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In this article, partial and full collapses of sawtooth activity are investigated with the high-resolution two-dimensional electron cyclotron emission (ECE) Imaging diagnostic in EAST. The partial ...

A homodyne Mach–Zehnder (M-Z) optical fiber interferometer (OFI) based on the optical coupler demodulation with 1.55 μm wavelength is firstly developed for high plasma density measurement on the theta pinch (IMP-θ) plasmas in the Institute of Modern Physics for the research on magnetic-inertial fusion. The evolutions of center line-averaged electron density in the theta pinch discharges are measured by the interferometer. The OFI is full-fiber structure and the line-average electron number density resolution is 10 18 m -2 . The minimum time resolution of OFI is 1ns, depended on the bandwidth of terminal InGaAs detectors. Finally, the dynamics of theta pinch plasmas are reported, which are analyzed from the density waveforms under different gas pressures and charging voltages of capacitor banks. It shows the OFI is a highly reliable and flexible routine high plasma density diagnostic tool for theta pinch plasmas.

Due to the discrete nature of the saddle coils for the active feedback control of the plasma instabilities, the emergence of the sideband modes is inevitable. In the Keda Torus eXperiment (KTX), the edge Mirnov coils are unevenly distributed on the inner surface of the vacuum vessel to suppress the sideband modes. These arrays of the Mirnov coils are used for the feedback control system of KTX. They are also used for the magnetohydrodynamics (MHD) mode identification, which is a fundamental and significant method to distinguish and describe the plasma instabilities. A set of suitable MHD mode analysis methods has been utilized to complete the mode detection, including the spatial discrete Fourier transform (SDFT) method, the singular value decomposition (SVD) method and the Lomb periodogram method. These methods can obtain comprehensive mode information of the plasma instabilities and their results can be used as the feedback of the feedback control system. These methods have been successfully applied to detect and characterize an impulsive mode of ( $m = 1$ , $n = 0$ ) and rotating modes of ( $m = 2$ , 3, 4, $n = 1$ ) in the tokamak plasma of KTX, while the matrix decomposition technique is not applicable for KTX.

Electron density fluctuation has been measured based on the collect forward scattering method using a single-chord microwave interferometer system on the Keda Torus eXperiment (KTX) reversed field pinch. The interferometer system is a heterodyne one with two terahertz solid-state sources with frequency around 650 GHz, and the transmitted beam and forward scattering beam are received simultaneously by a Schottky planar diode mixer with sensitivity about 500 mV/mW. Constrained by the beamwidth and geometry limitation, the measurable wave vector range is $k_{w}\le 5 {\mathrm {cm}}^{-1}$ with a resolution $\Delta k\approx 2.07\,{\mathrm {cm}}^{-1}$ . MHD activities are detected via this forward scattering technique. The results are consistent with other diagnostics, such as the chord averaged electron density, the edge magnetic probe, and high-speed charge-coupled device (CCD) camera.

Electron cyclotron emission imaging (ECEI), as an advanced diagnostic, provides two-dimensional (2D) images of electron temperature fluctuation in the core plasma region. The ECEI system on Experim...

A massive amount of positrons (plasma positrons), produced by the collision between runaway electrons and nuclei during fusion plasma disruption, was first predicted theoretically in 2003. To help confirm this prediction, we report here the design of an experimental system to detect tokamak plasma positrons. Because a substantial amount of positrons (material positrons) are produced when runaway electrons impact plasma-facing materials, we proposed maximizing the ratio of plasma to material positrons by inserting a thin carbon target at the plasma edge as a plasma positron bombing target and producing a plasma disruption scenario triggered by massive gas injection. Meanwhile, the coincidence detection of positron annihilation photons was used to filter out the noise of annihilation photons from locations other than the carbon target and that of bremsstrahlung photons near 511 keV. According to our simulation, the overall signal-to-noise ratio should be more than 10:1.

Through a systematically developed theory, we demonstrate that the motion of Instanton identified in Zhang et al (2017 Phys. Plasmas 24 122304) is highly correlated to the intermittent excitation and propagation of geodesic acoustic mode (GAM) that is observed in tokamaks. While many numerical simulations have observed the phenomena, it is the first theory that reveals the physical mechanism behind GAM intermittent excitation and propagation. The preceding work is based on the micro-turbulence associated with toroidal ion temperature gradient mode, and slab-based phenomenological model of zonal flow. When full toroidal effect is introduced into the system, two branches of zonal flow emerge: the torus-modified low frequency zonal flow (TLFZF), and GAM, necessitating a unified exploration of GAM and TLFZF. Indeed, we observe that the transition from the Caviton to Instanton is triggered by a rapid zero-crossing of radial group velocity of drift wave and is found to be strongly correlated with the GAM onset. Many features peculiar to intermittent GAMs, observed in real machines, are thus identified in the numerical experiment. The results will be displayed in figures and in a movie; first for single central rational surface, and then with coupled multiple central rational surfaces. The periodic bursting first shown disappears as being replaced by irregular one, more similar to the intermittent characteristics observed in GAM experiments.

