C. X. Yu

Investigation of rescaled range analysis, the Hurst exponent, and long-time correlations in plasma turbulence

A detailed investigation of rescaled range (R/S) analysis to search for long-time correlations (via the Hurst exponent, H) in plasma turbulence is presented. In order to elucidate important issues related to R/S analysis, structure functions (SFs), one of several techniques available for calculating H, are also applied, and comparisons between the two methods are made. Time records of both simulated data and fluctuation reflectometry data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] are analyzed. It is found that the R/S method can be used to accurately determine H, provided a long enough data record is used, and that H is an indicator of persistence in the data. In addition, subtleties of the correct application of both methods are discussed, and potential advantages of SFs are pointed out.

Structure function analysis of long-range correlations in plasma turbulence

Long-range correlations (temporal and spatial) have been predicted in a number of different turbulence models, both analytical and numerical. These long-range correlations are thought to significantly affect cross-field turbulent transport in magnetically confined plasmas. The Hurst exponent, H—one of a number of methods to identify the existence of long-range correlations in experimental data—can be used to quantify self-similarity scalings and correlations in the mesoscale temporal range. The Hurst exponent can be calculated by several different algorithms, each of which has particular advantages and disadvantages. One method for calculating H is via structure functions (SFs). The SF method is a robust technique for determining H with several inherent advantages that has not yet been widely used in plasma turbulence research. In this article, the SF method and its advantages are discussed in detail, using both simulated and measured fluctuation data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, ...

Spectral features of the geodesic acoustic mode and its interaction with turbulence in a tokamak plasma

The three-dimensional wavenumber and frequency spectrum for the geodesic acoustic mode (GAM) has been measured in the HuanLiuqi-2A tokamak for the first time. The spectrum provides definite evidence for the GAM, which is characterized by kθ=kϕ=0 and krρi≈0.04−0.09 with the full width at half-maximum Δkrρi≈0.03−0.07. The localized GAM packet is observed to propagate outward in the radial direction with nearly the same phase and group velocity. The envelopes of the radial electric field and density fluctuations are observed to be modulated by the GAM. By comparing the experimental result with that of the envelope analysis using model signals, the mechanism of the envelope modulation has been identified. The results strongly suggest that the envelope modulation of the Er fluctuations is dominantly caused by the direct regulation of the GAM during the GAM generation in the energy-conserving triad interaction, and the envelope modulation of the density fluctuations is induced by the GAM shearing effect, which...

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.

First results from H-mode plasmas generated by ICRF heating in the EAST

Deuterium high-confinement (H-mode) plasmas, lasting up to 3.45 s, have been generated in the EAST by ion cyclotron range of frequency (ICRF) heating. H-mode access was achieved by coating the molybdenum-tiled first wall with lithium to reduce the hydrogen recycling from the wall. H-mode plasmas with plasma currents between 0.4 and 0.6 MA and axial toroidal magnetic fields between 1.85 and 1.95 T were generated by 27 MHz ICRF heating of deuterium plasma with hydrogen minority. The ICRF input power required to access the H-mode was 1.6–1.8 MW. The line-averaged density was in the range (1.83–2.3) × 1019 m−3. 200–500 Hz type-III edge localized mode activity was observed during the H-mode phase. The H-mode confinement factor, H98IPB(y, 2), was ~0.7.

Cherenkov radiation generated by a beam of electrons revisited

Cherenkov radiation generated by a beam of electrons is theoretically investigated. In the case that the boundary effect is negligible, coherent Cherenkov radiation does not depend on the longitudinal bunch form of the electron beam, which is remarkably different from other kinds of coherent radiation like coherent transition radiation and coherent synchrotron radiation. The reason for this result is ascribed to the criterion of the emission of Cherenkov radiation. The angular distribution of coherent Cherenkov radiation is mainly determined by the transverse bunch form of the beam. The spectral intensity of incoherent Cherenkov radiation is proportional to the velocity distribution function of the electrons in the beams. Based on these results, some methods are suggested to study hot electrons with the measurement of Cherenkov radiation.

