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Velocity, density, and magnetic fluctuations of the geodesic acoustic mode (GAM) have been measured using the Doppler backscattering system, Faraday-effect polarimeter-interferometer, and external pick-up coils in the Experimental Advanced Superconducting Tokamak. Simultaneous measurements of density and velocity fluctuations at the midplane and top of plasmas demonstrate that m = 1 density fluctuations are quantitatively balanced by the compression of perpendicular flow fluctuations. Furthermore, internal magnetic fluctuations associated with GAM have now been directly measured by laser-based Faraday-effect polarimetry for the first time. Line-averaged magnetic fluctuations (up to 16 Gauss, B ̃ ¯ R , GAM B T ∼ 0.066 %) are significantly larger than those extrapolated from edge coils (a few Gauss) and that magnetic fluctuations increase with β. The observed discrepancy between finite β theory and experimental data indicates the need for further theoretical investigations. [ABSTRACT FROM AUTHOR]

Correlations between the edge fluctuations and the pedestal evolution during the relatively large edge localized mode (ELM) cycles at high pedestal normalized electron collisionality ( ν e , ped * > 1) on the EAST tokamak are investigated. Not only the edge electrostatic coherent mode (ECM, ∼50 kHz) and the low frequency magnetic coherent mode (MCM, ∼32 kHz) but also a high frequency electromagnetic mode (HFM, >150 kHz) are observed to be coexisting between ELMs. After the ELM crash, the pedestal electron temperature recovered faster than the pedestal electron density. It is found that the saturation of the ECM coincides more with the saturation of the pedestal electron density, while the saturation of the HFM and MCM coincides more with the saturation of the pedestal electron temperature. In addition, the characteristics of the electromagnetic fluctuations (the HFM and MCM) are studied in detail: the HFM propagates in the electron diamagnetic drift direction in the laboratory frame with an average poloidal wave number of k ¯ θ HFM ≈ 0.17 cm − 1 , while the MCM propagates in the ion diamagnetic drift direction in the laboratory frame with k ¯ θ MCM ≈ 0.12 cm − 1 and the toroidal mode number of n = 1. Furthermore, both the HFM and MCM have inward average radial wave numbers of k ¯ R HFM ≈ 0.13 cm − 1 and k ¯ R MCM ≈ 4.64 cm − 1 . The bispectral analysis shows that the HFM and MCM have strong nonlinear interactions. The HFM is clearly observed on both low and high field side Mirnov coils, which might suggest a feature beyond a ballooning type instability, e.g., the kinetic ballooning mode. These studies may contribute to a better understanding of the pedestal evolution. [ABSTRACT FROM AUTHOR]

An axisymmetric magnetic oscillation (AMO) of 5–10 kHz in the L-mode plasma edge has been studied on an experimental advanced superconducting tokamak. The analysis shows that this oscillation only occurs at the plasma very close to the L-H transition. The structure of magnetic oscillation is axisymmetric (n = 0) toroidally and demonstrates an m = 2 standing wave in the poloidal direction. The density fluctuation of AMO measured by reflectometry at the midplane is nearly in phase, indicating that the AMO is not a radial traveling wave. The AMO shows an obvious modulation effect on the background magnetic/density fluctuation, and as a result, the edge density/density gradient and particle flux onto the divertor are also modulated by the AMO. The frequency scaling of the AMO is f m ∝ (∇ n e / I p 2 ) − 1 . The bicoherence analyses show a strong nonlinear interaction between the AMO and the background turbulence, and other nonlinear interactions between the AMO and three other coherent fluctuations with frequencies (∼4, ∼11, and ∼15 kHz) are also found. [ABSTRACT FROM AUTHOR]

Poloidally localized density fluctuation with applied and spontaneous 3-D fields in high-confinement mode (H-mode) plasma is observed by using poloidally distributed multichannel interferometer diagnostics in the experimental advanced superconducting tokamak. With the application of an n = 1 rotating nonaxisymmetric magnetic perturbation (NAMP) field, a density fluctuation appears on different interferometer channels sequentially. Evidence shows that this fluctuation is located at the plasma edge. When an n = 1 static NAMP field is applied, the density fluctuation amplitude increases in parts of channels but decreases in other channels, implying a poloidal localization. This localization is enhanced with the increase in the NAMP field. In addition, when a spontaneous n = 1 external kink mode with a frequency of ∼3.5 kHz appears in the plasma edge, the amplitude of density fluctuation is modulated by the n = 1 kink. This modulation could be attributed to the localization effect on fluctuation by the 3-D fields of the spontaneous kink mode. [ABSTRACT FROM AUTHOR]