A front-end optics system has been developed for the EAST microwave imaging reflectometry for 2D density fluctuation measurement. Via the transmitter optics system, a combination of eight transmitter beams with independent frequencies is employed to illuminate wide poloidal regions on eight distinct cutoff layers. The receiver optics collect the reflected wavefront and project them onto the vertical detector array with 12 antennas. Utilizing optimized Field Curvature adjustment lenses in the receiver optics, the front-end optics system provides a flexible and perfect matching between the image plane and a specified cutoff layer in the plasma, which ensures the correct data interpretation of density fluctuation measurement.

A poloidal and toroidal magnetic field coupling circuit model is developed to simulate discharge waveforms of the Keda Torus eXperiment (KTX) reversed field pinch (RFP) to ensure that the design of the power supply systems is able to achieve the typical discharge parameters. The radial profiles of plasma current density and magnetic field used in the coupling model are characterized by modified Bessel function model (MBFM) derived from the well-known Taylor's theory. The power balance in the energy conservation of poloidal and toroidal fields is used to establish the connection between these two fields. The numerical solutions of the coupling show that the KTX power supply system is capable of achieving the designed discharge parameters, such as field reversal parameter ( F ), pinch parameter (Θ), magnetic field, plasma current and loop voltage. Furthermore, the delay of magnetic field penetration due to the effect of vacuum vessel is also considered in the model.

A feedback control system has been designed for the Keda Torus eXperiment device to achieve the following goals: (a) suppression of the error field at the poloidal gap, (b) three-dimensional plasma stability control, and (c) improvement of plasma discharge quality. The system consists of a boundary electromagnetic probe array, a field-programmable gate array, a linear power amplifier, and an active control coil array. The system adopts a compound control method that includes both active and passive control methods. To control the plasma, an active control coil array is used to generate a localized radial magnetic field as needed at two poloidal gaps of the device. In the open-loop control experiment, the active control coils near the poloidal gaps reduced the original error field and thus increased the plasma discharge current amplitude by 50% and effectively extended the plasma discharge duration.

We have developed a parametric method for eliminating the background component of the plasma current, which is measured by a Rogowski coil and polluted by the toroidal magnetic field in the vacuum vessel of the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device. The method considers the toroidal magnetic field windings, the KTX vacuum chamber, and the Rogowski coil as a linear time-invariant system; in this case, a constant frequency response function characterizes the system. Using this response function, the current component caused by pollution from the toroidal magnetic field can be predicted exactly for an arbitrary input current to the toroidal magnetic field windings. Compared with the traditional proportional compensation method, the proposed method has great flexibility and universality and it is potentially applicable to cases in which the toroidal field current signal changes over time with plasma feedback signals. Furthermore, the method can be applied to other similarly affected signals, such as magnetic field signals. As an example, we have corrected the poloidal and toroidal magnetic field signals better to reveal the true physical processes for the RFP state.

An automatic beam alignment system based on relative reference points is developed for the Thomson scattering system on Keda Torus eXperiment. Two critical apertures around the vacuum vessel are designed to shield stray light, and a probe beam is required to go through the centers of these two apertures, which are the reference points for alignment. Since these two apertures are coated with light absorbing materials, three fibers with glowing tips are employed to indicate the centers of two apertures. CMOS cameras are used to monitor beam deviations. The misalignment correction is achieved by driving piezomotor mirror mounts via a program developed with LabVIEW, which includes the image acquisition and processing module and the deviation correction module. As a result, this system can correct beam misalignment in less than 20 s and suppress the long-term drift of laser pointing in ±10 μrad. Also, this system has the capability to correct up to about 2.3 mm of camera shift with our experiment condition.

