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The Doppler reflectometer (DR), a powerful diagnostic for the plasma perpendicular velocity ($u_\perp$) and turbulence measurement has been widely used in various fusion devices. Many efforts have been put into extracting the Doppler shift from the DR signal. There are several commonly used methods for Doppler shift extraction, such as the phase derivative, the center of gravity (COG), and symmetric fitting (SFIT). However, the strong zero-order reflection component around $0 kHz$ may interfere with the calculation of the Doppler shift. To avoid the influence of the zero-frequency peak, the asymmetric fitting (AFIT) method was designed to calculate the Doppler shift. Nevertheless, the AFIT method may lead to an unacceptable error when the Doppler shift is relatively small compared to the half width at half maximum (HWHM). Therefore, an optimized method, which can remove the zero-frequency peak and fit the remaining Doppler peak with a Gaussian function, is devised to extract the Doppler shift. This method can still work reliably whether the HWHM is larger than the Doppler shift or not.

A compact torus injection system, KTX-CTI, has been developed for the planned injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device to investigate the physics and engineering issues associated with interaction between a compact torus (CT) and RFP. The key interests include fueling directly into the reactor center, confinement improvement, and the injection of momentum and helicity into the RFP discharges. The CT velocity and mass have been measured using a multichannel optical fiber interferometer, and for the first time the time evolution of the CT density profile during CT propagation is obtained. The effects of discharge parameters on the number of injected particles, CT velocity and CT density have been characterized: the maximum hydrogen CT plasma mass, m CT , is 50 μg, corresponding to 30% of the mass in a typical KTX plasma; the CT velocity exceeds 120 km s−1. It is observed for the first time that multiple CTs can be produced and emitted during a very short period (μs) in one discharge, which is significant for the future study of repetitive CT injection, even with an ultra-high frequency.

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.

On HL-2A, two different injections (supersonic molecular beam injection (SMBI) and pellet injection (PI)) are used to mitigate edge localized mode (ELM)-filament convective transport. The changes of their characteristics are studied in this paper. A high spatiotemporal resolution probe shows there are many similar phenomena, and the filament density amplitude and radial velocity are both suppressed. Our statistical results indicate that: the velocity suppression comes from the decrease of filament density and temperature; the transient particle and heat fluxes drop strongly; and long-range correlation along a magnetic flux surface also decreases, when the electron-ion collisionality increases significantly, which may have a role on the filament parallel current during ELM mitigation.

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.

The Vacuum Vessel (VV) system is a vital component of Keda Torus for eXperiment (KTX). Various accidental scenarios might occur on the VV. In this report, an extreme scenario is assumed and studied: plasma accidental termination during the flat-top stage. Numerical simulations based on finite element are performed as the major tool for analyses. The detailed distributions of eddy and the reaction forces on VV are extracted, and the total eddy current and the maximum reaction force due to electromagnetic load are figured out. In addition, according to the results, the VV can be approximately regarded as a centrally symmetric structure, even though its ports distribution is asymmetric.

KTX (Keda Torus for eXperiment) is a new reversed field pinch device. The KTX plasma control system (PCS) can provide real-time, stable, flexible plasma control which is designed by ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences), based on the Linux cluster system and EPICS (Experimental Physics and Industrial Control System) framework, and developed from DIII-D (Doublet III-D) PCS. The control of the equilibrium field in KTX uses a PID (Proportional-Integral-Derivative) feedback controller. The control of the gas injection is an open loop control. The plasma control simulation system is one part of the plasma control system, which is used to test the plasma control algorithm if it is revised and updated. The KTX PCS has been successfully tested using HT-7 (Hefei Torus 7) experiment data in simulation mode. In the next phase, an error field feedback control and KTX simulator will be added to the KTX PCS, and the KTX PCS will be applied in experiments in the future.

The Vacuum Vessel (VV) system is an essential component of Keda Torus for eXperiment (KTX), and various scenarios might take place on it. The VV's supports should be adequately strong to stand against various loads on VV, which might happen in extreme scenarios. Therefore, the design of VV supports is verified in a single extreme scenario and is subsequently optimized in this report. The numerical simulation based on Finite Element theory is performed as the major method for analysis and optimization. The electromagnetic force in previous analyses serves as the load for the mechanical analyses of supports. During the optimization, the stresses of the VV supports decrease remarkably after introducing cotters. Finally, the optimum design has been worked out. It satisfies the requirements regarding the strength and convenience in assembly.

The KTX device is a reversed field pinch (RFP) device currently under construction. Its maximum plasma current is designed as 1 MA with a discharge time longer than 100 ms. Its major radius is 1.4 m and its minor radius is 0.55 m. One of the most important problems in the magnet system design is how to reduce the TF magnetic field ripple and error field. A new wedge-shaped TF coil is put forward for the KTX device and its electromagnetic properties are compared with those of rectangular-shaped TF coils. The error field Bn/Bt of wedge-shaped TF coils with 6.4 degrees is about 6% as compared with 8% in the case of a rectangular-shaped TF coil. Besides, the wedge-shaped TF coils have a lower magnetic field ripple at the edge of the plasma region, which is smaller than 7.5% at R=1.83 m and 2% at R=1.07 m. This means that the tokamak operation mode may be feasible for this device when the plasma area becomes smaller, because the maximum ripple in the plasma area of the tokamak model is always required to be smaller than 0.4%. Detailed analysis of the results shows that the structure of the wedged-shape TF coil is reliable. It can serve as a reference for TF coil design of small aspect ratio RFPs or similar torus devices.