A supersonic molecular beam injection modulation experiment with a developed 11-channel polarimeter-interferometer has been performed on EAST. The particle deposition was determined by the edge density profile measured by microwave reflectometry. The particle transport coefficient profiles in low- and high-density discharges including low confinement and the high confinement mode (L mode and H mode) have been obtained by fitting the line-integrated modulation amplitude and phase profiles. The results show that in ohmic discharge, the difference between the edge and center diffusion coefficients is small compared to that in L and H modes. For convection velocity, the edge coefficients are several times larger than the center coefficients. However, this discrepancy is small in ohmic discharge. The H mode is characterized by a large inward convective velocity at the plasma edge which coincides with the steep pedestal. [ABSTRACT FROM AUTHOR]

A quasi-coherent mode (QCM) was measured by the tangential CO2 laser collective scattering diagnostic at high plasma electron density during both enhanced Dα/small edge-localized mode (ELM) and ELM-free H mode phases in Experimental Advanced Superconducting Tokamak (EAST). Experimental results from only local oscillator CO2 laser scattering prove that the QCM is measured by the scattering diagnostic in the far-forward mode. The driven QCM density fluctuation ( k ⊥ < 3 cm−1) and magnetic fluctuation suggest that the QCM is an electromagnetic mode. The typical frequency of the QCM is f ≈ 26.5 kHz. A combination analysis of scattering signals and Mirnov signals suggests that the QCM has toroidal mode number n ≈ 17 and rotates along with the electron diamagnetic drift velocity direction in the lab frame. The analysis of Mirnov and reflectometer signals supports that the QCM locates in the edge pedestal region. The QCM power has been found to be related to both the Dα signal and the pedestal density gradient. A comparison of the EAST QCM and C-Mod quasi-coherent mode has been given in detail. [ABSTRACT FROM AUTHOR]

Hot spots induced by lower hybrid wave in experimental advanced superconducting tokamak tokamak have caused high performance experiment disruption and serious damages to the guard limiters. Experimental and theoretical analyses have been carried out to study its physical mechanism. Plasma density scan experiments indicate that the wall temperature within the hot spots enhanced by a factor of 5 and increases with the plasma density near the antenna. A lower hybrid current drive (LHCD)-only density climb experiment shows that the carbon impurity decreases to a minimum value at certain plasma density and then increases with the line averaged plasma density. A model has been developed to explain the mechanism of sputtering of graphite tiles due to hot spots as the plasma density near the LHCD antenna and the time increases. A theoretical scaling of the heat flux driven by LHCD is also presented and is consistent with the experimental scaling in the Tore Supra tokamak. The simulation results show that the total sputtering flux density has a minimum at a certain plasma density and gradually increases as the plasma density increases or decreases away from the minimum value, and the increase in parallel heat flux near the antenna would enhance the sputtering flux density. The sputtering flux density trend is qualitatively consistent with the density scan experiments. The simulated temporal evolution of sputtered flux implies that the chemical sputtering could be a candidate for the carbon impurity explosion. [ABSTRACT FROM AUTHOR]

Both k1 = 12 cm−1 (intermediate-k) and k2 = 22 cm−1 (high-k) micro-turbulence (k⊥ρs ∼ 1–4) in gradient region plasmas were measured simultaneously by the EAST poloidal CO2 laser collective scattering system. Their behaviors have been studied in detail in both ELM-free/type III ELMy and enhanced Da (EDA) H mode plasma discharges. Compared with that in the previous L mode phase, it can be seen obviously that the mean frequency of lower-k (k1) micro-turbulence increases and that of higher-k (k2) decreases during the ELM-free or type III ELMy H mode phase, while frequency-integrated spectral power (Stot) of both k1 and k2 micro-turbulence seems almost the same. On the other hand, both Stot and mean frequency of k1/k2 micro-turbulence show continuous increase during the EDA H mode phase. These results imply that the response of (intermediate, high)-k micro-turbulence on different type H mode is different. Further cross-correlation analysis between k1 and k2 micro-turbulence shows that their cross-correlation value is a little lower in H mode phase than the previous L mode phase. It can also be found that the cross-correlation value decreases gradually during the ELM-free and subsequent type-III ELMy phases, and it is almost zero in the EDA H mode phase. These new results could shed some light on the comprehensive understanding of mechanisms under H mode physics. [ABSTRACT FROM AUTHOR]

A study of radial propagation and electric fields induced by charge separation in blob-like structures has been performed in a non-confined cylindrical electron cyclotron resonance heating plasma on Q-shu University Experiment with a Steady-State Spherical Tokamak using a fast-speed camera and a Langmuir probe. The radial propagation of the blob-like structures is found to be driven by E × B drift. Moreover, these blob-like structures were found to have been accelerated, and the property of the measured radial velocities agrees with the previously proposed model [C. Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)]. Although the dependence of the radial velocity on the connection length of the magnetic field appeared to be different, a plausible explanation based on enhanced short-circuiting of the current path can be proposed. [ABSTRACT FROM AUTHOR]

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao et al., Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-qa was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Zeff½fGW=0.9∼1.2, where fGW=ne/nGW. (Here ne is the maximum line average electron density and nGW is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter. [ABSTRACT FROM AUTHOR]