A series of discharges achieving high- β N ( > 1.5 ) H-mode phases were obtained on EAST with both LHW and NBI as the heating power, a tungsten divertor, an unfavorable ▽ B × B direction configuration for the L-H transition and a lower toroidal field ( B T ∼ 1.6 T ). The experimental power threshold with the unfavorable direction configuration appeared to be higher than previous results with favorable directions in 2014 and 2015, although the tungsten divertor configurations were believed to benefit the L-H transition compared to a graphite divertor. The proportions of LHW and NBI power modulated the ratio of electron temperature to ion temperature, accompanied with an optimal ratio that caused a minimum P l o s s . The WCM might have prolonged the dwell time. The power threshold increased with increasing L ∥ and lower OLL lead to lower power threshold. Better energy confinement was observed during the high- β N H-mode phases.

The Doppler backscattering system has been widely used for turbulence measurements, and the microwave beam will be backscattered near the cut-off layer when the Brag condition is fulfilled. In tokamak, the ray-tracing code is used to obtain the radial position and perpendicular wave number of the scattering layer for turbulence velocity measurement and the WKB (Wentzel-Kramers-Brillouin) approximation should be satisfied for optical propagation. To calculate the backscattering location and wave number at the cut-off layer only, a single ray tracing in the cross section is enough, while for spatial and wave number resolution calculation, multiple rays reflecting the microwave beam size should be used. Considering the angle between the wave vector and the magnetic field, a three-dimension quasi-optical Gaussian ray tracing is sometimes needed.

Doppler backscattering system can measure the perpendicular velocity and fluctuation amplitude of the density turbulence with intermediate wavenumber. An eight-channel Doppler backscattering system has been installed in the Experimental Advanced Superconducting Tokamak (EAST), which can probe eight different radial locations simultaneously by launching eight fixed frequencies (55, 57.5, 60, 62.5, 67.5, 70, 72.5, 75 GHz) into plasma. The quasi-optical system consists of circular corrugated waveguide transmission, a fixed parabolic mirror, and a rotatable parabolic mirror which are integrated with quasi-optics front-end of the profile reflectometer inside the vacuum vessel. The incidence angle can be chosen from 5° to 12°, and the wavenumber range is 2-15/cm with the wavenumber resolution Δk/k≤0.21. Ray tracing simulations are used to calculate the scattering locations and the perpendicular wavenumber. The dynamic range of this new eight-channel Doppler backscattering system can be as large as 40 dB in the EAST. In this article, the hardware design, the ray tracing, and the preliminary experimental results in the EAST will be presented.

The Keda Torus eXperiment (KTX) is a new reversed field pinch (RFP) device at the University of Science and Technology of China. The construction and assembly of KTX, including the vacuum chamber, conducting shell, magnetic field windings, power supply system, active control coils, vacuum pump and data acquisition system, have been completed on August 1, 2015. Immediately following that, the first plasma was obtained on August 15, 2015. Intensive conditioning of the machine is underway to ramp up the plasma current toward its full operation. An active feedback mode control system has been built and has been implemented to control the error field around the vertical gaps of the conducting shell. The pulsed power supply systems of ohmic heating field and toroidal field (TF), using thyristor and energy storage capacitors, have been tested and commissioned. The TF power supply has flexibility in being able to operate with a reversed TF configuration and stable TF configuration. The fundamental diagnostic tools are developed for early KTX operation. Currently, the plasma current is up to 205 kA and the maximum discharge length is 21 ms, approaching to the conducting shell penetration time. Furthermore, typical RFP discharges are being produced with RFP state lasting 2 ms. These initial operation results for KTX are described in detail.

In a reversed field pinch device, the conductive shell is placed as close as possible to the plasma so as to balance the plasma during discharge. Plasma instabilities such as the resistive wall mode and certain tearing modes, which restrain the plasma high parameter operation, respond closely with conditions in the wall, in essence the eddy current present. Also, the effect of eddy currents induced by the external coils cannot be ignored when active control is applied to control instabilities. One diagnostic tool, an eddy current probe array, detects the eddy current in the composite shell. Magnetic probes measuring differences between the inner and outer magnetic fields enable estimates of the amplitude and angle of these eddy currents. Along with measurements of currents through the copper bolts connecting the poloidal shield copper shells, we can obtain the eddy currents over the entire shell. Magnetic field and eddy current resolutions approach 2 G and 6 A, respectively. Additionally, the vortex electric field can be obtained by eddy current probes. As the conductivity of the composite shell is high, the eddy current probe array is very sensitive to the electric field and has a resolution of 0.2 mV/cm. In a bench test experiment using a 1/4 vacuum vessel, measurements of the induced eddy currents are compared with simulation results based on a 3D electromagnetic model. The preliminary data of the eddy currents have been detected during discharges in a Keda Torus eXperiment device. The typical value of toroidal and poloidal eddy currents across the magnetic probe coverage rectangular area could reach 3.0 kA and 1.3 kA, respectively.

