d35
Abstract42)      PDF (2043KB)(71)      

In order to understand the mechanism by which the resonant magnetic perturbation (RMP) mitigates or suppresses the edge-localized mode (ELM), the topological study of the edge magnetic field in ELM mitigation or suppression phase is a critical issue. To model the three-dimensional magnetic field topology superposed RMP on Experimental Advanced Superconducting Tokamak, a numerical model using the field line tracing method for both vacuum and ideal plasma response approximations is proposed. Using the numerical model, the topological change and the penetration depth of the stochastic field lines in the edge magnetic field are studied in an RMP experiment. Comparing profiles of minimum ρ on edge stochastic field lines and the particle flux pattern, the ideal plasma response changes the field line penetration depth while remaining similar profile relative to vacuum approximation. To mitigate and suppress ELM strongly, the deep penetration of RMP fields and topological changes of the edge magnetic field is a key from our modeling.

Influence of stationary driven helical current on the m=2/n=1 resistive tearing mode
Yun YUAN (袁赟) 1,2, Xingqiang LU (路兴强) 1, Jiaqi DONG (董家齐) 3, et al.
Plasma Sci. Technol.    2019, 21(5): 055101; doi: 10.1088/2058-6272/aafdc7
Abstract27)      PDF (1372KB)(44)      

The influence of stationary driven helical current on tearing mode instability in the m=2/n=1 rational surface is explored numerically using resistive magnetohydrodynamic simulation in cylindrical geometry. The results indicate that the flip instabilities result from the sustained injection of the sufficiently strong helical current driven in the island O-point. The driven helical current induces high order harmonics of instabilities due to the delay of suppressing timing and the increase of its current intensity. With the appropriate current density values, the development of the perturbed kinetic energy can be limited and the occurrence of the flip instabilities can be delayed for a long time. The radial deviation of the current deposition can lead to poor inhibition effect, and the effect of current bias on the boundary is greater than that on the axis.

Numerical study of equilibrium solutions for axisymmetric plasmas with toroidal flow obtained using Solovev approach
Yemin HU (胡业民) 1,2, M S ZHU (朱名盛) 1,2 and Wenfeng GUO (郭文峰) 1,2, et al.
Plasma Sci. Technol.    2019, 21(5): 055102; doi: 10.1088/2058-6272/ab004a
Abstract19)      PDF (10751KB)(49)      

Solovev approach of finding equilibrium solutions, which was extended to include the vacuum solutions provided by Zheng, Wooton, and Solano, was found extremely useful for the purpose of shaping studies. Its extension to toroidal equilibria with a general plasma flow was examined theoretically in a companion paper by Chu, Hu and Guo. The only meaningful extension was found for plasmas with a pure toroidal rotation and with a constant Mach number. A set of functions {SOLOVEV_ZWSm} was obtained which fixed location of the magnetic axis for equilibria with quasi-constant current density profile, with toroidal flow at constant Mach number and with specific heat capacity 1. The set {Solovev_ZWSm} should have complete shaping capability for plasma shapes with positive curvature at the boundary; but not for plasmas with negative curvature boundary points, i.e. the doublets or bean shaped tokamaks. We report here extensive numerical studies showing the shaping capability of {Solovev_ZWSm} for plasmas with pure toroidal rotations, including the change in topology of the solution when the rotation mach number changes. Included plasma topology are the sphere (spheromaks); and the tokamaks (including the doublets).

Spontaneous magnetic field multipolar structure in toroidal plasmas based on 2D equilibrium
Liuxiu HE (何柳秀) 1, Minghai LIU (刘明海) 2,3 and Shuangyun ZHAO (赵双云) 1
Plasma Sci. Technol.    2019, 21(4): 045101; doi: 10.1088/2058-6272/aaf78d
Abstract23)      PDF (775KB)(58)      

The multipolar magnetic field structure is investigated by the momentum conservation equation with self-consistent 3D sheared flows during transition of plasma properties from local paramagnetic to diamagnetic fields. Numerical results show that the traditional poloidal magnetic field (BP) is one part of equilibrium magnetic fields. The non-zero-order quantities are originated from the higher-order terms of 2D equilibrium treatment based on a Fourier expansion of ψ (r, θ). The distributions of magnetic field vectors of the order of 1, 2, and 3 terms are presented respectively in two, four, and six polar fields with the local vortex structures (spontaneous magnetic connection). The excitation mechanisms of the magnetic vortices are the coupling effects of the magnetic fluid structure pattern and the toroidal effects. These results can help us understand the physical mechanism of the interaction between the external perturbation fields and control tearing modes, as well as the radial plasma flow and magnetic vortices.

