Jinlin Xie

Experimental observation of ion-acoustic waves in an inhomogeneous dusty plasma

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.

Progress of the Keda Torus eXperiment Project in China: design and mission

The Keda Torus eXperiment (KTX) is a medium-sized reversed field pinch (RFP) device under construction at the University of Science and Technology of China. The KTX has a major radius of 1.4 m and a minor radius of 0.4 m with an Ohmic discharge current up to 1 MA. The expected electron density and temperature are, respectively, 2 × 1019 m−3 and 800 eV. A combination of a stainless steel vacuum chamber and a thin copper shell (with a penetration time of 20 ms) surrounding the plasma provides an opportunity for studying resistive wall mode instabilities. The unique double-C design of the KTX vacuum vessel allows access to the interior of the KTX for easy first-wall modifications and investigations of power and particle handling, a largely unexplored territory in RFP research leading to demonstration of the fusion potential of the RFP concept. An active feedback mode control system is designed and will be implemented in the second phase of the KTX program. The recent progress of this program will be presented, including the design of the vacuum vessel, magnet systems and power supplies.

The evolution of the magnetic structures in electron phase-space holes: Two-dimensional particle-in-cell simulations

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.

Observation of nonlinear couplings between coexisting kinetic geodesic acoustic modes in the edge plasmas of the HT-7 tokamak

Coexisting multi-geodesic acoustic modes (GAMs), especially coexisting dual GAMs, are observed and studied through Langmuir probe arrays at the edge plasmas of the HT-7 tokamak with lithium-coated walls. The dual GAMs are named a low-frequency GAM (LFGAM) and a high-frequency GAM (HFGAM), and it is found that within the measuring range, the HFGAM propagates outwards while the LFGAM propagates both inwards and outwards with their central frequencies nearly unchanged, and both modes have maximum amplitudes at positions with radial wavenumbers close to zero; meanwhile, the two positions happen to be where the continuum GAM frequency is closest to the central frequencies of the LFGAM and the HFGAM. These characteristics are consistent with those of a kinetic GAM converted from a continuum GAM. The nonlinear couplings between the LFGAM and the HFGAM are also analysed. In this study, we observed not only the interaction between the LFGAM and the HFGAM, but also the self-coupling of the GAM with the beat frequency between them, as well as the coupling between the LFGAM and an unknown mode at ?50?kHz. These nonlinear interactions may play important roles during the saturation process of GAMs. Additionally, amplitude correlation analyses of multi-GAMs indicate that second harmonic GAMs are probably generated from the self-interaction of fundamental GAMs.

Optics design for J-TEXT ECE imaging with field curvature adjustment lens.

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 general optics structure of millimeter-wave imaging diagnostic on TOKAMAK

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.

Design of interferometer system for Keda Torus eXperiment using terahertz solid-state diode sources.

A solid-state source based terahertz (THz) interferometer diagnostic system has been designed and characterized for the Keda Torus eXperiment (KTX). The THz interferometer utilizes the planar diodes based frequency multiplier (X48) to provide the probing beam at fixed frequency 0.650 THz, and local oscillator is provided by an independent solid-state diode source with tunable frequency (0.650 THz +/- 10 MHz). Both solid-state sources have approximately 1 mW power. The planar-diode mixers optimized for high sensitivity, ∼750 mV/mW, are used in the heterodyne detection system, which permits multichannel interferometer on KTX with a low phase noise. A sensitivity of ⟨nel⟩min = 4.5 × 10(16) m(-2) and a temporal resolution of 0.2 μs have been achieved during the initial bench test.

Quasi-optics design of the dual-array ECE imaging system on the EAST Tokamak

A novel large aperture quasi-optical imaging system is designed for the new dual-array electron cyclotron emission (ECE) imaging (ECEI) instrument on the EAST tokamak. The zoom doublet scheme is used in the microwave imaging system on a super-conducting tokamak for the first time, and the focal plane can reach the high magnetic field side region even in the narrowest zoom configuration. The best spatial resolution in the vertical direction is 1.1 cm and the maximum vertical coverage can reach 80 cm. The field curvature is largely reduced in the narrow zoom configuration by the parabolic correction of a single lens surface. The imaging performance is fully characterized in the laboratory, and the characterized beam patterns show good agreements with the Gaussian beam specifications in the simulation results of the design.

Observations of compound sawteeth in ion cyclotron resonant heating plasma using ECE imaging on experimental advanced superconducting tokamak

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.

Evolutions of zonal flows and turbulence in a tokamak edge plasma during electron cyclotron resonance heating

Geodesic acoustic mode (GAM) and low-frequency zonal flow (LFZF) are both observed through Langmuir probe arrays during electron cyclotron resonance heating (ECRH) on the HL-2A tokamak edge. The radial distributions of the amplitude and peak frequency of GAM in floating potential fluctuations are investigated through rake probe arrays under different ECRH powers. It is observed that the GAM frequency would decrease and the intensity of carbon line emission would increase as the ECRH power exceeds a certain threshold. The analyses suggest that the impurity ions may play an important role in the GAM frequency at the edge region. It is also found that during the ECRH phase besides the mean flow, both GAM and LFZF are strengthened. The total fluctuation power and the fraction of that power associated with zonal flows both increase with the ECRH power, consistent with a predator–prey model. The auto- and cross-bicoherence analyses show the coupling between GAM and its second harmonic during the ECRH phase. Moreover, the results also suggest that the couplings between GAM and the components with multiple GAM frequency are strengthened. These couplings may be important for GAM saturation during the ECRH phase.