A 10-channel H α diagnostic system has been designed with the rapid response rate of 300 kHz, spatial resolution of about 40 mm, and overlap between adjacent channels of about 3%, and it has been implemented successfully on Keda Torus eXperiment (KTX), a newly constructed, reversed field pinch (RFP) experimental device at the University of Science and Technology of China (USTC). This diagnostic system is a very important tool for the initial KTX operations. It is compact, with an aperture slit replacing the traditional optical lens system. A flexural interference filter is designed to prevent the center wavelength from shifting too much as the increase of angle from vertical incidence. To eliminate the stray light, the interior of the system is covered with the black aluminum foil having a very high absorptivity. Using the H α emission data, together with the profiles of electron temperature and density obtained from the Langmuir probe, the neutral density profiles have been calculated for KTX plasmas. The rapid response rate and good spatial resolution of this H α diagnostic system will be beneficial for many studies in RFP plasma physics.

By analyzing large quantities of discharges in the unfavorable ion $ \vec B\times \nabla B $ drift direction, the I-mode operation has been confirmed in EAST tokamak. During the L-mode to I-mode transition, the energy confinement has a prominent improvement by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with the I-mode observation on other devices, the $ E_r $ profiles obtained by the eight-channel Doppler backscattering system (DBS8)\cite{J.Q.Hu} show a deeper edge $ E_r $ well in the I-mode than that in the L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the $ E_r $ fluctuation and the caused flow shear from WCM should play an important role during I-mode. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where GAM intensity is decreased or disappeared. Many evidences show that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigations are needed to explain the new properties such as the harmonics and obvious magnetical perturbations., Comment: 14 pages, 7 figures

The Keda Torus eXperiment (KTX) is still operated in the commissioning phase, and preparation for the operation capability of the KTX phase II upgrade is underway. The diagnostics in the KTX have been greatly developed: (1) the terahertz interferometer has been upgraded to seven chords for electron density profile inversion; (2) a Thomson scattering system with a 5 Joule laser has been installed and commissioning is in progress; (3) a 3D movable probe system has been developed for the electromagnetic turbulence measurement; (4) double-foil soft x-ray imaging diagnostics have been set up and a bench test has been completed; (5) an edge capacitive probe has been installed for the radial electrical field measurement; (6) a multi-channel spectrograph system has been built for detecting impurities of carbon and oxygen. In addition, the design of a new compact torus injection system has been completed for feeding and momentum driving. Pilot research, such as the 3D reversed field pinch physics and electromagnetic turbulence, etc, have been conducted in the discharge status of the KTX. The 3D spectra characters of electromagnetic turbulence are firstly measured using a classical two-point technique by Langmuir probe arrays set on the 3D movable probe system and edge magnetic sensors. The forward scattering is collected by the interferometer system, which shows the potential for turbulence research. The electromagnetic turbulence is tentatively investigated in the KTX. The formation of a quasi-single-helicity state in the KTX regime is also preliminarily explored in simulation.

We conduct an electron magnetohydrodynamics magnetic reconnection experiment with guide-field in our Keda linear magnetized plasma device, in which two pulsed currents with the same direction are conducted in parallel with the axial direction of the main chamber of the device using two long aluminum sticks. After approximately , an X-type magnetic field line topology is formed at the center of the chamber. With the formation of the X-type topology of magnetic field lines, we can also find the rapid increase of the current and ratio of the common flux to the private flux in this area. Additionally, a reduction in the plasma density and the plasma density concentration along one pair of separatrices can also be found.

An extraordinary-mode (X-mode) frequency-modulated continuous-wave (FMCW) profile reflectometer has been built on EAST. In the reflectometer, continuous waves with frequency sweeping from 12.5 GHz to 18 GHz were generated through a Hyperabrupt Tuned-varactor Oscillator (HTO) source and a four times active multiplier was used to increase the frequency to V-band (50 GHz to 72 GHz). The polarization of horn lens antenna is perpendicular to the magnetic field line at the edge plasmas. According to the V-band frequency range and polarization, the system cover density range from 0.5 × 1019m−3 to 3.0 × 1019m−3 (when toroidal magnetic field is 1.8 T), with time resolution of 12.5 ~ 50 μs. The density profile could be calculated by assuming the edge profile through an empirical equation. The maximum spatial error deduced by the method is about 4 cm. This reflectometer has been successfully applied in 2010 autumn EAST campaign, the temporal evolution of density profiles was acquired during the low confinement mode to high confinement mode transition. The density pedestal of EAST Tokamak was observed and the top value and gradient of the density pedestal were estimated.

