Yongcai Shen

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...

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.

The Edge Rotation and Temperature Diagnostic System on EAST Tokamak

The Edge rotation and temperature diagnostic system (ERD) on EAST tokamak is designed to obtain profiles of the ion temperature and rotation velocity at the edge of the plasma. Multiple toroidal and poloidal chords are coupled to the entrance multi-slit of one spectrometer equipped with back-illuminated charged coupled device (CCD) camera that has a high quantum efficiency and high-readout speed. The cameral records spatially and spectrally resolved emission profiles with real time digitization. Line-integrated radiation from different ionization stages in the visible range is chosen as representative of the plasma in their regions of maximum emission. The diagnostic including the diagnostic cannula, collection assembly and spectrometer as well as CCD is discussed. The initial test results of the system are given.

Flat-field EUV spectrometer and its performance test by Penning discharge source

A high-resolution extreme ultraviolet (EUV) spectrometer is developed to diagnose the magnetically confined plasmas in the spectral region of 5-50 nm. The spectrometer consists a holographic aberration-corrected concave grating, an entrance slit, an x-ray shutter and a deep-cooled back-illuminated CCD camera. This paper presents the spectrometer structure, the spectral resolution simulation. The spectrometer is tested by a Penning discharge light source which is a convenient wavelength calibration source working at EUV spectral range. This light source emits a number of standard spectral lines with different combinations of discharge gas and cathode material, which will provide different standard spectral lines for wavelength calibration and spectral resolution evaluation. Results show that the spectral resolution is about 0.015 nm at 20 nm which agrees with the design goal.

Observation of Molybdenum Emission from Impurity-Induced Long-Lived m = 1 Mode on the Experimental Advanced Superconducting Tokamak*

We observe the spectra of molybdenum for the first time since the first wall of our experimental advanced superconducting tokamak (EAST) was changed mainly to molybdenum tiles. A large amount of molybdenum accumulated in the central plasma where the long-lived m = 1 mode instability bursts is shown. Molybdenum is proved to be the main impurity species observed during the formation and lifetime of impurity-induced long-lived m = 1 mode. This may indicate that a close relationship exists between the high-Z impurity accumulation and the occurrence of long-lived m = 1 mode in EAST plasmas.

A quantitative evaluation of various deconvolution methods and their applications in the deconvolution of plasma spectra

A quantitative evaluation of various deconvolution methods and their applications in processing plasma emitted spectra was performed. The iterative deconvolution algorithms evaluated here include Janssons method, Richardson–Lucys method, the maximum a posteriori method and Golds method. The evaluation criteria include minimization of the sum of squared errors and the sum of squared relative error of parameters, and their rate of convergence. After comparing deconvolved results using these methods, it was concluded that Janssons and Golds methods were able to provide good profiles that are visually close to the original spectra. Additionally, Golds method generally gives the best results when considering all the criteria above. The applications to the actual plasma spectra obtained from the EAST tokamak with these methods are also presented in this paper. The deconvolution results with Golds and Janssons methods show that the effects of instruments can be satisfactorily eliminated and clear spectra are recovered.