Liqing Xu

Comparative study on in vitro inhibition of grass carp (Ctenopharyngodon idellus) and mouse protein phosphatases by microcystins

Microcystins isolated from toxic cyanobacteria are potent inhibitors of protein phosphatases 1 and 2A (PP1 and PP2A). The inhibitory effects of three structural variants of microcystins (microcystin‐LR, ‐YR, and ‐RR) on protein phosphatases isolated and purified from the liver and kidney of grass carp (Ctenopharyngodon idellus) were investigated using the 32P radiometric assay. The relationships between percentage inhibition of protein phosphatase activity and microcystin levels followed a typical dose‐dependent sigmoid curve. These results were compared to those obtained from mouse PP2A. The degree and pattern of inhibition of both fish and mouse protein phosphatases by microcystins were similar. Protein phosphatases in crude fish tissue homogenates showed similar inhibition patterns as purified fish PP2A toward microcystins. © 2000 John Wiley & Sons, Inc. Environ Toxicol 15: 71–75, 2000

Excitation of (2,1) neoclassical tearing modes by mode coupling with (1,1) internal mode in EAST

Neoclassical tearing modes (NTM) are observed in discharges with auxiliary heating LH+ICRF and LH only during H-mode in EAST. The m/n?=?2/1 NTM is triggered by strongly coupling with an m/n?=?1/1 internal mode. Here, LH and ICRF are the abbreviations of lower hybrid resonance heating and ion cyclotron resonance frequency heating, respectively. The mode number of the NTM is m/n?=?2/1, where m is the poloidal mode number and n is the toroidal mode number. Just before the triggering of NTMs, an m/n?=?1/1 internal mode appears in the soft x-ray emission at plasma centre when the intensity of hard x-ray (IHX) reaches a critical value. The mode, characterized by frequency chirping in the spectrum, may be related to suprathermal electrons produced by LH. The saturated magnetic island width wsat of the NTM is strongly correlated with poloidal ?p. Normalized ?N,onset and the magnetic island critical width wcrit increase with electron temperature Te.

Observations of pressure gradient driven m = 1 internal kink mode in EAST tokamak

Pressure gradient driven m = 1 internal kink mode destabilization that follows an L-H transition is observed in the operational region of the EAST tokamak, which manifests in periodic oscillations in soft x-ray (SXR) and Mirnov coil signals. Using tomography with the high resolution soft x-ray detection array, we find that the rotation direction of the 1/1 kink mode is in the ion diamagnetic drift direction in poloidal cross-section. A large displacement of the hot core is attributable to the shift of the 1/1 internal kink mode. In contrast to stationary oscillations with fixed frequency, various frequency chirping behavior is observed with this 1/1 kink mode. Furthermore, we also occasionally observe that a 2/1 neoclassical tearing mode (NTM) is triggered by a 1/1 internal kink mode via mode coupling in a high-performance plasma. The spatial structure of a 2/2 mode, which is the harmonic mode of the 1/1 kink mode, is also presented in this paper. Large amounts of medium-Z impurities accumulate in the cen...

Observation of 1/1 impurity-related ideal internal kink mode locking in the EAST tokamak

m/n = 1/1 pressure gradient driven impurity-related ideal internal kink mode locking is observed in EAST limit-cycle state H-mode plasmas. This 1/1 mode has a radial structure with no inversion around the resonance, which implies that the mode has an ideal internal kink structure. The rotation direction of the 1/1 internal kink mode, as obtained by tomography of a high-resolution multi-array soft x-ray system, is the ion diamagnetic drift velocity direction. Impurities accumulated in the central region play a vital role in mode excitation. Remarkable loss of the yield of neutron rate due to 1/1 mode is also observed. A notch at the location in the vicinity of q = 1 is also observed in the local toroidal rotation profile. Furthermore, the frequency chirping behavior of the 1/1 kink mode is related to the toroidal rotation velocity of the plasma. (Some figures may appear in colour only in the online journal)

Density limits investigation and high density operation in EAST tokamak

Increasing the density in a tokamak is limited by the so-called density limit, which is generally performed as an appearance of disruption causing loss of plasma confinement, or a degradation of high confinement mode which could further lead to a H → L transition. The L-mode and H-mode density limit has been investigated in EAST tokamak. Experimental results suggest that density limits could be triggered by either edge cooling or excessive central radiation. The L-mode density limit disruption is generally triggered by edge cooling, which leads to the current profile shrinkage and then destabilizes a 2/1 tearing mode, ultimately resulting in a disruption. The L-mode density limit scaling agrees well with the Greenwald limit in EAST. The observed H-mode density limit in EAST is an operational-space limit with a value of . High density H-mode heated by neutral beam injection (NBI) and lower hybrid current drive (LHCD) are analyzed, respectively. The constancy of the edge density gradients in H-mode indicates a critical limit caused perhaps by e.g. ballooning induced transport. The maximum density is accessed at the H → L transition which is generally caused by the excessive core radiation due to high Z impurities (Fe, Cu). Operating at a high density () is favorable for suppressing the beam shine through NBI. High density H-mode up to could be sustained by 2 MW 4.6 GHz LHCD alone, and its current drive efficiency is studied. Statistics show that good control of impurities and recycling facilitate high density operation. With careful control of these factors, high density up to 0.93 stable H-mode operation was carried out heated by 1.7 MW LHCD and 1.9 MW ion cyclotron resonance heating with supersonic molecular beam injection fueling.