Solar-cycle related variation of differential rotation is investigated through analyzing the rotation rates of magnetic fields, distributed along latitudes and varying with time at the time interval of August 1976 to April 2008. More pronounced differentiation of rotation rates is found to appear at the ascending part of a Schwabe cycle than at the descending part on an average. The coefficient $B$ in the standard form of differential rotation, which represents the latitudinal gradient of rotation, may be divided into three parts within a Schwabe cycle. Part one spans from the start to the $4^{th}$ year of a Schwabe cycle, within which the absolute $B$ is approximately a constant or slightly fluctuates. Part two spans from the $4^{th}$ to the $7^{th}$ year, within which the absolute $B$ decreases. Part three spans from the $7^{th}$ year to the end, within which the absolute $B$ increases. Strong magnetic fields repress differentiation of rotation rates, so that rotation rates show less pronounced differentiation, but weak magnetic fields seem to just reflect differentiation of rotation rates. The solar-cycle related variation of solar differential rotation is inferred to the result of both the latitudinal migration of the surface torsional pattern and the repression of strong magnetic activity to differentiation of rotation rates., 19 pages, 8 figures, accepted by MNRAS. arXiv admin note: text overlap with arXiv:1305.3970

The daily sunspot numbers of the whole disk as well as the northern and southern hemispheres from January 1, 1945 to December 31, 2010 are used to investigate the temporal variation of the rotational cycle length through the continuous wavelet transformation analysis method. The auto-correlation function analysis of daily hemispheric sunspot numbers shows that the southern hemisphere rotates faster than the northern hemisphere. The results obtained from the wavelet transformation analysis are: there exists no direct relationship between the variation trend of the rotational cycle length and the variation trend of solar activity in the two hemispheres; the rotational cycle length of both hemispheres has no significant period appearing at the 11 years, but has significant period of about 7.6 years. Analysis concerning the solar cycle dependence of the rotational cycle length shows that in the whole disk and the northern hemisphere acceleration seems to appear before the minimum time of solar activity. Furthermore, the cross-correlation study indicates that the rotational cycle length of the two hemispheres has different phases, and the rotational cycle length of the whole disk as well as the northern and southern hemispheres also has phase shifts with the corresponding solar activity. What's more, the temporal variation of North-South (N-S) asymmetry of the rotational cycle length is also studied; it displays the same variation trend as the N-S asymmetry of solar activity in a solar cycle as well as in the considered time interval, and it has two significant periods of 7.7 and 17.5 years. Moreover, the N-S asymmetry of the rotational cycle length and the N-S asymmetry of solar activity are highly correlated. It's inferred that the northern hemisphere should rotate faster at the beginning of solar cycle 24.

In the Linear Magnetized Plasma (LMP) device of University of Science and Technology of China and by exerting parallel currents on two parallel copper plates, we have realized the magnetic reconnection in laboratory plasma. With the emissive probes, we have measured the parallel (along the axial direction) electric field in the process of reconnection, and verified the dependence of reconnection current on passing particles. Using the magnetic probe, we have measured the time evolution of magnetic flux, and the measured result shows no pileup of magnetic flux, in consistence with the result of numerical simulation.

In order to investigate electron dynamics near the electron diffusion region in magnetic reconnection process, an upgrade in the linear magnetized plasma (LMP) device is accomplished at the University of Science and Technology of China. Radio frequency (RF) helicon discharge is used to generate a quasi-stationary plasma, and a time-dependent magnetic field is applied to the plasma, which exhibits an X-type neutral point in vacuum. A two-dimensional sophisticated mobile platform is built up, providing a high spatial resolution, below 0.5 mm, for the diagnostics.

An upgraded electron cyclotron emission imaging (ECEI) system consisting of new optics lenses with necessary electronics for receiving and processing signals for two dimension (2D) ECEI diagnostics was installed on EAST. Hyperboloid lens were adopted in the new system to optimize the spatial resolutions. The mixers array of sixteen elements measured the plasma electron cyclotron emission at eight frequencies simultaneously, and the profiles of the electron temperature and its fluctuation in an area of 20 cm (vertical) × 6 cm (horizontal) could then be analyzed. Evolution of sawtooth precursor and crash in EAST was observed.

Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.