Overview of HL-2A experiment results

Recent experiment results from the HL-2A tokamak are presented in this paper. Supersonic molecular beam injection (SMBI) with liquid nitrogen temperature propellant is used. Low temperature SMBI can form hydrogen clusters that penetrate into the plasma more deeply and efficiently. Particle diffusion coefficient and convection velocity (D = 0.5?1.5?m2?s?1 and Vconv < 40?m?s?1, respectively) are obtained at the plasma periphery using modulated SMBI. Multi-probe measurements reveal the m = 0?1, n = 0 symmetries of directly measured low frequency (7?9?kHz) electric potential and field are simultaneously observed for the first time. Impurity transport is determined with the laser blow-off system and transport code. A disruption predictor has been derived based on MHD activity observations and statistical analysis. Sawtooth characteristics during ECRH are investigated and coupling between m = 1 and m/n = 2/1 modes is studied. Detachment features of HL-2A divertor are numerically and experimentally studied using the code SOLPS5.0 and measured data. The long divertor legs and thin divertor throats in HL-2A pose MHD shaping problems resulting in momentum losses even at low densities and strongly enhanced main chamber losses.

Overview of experimental results on HL-2A

Significant experimental advances have been made on the HL-2A tokamak along with substantial improvement and development of the hardware. A spontaneous particle transport barrier has been observed in Ohmic discharges without any external momentum input. The barrier was evidenced by a density perturbation study using modulated supersonic molecular beam injection (SMBI) and microwave reflectometry. The new features of the non-local transport effect induced with SMBI have been analysed. The three-dimensional spectral structures of the low frequency zonal flow, the geodesic acoustic mode (GAM) and the quasi-mode-like low frequency fluctuations have been observed simultaneously for the first time. In addition, the spectral structure of the density fluctuations of GAM was also identified. The e-fishbone instability excited by energetic electrons deviated from Maxwellian distribution has been investigated via a 10-channel CdTe hard x-ray detector. It was found that the e-fishbone was correlated with the existence of energetic electrons of 30–70 keV. The MHD experiment has indicated that the suppression of m/n = 2/1 tearing modes may be sustained by ECRH modulation at a frequency of about 10 Hz.

Overview of experimental results on the HL-2A tokamak

The physics experiments on the HL-2A tokamak have been focused on confinement improvement, particle and thermal transport, zonal flow and turbulence, filament characteristics, energetic particle induced modes and plasma fuelling efficiency since 2008. ELMy H-mode discharges are achieved in a lower density regime using a combination of NBI heating with ECRH. The power threshold is found to increase with a decrease in density, almost independent of the launching order of the ECRH and NBI heating power. The pedestal density profiles in the H-mode discharges are measured. The particle outward convection is observed during the pump-out transient phase with ECRH. The negative density perturbation (pump-out) is observed to propagate much faster than the positive one caused by out-gassing. The core electron thermal transport reduction triggered by far off-axis ECRH switch-off is investigated. The coexistence of low frequency zonal flow (LFZF) and geodesic acoustic mode (GAM) is observed. The dependence of the intensities of LFZFs and GAMs on the safety factor and ECRH power is identified. The 3D spatial structures of plasma filaments are measured in the boundary plasma and large-scale structures along a magnetic field line analysed for the first time. The beta-induced Alfven eigenmodes (BAEs), excited by large magnetic islands (m-BAE) and by energetic electrons (e-BAE), are observed. The results for the study of fuelling efficiency and penetration characteristics of supersonic molecular beam injection (SMBI) are described.

Spectral characteristics of geodesic acoustic mode in the HL-2A tokamak

The spectral characteristics of the geodesic acoustic mode (GAM) are investigated systematically by applying the two-point correlation technique and bispectral analysis to electric field fluctuations measured by electrostatic probe arrays on the HuanLiuqi-2A (HL-2A) tokamak. The three-dimensional wavenumber and frequency spectrum for the GAM has been measured for the first time. The spectrum provides definite evidence for the GAM which is characterized by kθ = k = 0 and krρi ≈ 0.04–0.09 with the full width at half maximum Δkrρi ≈ 0.03–0.07. The radial wavenumber spectrum shows that the localized GAM packet propagates in the outward direction with approximately the same phase and group velocity. The cross-bicoherences involving the Reynolds stress and auto-bicoherences of potential ( ), radial electric field ( ) and density ( ) fluctuations have been estimated for comparisons. Strong nonlinear coupling between the GAM and broadband turbulence is observed in all summed bicoherences, except for the summed auto-bicoherence of density fluctuations. All cross- and auto-bicoherences, except for the auto-bicoherence of density fluctuations, for interactions satisfying f1 + f2 = fGAM are found to have a peaked feature in the frequency range f1 < 100 kHz. This peaked feature might reflect the resonance property in the nonlinear coupling between the GAM and ambient turbulence.