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.

In this article, the design idea of a 57-pin Langmuir probe array is presented, together with some experimental results. The manufacture technique of the probe array is discussed. Two-dimensional distributions of the plasma density, the floating potential and their fluctuations in the poloidal cross section are obtained. The blobs, which move radially and poloidally, cause great particle and energy transport. The charge neutralization by the poloidal movement of particles greatly decreases the vertical electric field in the helimak plasma. This leads to a confinement time which is two orders of magnitude larger than the theoretical prediction.

Continuous emission spectrum measurement is applied for the inconvenient diagnostics of low-temperature collisional plasmas. According to the physical mechanism of continuous emission, a simplified model is presented to analyze the spectrum in low temperature plasma. The validity of this model is discussed in a wide range of discharge parameters, including electron temperature and ionization degree. Through the simplified model, the continuous emission spectrum in a collisional argon internal inductively coupled plasma is experimentally measured to determine the electron temperature distribution for different gas pressures and radio-frequency powers. The inverse Abel transform is also applied for a better spatially resoluted results. Meanwhile, the result of the continuous emission spectrum measurement is compared to that of the electrostatic double probes, which indicates the effectiveness of this method.

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.

A simple diagnostic tool for density measurements in plasma with a certain spatial resolution is proposed in this paper. It uses the emission characteristics of monopole antenna to determine the dielectic property of plasma = 1 − fp2/f2, with the fp electron plasma frequency related to plasma density. We immersed a monopole antenna probe into plasma and introduced a microwave signal via a network analyzer. When the emitted power is maximized, the reflected power is minimized and there occurs a resonance. Since can be derived from the resonant frequency, this is actually a method to measure the absolute electron density. Validated by a comparison with the amended Langmuir double-probe method, the monopole antenna probe is valuable. In addition, it is free from the difficulties, such as fluctuation in plasma potential.

A method is proposed to built up plasma based on a nonlinear enhancement phenomenon of plasma density with discharge by multiple internal antennas simultaneously. It turns out that the plasma density under multiple sources is higher than the linear summation of the density under each source. This effect is helpful to reduce the fast exponential decay of plasma density in single internal inductively coupled plasma source and generating a larger-area plasma with multiple internal inductively coupled plasma sources. After a careful study on the balance between the enhancement and the decay of plasma density in experiments, a plasma is built up by four sources, which proves the feasibility of this method. According to the method, more sources and more intensive enhancement effect can be employed to further build up a high-density, large-area plasma for different applications.

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.

An intelligent remote control system based on a power distribution unit (PDU) and Arduino has been designed for the electron cyclotron emission imaging (ECEI) system on Experimental Advanced Superconducting Tokamak (EAST). This intelligent system has three major functions: ECEI system reboot, measurement region adjustment and signal amplitude optimization. The observation region of ECEI can be modified for different physics proposals by remotely tuning the optical and electronics systems. Via the remote adjustment of the attenuation level, the ECEI intermediate frequency signal amplitude can be efficiently optimized. The remote control system provides a feasible and reliable solution for the improvement of signal quality and the efficiency of the ECEI diagnostic system, which is also valuable for other diagnostic systems.

A new integrated technique for fast and accurate measurement of the quasi-optics, especially for the microwave/millimeter wave diagnostic systems of fusion plasma, has been developed. Using the LabVIEW-based comprehensive scanning system, we can realize not only automatic but also fast and accurate measurement, which will help to eliminate the effects of temperature drift and standing wave/multi-reflection. With the Matlab-based asymmetric two-dimensional Gaussian fitting method, all the desired parameters of the microwave beam can be obtained. This technique can be used in the design and testing of microwave diagnostic systems such as reflectometers and the electron cyclotron emission imaging diagnostic systems of the Experimental Advanced Superconducting Tokamak.

The time evolution of the argon electron-beam plasma at intermediate pressure and low electron beam intensity was presented. By applying the amplitude modulation with the frequency of 20 Hz on the stable beam current, the plasma evolution was studied. A Faraday cup was used for the measurement of the electron beam current and a single electrostatic probe was used for the measurement of the ion current. Experimental results indicated that the ion current was in phase with the electron beam current in the pressure range from 200 Pa to 3000 Pa and in the beam current range lower than 20 mA, the residual density increased approximately linearly with the maximum density in the log-log plot and the fitting coefficient was irrelative to the pressure. And then three kinds of kinetic models were developed and the simulated results given by the kinetic model, without the consideration of the excited atoms, mostly approached to the experimental results. This indicated that the effect of the excited atoms on the plasma density can be ignored at intermediate pressure and low electron beam current intensity, which can greatly simplify the kinetic model. In the end, the decrease of the plasma density when the beam current was suddenly off was studied based on the simplified model and it was found that the decease characteristic at intermediate pressure was approximate to the one at high pressure at low electron beam intensity, which was in good accordance with the experimental results.