Loss-cone instabilities for compact fusion reactor and field-reversed configuration
Zhongtian WANG (王中天)1,2, Huidong LI (李会东)1 and Xueke WU (吴雪科)1
Plasma Sci. Technol.    2019, 21(2): 025101; doi: 10.1088/2058-6272/aaead9
Abstract24)      PDF (617KB)(328)      

Loss-cone instabilities are studied for linear fusion devices. The gyro-kinetic equation for such a configuration is rigorously constructed in terms of action-angle variables by making use of canonical transformation. The dispersion relation, including for the first time, finite bounce frequency is obtained and numerically solved. The loss-cone modes are found near ion-cyclotron frequency. The growth rates are greatly reduced and approaching zero with increasing beta value. The results suggest that loss-cone instabilities are unlikely to be threatening to linear fusion devices since a new longitudinal invariant is found and gives a constraint which helps confinement.

Study of first orbit losses of 1 MeV tritons using the Lorentz orbit code in the LHD
Kunihiro OGAWA 1,2, Mitsutaka ISOBE 1,2, Takeo NISHITANI 1, et al.
Plasma Sci. Technol.    2019, 21(2): 025102; doi: 10.1088/2058-6272/aaeba8
Abstract22)      PDF (975KB)(126)      

Shot-integrated measurement of the triton burnup ratio has been performed in the Large Helical Device. It was reported that the triton burnup ratio, defined as total DT neutron yield divided by total DD neutron yield, increases significantly in inward shifted configurations. To understand the magnetic configuration dependence of the triton burnup ratio, the first orbit loss fraction of 1MeV tritons is evaluated by means of the Lorentz orbit code for various magnetic configurations. The first orbit loss of 1 MeV tritons is seen at t of less than 10−5 s and loss points of the triton are concentrated on the side of the helical coil case where the magnetic field is relatively weak. The significant decrease of the first orbit loss fraction by 15% is obtained with the inward shift of the magnetic axis position from 3.90 to 3.55 m. It is found that the decrease of first orbit loss is due to the reduction of the first orbit loss of transition and helically trapped tritons.

The residual zonal flow in tokamak plasmas with a poloidal electric field
Wenjia WANG (王文家)1,2, Deng ZHOU (周登)1 and Yue MING (明玥)1,2
Plasma Sci. Technol.    2019, 21(1): 015101; doi: 10.1088/2058-6272/aadd8e
Abstract26)      PDF (446KB)(135)      

In a tokamak plasma with auxiliary heating by cyclotron waves, a poloidal electric field will be produced, and as a consequence influence the residual zonal flow (RZF) level. The poloidal electric field can also be induced through biasing electrodes at the edge region of tokamaks. Numerical evaluation for a large aspect ratio circular cross section tokamak for the electron cyclotron wave heating indicates that the RZF level decreases significantly when the poloidal electric field increases. Qualitatively, the ion cyclotron wave heating is able to increase the RZF level. It is difficult to apply the calculation to the real cyclotron wave heating experiments since we need to know factors such as the plasma profiles, the exact power deposition and the cross section geometry, etc. It is possible to use the cyclotron wave heating to control the zonal flow and then to control the turbulence level in tokamak experiments.

Estimation of magnetic island width by the fluctuations of electron cyclotron emission radiometer on J-TEXT
Bowen RUAN (阮博文), Zhoujun YANG (杨州军), Xiaoming PAN (潘晓明), et al.
Plasma Sci. Technol.    2019, 21(1): 015102; doi: 10.1088/2058-6272/aae382
Abstract32)      PDF (1257KB)(156)      

The width of a magnetic island is an important parameter for the quantitative analysis of magnetohydrodynamic-related physics. An electron cyclotron emission radiometer (ECE) is a powerful tool that can be used to obtain this width, which can usually be determined from the flat temperature distribution at the O-point phase or the maxima temperature perturbation. An improved method to estimate the width of a magnetic island is proposed in this paper, and it is independent of calibration. With this method and the existing 24-channel ECE system, the width of a rotation magnetic island can be estimated. Additionally, by filtering the fluctuation ECE signal, the evolution of the magnetic island can be obtained. The results of this method are consistent with those of the integrated magnetic probe signals, which represent the relative change of the magnetic island.

Overview of the J-TEXT progress on RMP and disruption physics
Yonghua DING (丁永华) 1, Zhongyong CHEN (陈忠勇) 1, Zhipeng CHEN (陈志鹏) 1, et al.
Plasma Sci. Technol.    2018, 20(12): 125101; doi: 10.1088/2058-6272/aadcfd
Abstract54)      PDF (11195KB)(139)      

The J-TEXT tokamak has been operated for ten years since its first plasma obtained at the end of 2007. The diagnostics development and main modulation systems, i.e. resonant magnetic perturbation (RMP) systems and massive gas injection (MGI) systems, will be introduced in this paper. Supported by these efforts, J-TEXT has contributed to research on several topics, especially on RMP physics and disruption mitigation. Both experimental and theoretical research show that RMP could lock, suppress or excite the tearing modes, depending on the RMP amplitude, frequency difference between RMP and rational surface rotation, and initial stabilities. The plasma rotation, particle transport and operation region are influenced by the RMP. Utilizing the MGI valves, disruptions have been mitigated with pure He, pure Ne, and a mixture of He and Ar (9:1). A significant runaway current plateau could be generated with moderate amounts of Ar injection. The RMP has been shown to suppress the generation of runaway current during disruptions.