The magnetic fluctuations of geodesic acoustic modes (GAMs) have been investigated using a Doppler backscattering system and Mirnov probes during ion cyclotron resonance heating and lower hybrid wave heating in the edge plasma in the experimental advanced superconducting tokamak, and the magnetic component of the GAM was observed. The frequency of the GAM has a clear relationship with the edge electron temperature, and the mode numbers of the poloidal component of the magnetic structure of the GAM are approximately m = 2 ( sin ( 2 θ ) ) and n = 0. A more detailed investigation shows that the maximum values of the amplitude of the poloidal magnetic field fluctuations and poloidal electric field fluctuations are approximately 10 − 6 T and 200 V/m, respectively.

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, e = 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.

Electron cyclotron emission imaging on EAST provides direct measurements of the 2-D electron temperature dynamics in a continuous large observation area with high temporal and spatial resolution. Besides the normal MHD investigation, a system with a view field large enough to cover the core plasma region has been applied to extract more plasma information, such as the plasma center location, the deposition location of auxiliary heating, and the core toroidal rotation speed. These results solely based on electron cyclotron emission imaging (ECEI) data are consistent with the results of the equilibrium fitting (EFIT), numerical code, and other diagnostics, which indicate the powerful diagnostic capacity of this ECEI system.

Drift instabilities contribute to the formation of edge turbulence and zonal flows, and thus the anomalous transport in tokamaks. Experiments often found micro-scale tur- bulence strongly coupled with large-scale magnetohydrodynamic (MHD) processes, whereas a general framework has been lacking that can cover both regimes, in partic- ular, their coupling. In this paper, the linear resistive drift wave instability (DWI) is investigated using a full 2-fluid MHD model, as well as its numerical implementation in NIMROD code. Both analytical and numerical analyses reveal a macro-scale global drift wave eigenmode coupled with MHD dynamics and illustrate a non-monotonic dispersion relation with respect to both perpendicular and parallel wavenumbers. NIMROD results also reveal an edge-localized behavior in the radial mode structure as the azimuthal mode number increases, implying the dependence of the 2-fluid ef- fects due to the inhomogeneous density profile. The edge-localization introduces a non-trivial dependence of the effective perpendicular wavenumber to the perpendicular mode number, which may explain the quantitative difference between the global dispersion relation and its local approximation from the conventional local theory., 22 pages, 8 figures

A 384 channels (24 vertical × 16 radial) Electron Cyclotron Emission Imaging (ECEI) system has been installed on Experimental Advanced Superconducting Tokamak (EAST). With the aid of advanced front optics, high spaital resolution around 1.1cm and flexible vertical coverage 30 ~ 70 cm have been realized. The well-designed optics also provides a long Rayleigh length up to 70 cm even with an object length larger than 2.6 m. The electronic system has a wide immediate frequency (IF) bandwidth 2 ~ 16.5 GHz, which enables a continuous radial coverage up to 25 cm. Benefited from advanced optics design and the wide IF bandwidth, the ECEI system with high spatial resolution can provide a large and continuous view field, especially in the radial direction, which is enough for the observation of the whole q = 1 surface. The comprehensive details of the ECEI system will be presented in this paper, along with some experimental results.

Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas. Of particular importance has been microwave electron cyclotron emission imaging (ECEI) for imaging Te fluctuations. Key to the success of ECEI is a large Gaussian optics system constituting a major portion of the focusing of the microwave radiation from the plasma to the detector array. Both the spatial resolution and observation range are dependent upon the imaging optics system performance. In particular, it is critical that the field curvature on the image plane is reduced to decrease crosstalk between vertical channels. The receiver optics systems for two ECEI on the J-TEXT device have been designed to ameliorate these problems and provide good performance with additional field curvature adjustment lenses with a meniscus shape to correct the aberrations from several spherical surfaces.

The measurement of plasma displacement is one of the most basic diagnostic tools in the study of plasma equilibrium and control in a toroidal magnetic confinement configuration. During pulse discharge, the eddy current induced in the vacuum vessel and shell will produce an additional magnetic field at the plasma boundary, which will have a significant impact on the measurement of plasma displacement using magnetic probes. In the newly built Keda Torus eXperiment (KTX) reversed field pinch device, the eddy current in the composite shell can be obtained at a high spatial resolution. This device offers a new way to determine the plasma displacement for KTX through the multipole moment expansion of the eddy current, which can be obtained by unique probe arrays installed on the inner and outer surfaces of the composite shell. In an ideal conductor shell approximation, the method of multipole moment expansion of the poloidal eddy current for measuring the plasma displacement in toroidal coordinates, is more acc...