Control of sawtooth via ECRH on EAST tokamak

Localized electron heating produced by electron cyclotron resonant heating (ECRH) system has been proven to be powerful tools for controlling sawtooth instabilities, because such system allows to directly modify the local plasma parameters that determine the evolution of sawtooth periods. In this paper, we present the experimental results carried out on experimental advanced superconducting tokamak (EAST) with regard to sawtooth period control via ECRH. The electron cyclotron heating system on EAST was capable of inject electron cyclotron wave toward certain locations inside or outside q = 1 magnetic surface on the poloidal cross section, which renders us able to investigate the evolution of sawtooth period against the ECRH deposition position. It is found that when ECRH deposition position is inside the q = 1 surface, the sawtooth oscillation is destabilized (characterized by reduced sawtooth period). So far, inside the q = 1 surface, there are not enough EAST experiment data that can reveal more detaile...

Kinetic-MHD hybrid simulation of fishbone modes excited by fast ions on the experimental advanced superconducting tokamak (EAST)

Kinetic-MagnetoHydroDynamic hybrid simulations are carried out to investigate fishbone modes excited by fast ions on the Experimental Advanced Superconducting Tokamak. The simulations use realistic equilibrium reconstructed from experiment data with the constraint of the q = 1 surface location (q is the safety factor). Anisotropic slowing down distribution is used to model the distribution of the fast ions from neutral beam injection. The resonance condition is used to identify the interaction between the fishbone mode and the fast ions, which shows that the fishbone mode is simultaneously in resonance with the bounce motion of the trapped particles and the transit motion of the passing particles. Both the passing and trapped particles are important in destabilizing the fishbone mode. The simulations show that the mode frequency chirps down as the mode reaches the nonlinear stage, during which there is a substantial flattening of the perpendicular pressure of fast ions, compared with that of the parallel ...

Snake perturbation during pellet injection in the EAST tokamak

The pellet-induced snake oscillation was observed by soft x-ray (SXR) diagnostic in EAST for the first time after a fueling-sized pellet penetrated the q = 1 surface. The snake phenomenon has a long lifetime with a helicity of m = 1 and n = 1. Basic behaviors of the snake, including the triggering condition, interaction with the sawtooth and snake rotation frequency, were discussed in detail by multiple core diagnostics. The snake location was also analyzed through observation of the vertical SXR arrays and raw SXR brightness profiles. It is clear that the snake resided in a broad region between the magnetic axis and the q = 1 surface derived from equilibrium reconstruction. This investigation is beneficial for the understanding of the snake formation for EAST and future devices, like ITER and DEMO.

Simulation of fast-ion-driven Alfvén eigenmodes on the Experimental Advanced Superconducting Tokamak

Kinetic-MHD hybrid simulations are carried out to investigate possible fast-ion-driven modes on the Experimental Advanced Superconducting Tokamak. Three typical kinds of fast-ion-driven modes, namely, toroidicity-induced Alfven eigenmodes, reversed shear Alfven eigenmodes, and energetic-particle continuum modes, are observed simultaneously in the simulations. The simulation results are compared with the results of an ideal MHD eigenvalue code, which shows agreement with respect to the mode frequency, dominant poloidal mode numbers, and radial location. However, the modes in the hybrid simulations take a twisted structure on the poloidal plane, which is different from the results of the ideal MHD eigenvalue code. The twist is due to the radial phase variation of the eigenfunction, which may be attributed to the non-perturbative kinetic effects of the fast ions. By varying the stored energy of fast ions to change the fast ion drive in the simulations, it is demonstrated that the twist (i.e., the radial phas...

Fishbone activity in experimental advanced superconducting tokamak neutral beam injection plasma

Repetitive fishbones near the trapped ion procession frequency were observed for the first time in the neutral beam injection high confinement plasmas in Experimental Advanced Superconducting Tokamak (EAST) tokamak, and diagnosed using a solid-state neutral particle analyzer based on a compact silicon photodiode together with an upgraded high spatial-temporal-resolution multi-arrays soft X-ray (SX) system. This 1/1 typical internal kink mode propagates in the ion-diamagnetism direction with a rotation speed faster than the bulk plasma in the plasma frame. From the SX measurements, this mode frequency is typical of chirping down and the energetic particle effect related to the twisting mode structure. This ion fishbone was found able to trigger a multiple core sawtooth crashes with edge-2/1 sideband modes, as well as to lead to a transition from fishbone to long lived saturated kink mode to fishbone. Furthermore, using SX tomography, a correlation between mode amplitude and mode frequency was found. Finally, a phenomenological prey–predator model was found to reproduce the fishbone nonlinear process well.