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
Abstract47)      PDF (2094KB)(216)      

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.

Dependence of the internal inductance on the radial distance between the primary and secondary X-point surfaces in the EAST tokamak
Qi WANG (汪启) 1,2, Qingquan YANG (杨清泉) 2,3,Zhengmao SHENG (盛正卯) 1, et al.
Plasma Sci. Technol.    2018, 20(10): 105101; doi: 10.1088/2058-6272/aad326
Abstract34)      PDF (1183KB)(170)      

In view of their advantage in long-pulse high-performance operations, low internal inductance scenarios are being developed and tested for the EAST and other tokamaks. Dependence of the internal inductance on several main plasma configuration parameters are statistically analyzed. It is found that the internal inductance of the plasma is closely related to the radial distance  dRsep between the primary and secondary X-point surfaces on the outer mid-plane as well as the poloidal field of the last closed flux surface. Moreover, dRsep is also related to the distribution of the fast electrons driven by the injected lower-hybrid waves, which is partially responsible for the observed relation between dRsep and the internal inductance. The results here should therefore be helpful as a guide for future experiments on internal inductance control and long-pulse operations of the EAST and other tokamaks, as well as for detailed theoretical study of the underlying physics.

Electron density calculation based on Stark broadening of D Balmer line from detached plasma in EAST tungsten divertor
Feng XU (徐峰) 1,2, Fang DING (丁芳) 1, Xiahua CHEN (陈夏华) 1,2, et al.
Plasma Sci. Technol.    2018, 20(10): 105102; doi: 10.1088/2058-6272/aad226
Abstract55)      PDF (1445KB)(173)      

The electron density within the volume of the tungsten divertor of the Experimental Advanced Superconducting Tokamak (EAST) is calculated based on Dε line (396.9 nm) Stark broadening (SB) measurements. The quasistatic approximation is employed in the SB calculation of the Dε line. The influences of other broadening mechanisms on the calculation error of electron density have been evaluated. The SB method is applied to the study of spatial distribution and time evolution of the electron density in the W divertor. Two electron density bands are observed in the detached divertor plasma during an L-mode discharge sustained by low hybrid wave (LHW) heating, which could be related to the striated particle flux distribution induced by LHW. After the onset of detachment, the upper electron density band corresponding to outer strike point firstly increases then decreases, while the lower density band corresponding to striated particle flux increases continually although the electron densities from Langmuir Probes at the divertor plate keep a descending trend. This could indicate a downward movement of the radiation region that approximately moves along the magnetic field lines after the onset of detachment.

High-speed VUV spectroscopy for edge impurity line emission measurements in HL-2A tokamak
Dianlin ZHENG (郑典麟), Kai ZHANG (张凯), Zhengying CUI (崔正英), et al.
Plasma Sci. Technol.    2018, 20(10): 105103; doi: 10.1088/2058-6272/aacf3d
Abstract40)      PDF (1995KB)(209)      

A 20 cm focal length normal incidence vacuum ultraviolet (VUV_20 cm) monochromator with a fast time response has been developed for measuring edge impurity line emission in the wavelength range of 300–2000 Å on an HL-2A tokamak. An aberration corrected concave holographic grating with 1200 grooves/mm is adopted in the monochromator, which provides a wavelength dispersion of 40 Åmm −1 . The aperture is f/4.5. A channel electron multiplier is used as a detector. The time resolution of the system is 17 μs. Wavelength calibration of the system has been done by using a hollow cathode light source in the laboratory with helium and argon gases. The obtained signals of helium and argon spectra are very strong since the inner surface of the monochromator vacuum chamber is blackened and the stray light level is then significantly reduced. The optical property of the system has been examined by scanning the width of the entrance and exit slits. The system is then installed at the mid-port of the HL-2A tokamak and typical line emissions from the HL-2A plasma are measured. Time behaviors of edge impurity line emissions are observed with the fast time response system in different plasma confinement regimes, especially in the H-mode discharges. The result shows that the VUV_20 cm system works very well to measure the edge impurity line emissions in the edge localized modes phase of H-mode discharges.

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
Abstract78)      PDF (368KB)(228)      

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.