The propagation of a dust-ion-acoustic wave (DIAW) wave down the steep density gradient in an inhomogeneous diffusive dusty plasma is experimentally studied. It is observed that the presence of the dust enhances the plasma inhomogeneity. The phase velocity of the DIAW increases rapidly with distance and becomes supersonic. The interplay between the effects of the density nonuniformity and collisional damping results in the continuous transition of the relative amplitude of the DIAW from damping to growth. The experimental data are in agreement with theoretical prediction and confirm the importance of density-dependent ion–dust collisions.

In this paper, with a two-dimensional particle-in-cell simulation model, we compare the structures of a primary magnetic island to those of a secondary magnetic island, and these islands are formed during collisionless magnetic reconnection in a force-free current sheet. The out-of-plane magnetic field By is enhanced in the center of both the primary and secondary islands; however, a quadrupole structure of By with a good symmetry may be formed at the ends of the primary island. The in-plane electric field also exists in both the primary and secondary islands. The electric field Ex has a positive value in the left and a negative value in the right of the islands, while the electric field Ez has a positive value in the upper part and a positive value in the lower part of the islands. However, the in-plane electric field exists in the outer part of the primary island, while it exists in the center of the secondary island.

A Doppler reflectometer system has recently been installed in the Experimental Advanced Superconducting (EAST) Tokamak. It includes two separated systems, one for Q-band (33–50 GHz) and the other for V-band (50–75 GHz). The optical system consists of a flat mirror and a parabolic mirror which are optimized to improve the spectral resolution. A synthesizer is used as the source and a 20 MHz single band frequency modulator is used to get a differential frequency for heterodyne detection. Ray tracing simulations are used to calculate the scattering location and the perpendicular wave number. In EAST last experimental campaign, the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated.

The latitudinal distributions of the yearly mean rotation rates measured respectively by Suzuki in 1998 and 2012 and Pulkkinen $\&$ Tuominen in 1998 are utilized to investigate internal-cycle variation of solar differential rotation. The rotation rate at the solar Equator seems to decrease since cycle 10 onwards. The coefficient $B$ of solar differential rotation, which represents the latitudinal gradient of rotation, is found smaller in the several years after the minimum of a solar cycle than in the several years after the maximum time of the cycle, and it peaks several years after the maximum time of the solar cycle. The internal-cycle variation of the solar rotation rates looks similar in profile to that of the coefficient $B$. A new explanation is proposed to address such a solar-cycle related variation of the solar rotation rates. Weak magnetic fields may more effectively reflect differentiation at low latitudes with high rotation rates than at high latitudes with low rotation rates, and strong magnetic fields may more effectively repress differentiation at relatively low latitudes than at high latitudes. The internal-cycle variation is inferred to the result of both the latitudinal migration of the surface torsional pattern and the repression of strong magnetic activity to differentiation., 10 pages, 8 figures, accepted by ApJ Supplement Series, 206:1 (6pp), 2013

An electron cyclotron emission imaging (ECEI) system is being developed for J-TEXT. It is comprised of two 16-channel antenna arrays that share the same toroidal window for the purpose of observing separate radial regions of the tokamak plasma simultaneously. Two imaging optic units have been designed, which share the same zoom lenses, but have different focus lenses. The setup is flexible and achieves good spatial resolution. In particular, the poloidal zoom factor can achieve 1.17 to 2.20. The desired focal plane can range anywhere from high field side (HFS) edge to low field side (LFS) edge. The effective field curvature adjustment (FCA) lenses have been adopted for the ECEI imaging optic system, which make the image plane flat enough to match the emission layer in order to increase the image quality.

The spatial and temporal evolutions of compound sawteeth were directly observed using 2D electron cyclotron emission imaging on experimental advanced superconducting tokamak. The compound sawtooth consists of partial and full collapses. After partial collapse, the hot core survives as only a small amount of heat disperses outwards, whereas in the following full collapse a large amount of heat is released and the hot core dissipates. The presence of two q = 1 surfaces was not observed. Instead, the compound sawtooth occurs mainly at the beginning of an ion cyclotron resonant frequency heating pulse and during the L-H transition phase, which may be related to heat transport suppression caused by a decrease in electron heat diffusivity.

Advanced imaging optics techniques have significantly improved the performance of millimeter-wave imaging diagnostics, such as Electron Cyclotron Emission imaging and Microwave Imaging of Reflectometry. The fundamental functions of millimeter-wave imaging optics are focusing, collecting the emission or reflected microwave signal from the target area in the plasma and focusing the emitted (reflected) signal on the detector array. The location of the observation area can be changed using the focus lens. Another important function of the imaging optics is zooming. The size of the observation area in poloidal direction can be adjusted by the zoom lenses and the poloidal spatial resolution is determined by the level of zoom. The field curvature adjustment lenses are employed to adjust the shape of the image plane in the poloidal direction to reduce crosstalk between neighboring channels. The incident angle on each channel is controlled using the specific surface type of the front-side lenses to increase the signal-to-noise ratio. All functions are decoupled with the minimum number of lenses. Successful applications are given.

