Yanmin Duan

EAST alternative magnetic configurations: modelling and first experiments

Heat and particle loads on the plasma facing components are among the most challenging issues to be solved for a reactor design. Alternative magnetic configurations may enable tokamak operation with a lower peak heat load than a standard single null (SN) divertor. This papers reports on the creation and control of one of such alternatives: a two-null nearby divertor configuration. An important element of this study is that this two-null divertor was produced on a large superconducting tokamak as an experimental advanced superconducting tokamak. A preliminary experiment with the second null forming a configuration with significant distance between the two nulls and a contracting geometry near the target plates was performed in 2014. These configurations have been designed using the FIXFREE code and optimized with CREATE-NL tools and are discussed in the paper. Predictive edge simulations using the TECXY code are also presented by comparing the advanced divertor and SN configuration. Finally, the experimental results of ohmic and low confinement (L-mode) two-null divertor and SN discharges and interpretative two-dimensional edge simulations are discussed. Future experiments will be devoted to varying the distance between the two nulls in high confinement (H-mode) discharges.

First results of LHCD experiments with 4.6 GHz system toward steady-state plasma in EAST

A 4.6 GHz lower-hybrid current drive (LHCD) system has been firstly commissioned in EAST in the 2014 campaign. The first LHCD results with 4.6 GHz show that LHW can be coupled to plasma with a low reflection coefficient, drive plasma current and plasma rotation, modify the plasma current profile, and heat plasma effectively. By means of configuration optimization and local gas puffing near the LHW antenna, good LHW–plasma coupling with a reflection coefficient less than 5% is obtained. The maximum LHW power coupled to plasma is up to 3.5 MW. The current drive (CD) efficiency is up to 1.1 × 1019 A m−2 W−1 and the central electron temperature is above 4 keV, suggesting that LH power could be mainly deposited in the core region, which is in agreement with code simulation. Experiments show that the current profile is effectively modified and toroidal rotation in the co-current direction is driven by the LHCD. Also, the CD efficiency and current profile depend on the launched wave spectrum, suggesting the possibility of controlling the current profile by changing the phase difference. Repeatable H-mode plasma is obtained by either the 4.6 GHz LHCD system alone, or together with a 2.45 GHz LHCD system, the NBI (neutral beam injection) system. The different ELM features of H-mode between the different heating methods are under investigation.

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.

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...

Characteristics of edge-localized modes in the experimental advanced superconducting tokamak (EAST)

Edge-localized modes (ELMs) are the focus of tokamak edge physics studies because the large heat loads associated with ELMs have great impact on the divertor design of future reactor-grade tokamaks such as ITER. In the experimental advanced superconducting tokamak (EAST), the first ELMy high confinement modes (H-modes) were obtained with 1?MW lower hybrid wave power in conjunction with wall conditioning by lithium (Li) evaporation and real-time Li powder injection. The ELMs in EAST at this heating power are mostly type-III ELMs. They were observed close to the H-mode threshold power and produced small energy dumps (1?2% of the stored energy). Type-III ELMs produced a time-averaged peak heat flux of about 2?MW?m?2 on the target plate, a value which is ?10 times larger than that of ELM-free phases. A few isolated and large type-I-like ELM events were also observed in EAST with an energy loss of up to 5% of the stored energy. Statistically, the ELM frequencies are several hundred hertz and the frequency appears to decrease with q95, the safety factor at 95% of the flux surface. When an ion cyclotron resonance frequency wave was injected during the H-mode phases, the ELM repetition frequency increased immediately. The frequency and amplitude of type-III ELMs can be effectively influenced by puffing impurity argon gas.

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...

Modeling of radiative divertor experiments with argon seeding for H-mode plasma in EAST

To obtain a quantitative understanding of the recent radiative divertor experiments and to give an instructive prediction for the future relative work performed on EAST, the 2D numerical tool SOLPS has been used to investigate the argon seeded EAST H-mode plasmas. The simulations were mainly based on a typical H-mode discharge with lower single null divertor configuration, in which the partially detached divertor plasma has been achieved due to the argon seeding. First the perpendicular particle and energy transport coefficients for particle density diffusivity D⊥, electron and ion conduction, χ⊥i,e were radially varied to determine the edge transport barrier in H-mode plasma by comparing the simulated upstream profiles of electron temperature (Te) and electron density (ne) with the edge Thomson scattering data. Then the reduction of the particle flux, static pressure, and the peak heat load onto the lower outer divertor target have been reproduced by radiative divertor simulations in agreement with the e...

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.

Investigation of the effects of impurity seeding under different magnetic configurations in L-mode plasma in EAST tokamak

Experiments with a small amount of Ar puffed from the lower outer target into L-mode plasma in double null (DN), lower single null (LSN), and upper single null (USN) configurations studied the effects of magnetic configurations and the seeded location relative to the X-point on the performance of Ar seeding. With Ar seeded, the peak heat flux and the integrated power load onto the target decreased most significantly in the LSN configuration, although the increase of the line integrated radiation in the main plasma was the lowest in this case and the highest in the USN configuration. The radiation fraction in the divertor rose from about 10% to 23% of the heating power in LSN, and from around 15% to 22% in DN, while it almost did not change in the USN configuration with Ar seeding. The line emission of Ar XVI increased following the puff and then started to degrade near the end of LSN and USN phases. No such degradation was seen in the DN phase, meaning that some Ar dwelt in the core till the LSN phase beg...