Kaiyun Chen

Impurity effects on the Ni/Ni3Al interface cohesion

The effects of B, C, N, O, H, P and S impurities on the Ni/Ni3Al interface cohesion have been investigated by employing first-principles electronic structure calculations based on the discrete variational method. The binding energy, bond order, difference electron density, orbital occupations and density of states have been calculated to study the impurity-induced changes in the energetics and electronic structure. The impurities promote the Ni/Ni3Al interface cohesion and prefer to occupy the interface interstitial sites in the order S < P < H < O < N < B < C. The impurity-nickel covalent-like bonds form mainly due to impurity-p/Ni-d hybridization (except H-s/Ni-p hybridization in the H case). Meanwhile, the Ni-Ni bonding becomes weaker because of charge depletion on Ni atoms and bond misorientation resulting from the more homogeneous electron redistribution. In the order B, C, N and O, the impurity-metal bond varies from being homopolar to being much more heteropolar with increasing ionicity percentage, which results in decreasing p-d hybridization effects

Observation of heat transfer across x point of the islands during sawtooth crash on the HT-7 tokamak

It is observed that a large heat flow abruptly transfers across the x point (which can be located at both the high field side and the low field side) of the 1/1 or 2/2 magnetic islands during a sawtooth crash on the HT-7 tokamak by using tomography of the high-resolution soft-x-ray emission together with the singular value decomposition technique. Although the poloidally asymmetric heat flow contributes an m = 1 component of perturbation in the signals, the heat flow is obviously different from the growth of the m = 1 magnetic island. Furthermore, it is also shown that the large heat flow or the displacement of the hot core occurs only after the crash. Hence, the large displacement of the hot core is just a result of the crash rather than the cause. The heat flow from both the x points of the m = 2 island is also observed. This observation demonstrates that a purely fast reconnection of the m = 1 magnetic island is not responsible for the crash, but that it may be due to the rapid energy and particle diffusion in the stochastic region.

Measurement of Radiated Power Loss on EAST

A type of silicon detector known as AXUV (absolute extreme ultraviolet) photodiodes is successfully used to measure the radiated power in EAST. The detector is characterized by compact structure, fast temporal response (<0.5 s) and flat spectral sensitivity in the range from ultra-violet to X-ray. Two 16-channel AXUV arrays are installed in EAST to view the whole poloidal cross-section of plasma. Based on the diagnostic system, typical radiation distributions for both limiter and divertor plasma are obtained and compared. As divertor detachment occurs, the radiation distribution in X-point region is observed to vary distinctly. The total radiation power losses in discharges with different plasma parameters are briefly analyzed.

Primary Results of Lithium Coating for the Improvement of Plasma Performance in EAST

First lithium coating associated with ion cyclotron range of frequency (ICRF) plasma was performed successfully in EAST. Results in reduction of both residual impurity and deuterium in the vacuum vessel were obtained. Particularly the partial pressure of deuterium after the lithium coating was reduced by about a factor of 5. Impurity radiation in the plasma was reduced and electron temperature increased by about 50%. Moreover, reproducible plasma discharges with high parameters, such as higher plasma current and density, could be easily obtained. These results showed that plasma performance was improved. Even though only 2 g of lithium were injected, the effective lifetime of the Li film was raised up to 40 shots.

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)

Preliminary observation of medium-Z impurities by means of the soft x-ray pulse height analysis diagnostic on EAST

A soft x-ray pulse height analysis (PHA) system, based on a silicon drift detector (SDD), has been successfully implemented on EAST to measure spectra of soft x-ray emission. Utilizing the soft x-ray PHA diagnostic, the behavior of intrinsic and injected medium-Z impurities is first investigated on EAST and the preliminary observations are presented. There are hardly any K? emission peaks of metallic impurities including Ti (Z = 22), Cr (Z = 24), Fe (Z = 26), Ni (Z = 28) and Cu (Z = 29) in the soft x-ray spectra for an ohmic heating plasma due to lower electron temperatures, while remarkable peaks can be seen in a lower hybrid wave (LHW) heated plasma. The profiles of K? intensities of these metallic elements have been obtained with about 7?cm spatial resolution. It is observed that the K? emissions come mainly from the core plasma column of ? ? 0.3 and found that their intensities are roughly proportional to the power of the LHW launched into the plasma. The Ar K? peaks can also be monitored when argon gas is puffed into the plasma. It is qualitatively confirmed that the K? lines of argon emitted from the lower ionic states are predominantly located in the outer plasma region, whereas the K? lines with higher energies are mainly from high ionization stages in the central region.

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...

Understanding complex magnetohydrodynamic activities associated with a relaxation in the HT-7 tokamak

A new relaxation instability with complex magnetohydrodynamics (MHD) activities is found in the HT-7 tokamak operational region, which manifests itself in bursts of hydrogen alpha-ray radiations, electron cyclotron emission and soft x-ray (SX) radiations on outer channels, as well as complex MHD perturbations, but without hard disruptions. The MHD modes are identified as m/n = 2/1, m/n = 3/2 and m/n = 5/3 just before the relaxation (m and n are poloidal and toroidal mode numbers, respectively). The Poincare mapping, according to the measured perturbations of the MHD modes at the edge, is used to show the topology of magnetic field lines. It is found that a stochastic annular belt resulted just before the relaxation due to the m/n = 5/3 island overlapping with m/n = 2/1 and m/n = 3/2 islands. This qualitatively coincides with the results of SX tomography.

Understanding the destabilization of a resistive drift mode in EAST core plasmas

A low-frequency oscillation driven by heavy impurities is frequently observed in the Experimental Advanced Superconducting Tokamak (EAST) H-mode discharges accompanied by an Edge Localized Mode-free duration. This instability has been identified as a resistive drift mode according to a two-fluid simulation, showing that the mode stability is determined by the gradient of resistivity profile. Furthermore, the analytic theory also demonstrates that the mode is destabilized by the sharp impurity density gradient agreeing to the observation of intrinsic Mo (Molybdenum) concentration. The destabilization conditions have been derived like LN,z<23LT,z ( LN,z and LT,z are the scale lengths of density and temperature of impurity, respectively). Calculations based on the analytical theory reproduce the characteristic of the mode, which is in agreement with the experimental observations.