Observations have shown that electron phase-space holes (electron holes) possess regular magnetic structures. In this paper, two-dimensional (2D) electromagnetic particle-in-cell (PIC) simulations are performed in the (x, y) plane to study magnetic structures associated with electron holes under different plasma conditions. In the simulations, the background magnetic field (B(0) = B(0)(e(x)) over right arrow) is along the x direction. The combined actions between the transverse instability and stabilization by the background magnetic field lead to the generation of the electric field E(y). Then electrons suffer the electric field drift and produce the current in the z direction, which leads to the fluctuating magnetic field along the x and y directions. Meanwhile, the motion of the electron holes along the x direction and the existence of the electric field E(y) generate the fluctuating magnetic field along the z direction. In very weakly magnetized plasma (Omega(e) omega(pe)), electrostatic whistler waves with streaked structures of E(y) are excited. The fluctuating magnetic field delta B(x) and delta B(z) also have streaked structures. The relevance between our simulation results and the magnetic structures associated with electron holes observed in the plasma sheet is also discussed.

[1] In collisionless magnetic reconnection, the in-plane Hall currents are carried mainly by the magnetized electrons. The in-plane Hall currents are directed toward the X line along the magnetic field lines just inside the separatrices and away from the X line along the separatrices. Such a current system leads to the quadrupole out-of-plane magnetic field with the peaks between the regions carrying the in-plane currents. Simultaneously, the electron flow toward the X line along the separatrices causes electron density depletions along the separatrices. In this paper, the features of separatrix regions in magnetic reconnection and the relations between the electron density depletions and the out-of-plane magnetic field are investigated with both two-dimensional particle-in-cell simulations and Cluster observations. We conclude that the electron density depletions are formed because of the magnetic mirror, and they are outside the peaks of the out-of-plane magnetic field. Such a theoretical prediction is confirmed by both simulations and observations.

Two-dimensional particle-in-cell simulation is performed to investigate magnetic reconnection in a force-free current sheet. The results show that the evolution of the ion diffusion region has two different phases. In the first phase, the electrons flow toward the X line along one pair of separatrices and away from the X line along the other pair of separatrices. Therefore, in the ion diffusion region, a distorted quadrupole structure of the out-of-plane magnetic field is formed, which is similar to that of a typical guide field reconnection in the Harris current sheet. In the second phase, the electrons move toward the X line along the separatrices and then flow away from the X line at the inner side of the separatrices. In the ion diffusion region, the out-of-plane magnetic field exhibits a characteristic quadrupole pattern with a good symmetry, which is similar to that of antiparallel reconnection in the Harris current sheet.

The poloidal rotation velocities in the core plasma region are studied using the instantaneous frequency method (IFM) with the density fluctuations measured by the CO2 laser collective scattering diagnostics on the HT-7 tokamak. A coherent mode is observed in the fluctuations of poloidal velocities with the mode frequency from 10 to 20?kHz. It is identified as geodesic acoustic mode (GAM) zonal flow with poloidal symmetry (m = 0) and its mode frequency coinciding with the theoretical expected GAM frequency. The nonlinear interactions are investigated by applying the envelope analysis on the density fluctuations. The results confirm that the envelope modulation in the high frequency density fluctuations only comes from the shearing by GAM. The comparison between IFM and envelope analysis is also discussed.

The m/n = 1/1 and its higher harmonic modes are observed in sawtooth oscillations by using the novel high-resolution 2D ECE imaging system on the experimental advanced superconducting Tokamak (EAST). Higher harmonic modes are appearing for a short time during the crash phase of sawtooth oscillation in lower βp plasma, which is not the preferable position in the poloidal cross section. These modes generate sharp pressure points on the inversion radius during the crash phase of sawtooth oscillation. Furthermore, reconnection events proceed in two distinctive phases. In the first phase, a small amount of heat is expelled through the weak reconnection while in the second phase the remaining large quantity of heat and particles emerged rapidly from the hot core to the peripheral region of the inversion radius. In addition, these harmonic modes are only found before and after the ICRF pulse, while in the ICRF pulse only the (1,1) mode exists in the sawtooth oscillation.