Experimental study of core MHD behavior and a novel algorithm for rational surface detection based on profile reflectometry in EAST

Microwave reflectometry is a powerful diagnostic that can measure the density profile and localized turbulence with high spatial and temporal resolution and will be used in ITER,so understanding the influence of plasma perturbations on the reflect signal is important.The characteristics of the reflect signal from profile reflectometry,the time-of-flight(TOF)signal associated with the MHD instabilities,are investigated in EAST.Using a 1D full-wave simulation code by the Finite-DifferenceTime-Domain(FDTD)method,it is well validated that the local density flattening could induce the discontinuity of the simulated TOF signal and an obvious change of reflect amplitude.Experimental TOF signals under different types of MHD instabilities(sawtooth,sawtooth precursors and tearing mode)are studied in detail and show agreement with the simulation.Two new improved algorithms for detecting and localizing the radial positions of the low-order rational surface,the cross-correlation and gradient threshold(CGT)method and the 2D convolutional neural network approach(CNN)are presented for the first time.It is concluded that TOF signal analysis from profile reflectometry can provide a straightforward and localized measurement of the plasma perturbation from the edge to the core simultaneously and may be a complement or correction to the q-profile control,which will be beneficial for the advanced tokamak operation.

Observation of Doppler shift f_(D) modulated by the internal kink mode using conventional reflectometry in the EAST tokamak

In this paper we present a new experimental observation using a conventional reflectometry technique,poloidal correlation reflectometry(PCR),in the Experimental Advanced Superconducting Tokamak(EAST).The turbulence spectrum detected by the PCR system exhibits an asymmetry and induced Doppler shift f_(D)during the internal kink mode(IKM)rotation phase.This Doppler shift f_(D)is the target measurement of Doppler reflectometry,but captured by conventional reflectometry.Results show that the Doppler shift f_(D)is modulated by the periodic changes in the effective angle between the probing wave and cutoff layer normal,but not by plasma turbulence.The fishbone mode and saturated long-lived mode are typical IKMs,and this modulation phenomenon is observed in both cases.Moreover,the value of the Doppler shift f_(D)is positively correlated with the amplitude of the IKM,even when the latter is small.However,the positive and negative frequency components of the Doppler shift f_(D)can be asymmetric,which is related to the plasma configuration.A simulated analysis is performed by ray tracing to verify these observations.These results establish a clear link between f_(D)and IKM rotation,and are helpful for studying the characteristics of IKM and related physical phenomena.

Experimental investigation on divertor tungsten sputtering with neon seeding in ELMy H-mode plasma in EAST tokamak

Neon(Ne)seeding is used to cool the edge plasma by radiation to protect the divertor tungsten(W)target in the Experimental Advanced Superconducting Tokamak(EAST).The W sputtering in the outer divertor target with Ne seeding is assessed by the divertor visible spectroscopy system.It is observed that the W sputtering flux initially increases with Ne concentration in the divertor despite the decreasing plasma temperature.After reaching a maximum around 25 eV,the W sputtering rate starts to decrease,presenting a suppression effect.The effect on the divertor W sputtering flux and yield due to the competition between the increase of the Ne concentration and the decrease of the plasma temperature is discussed.The results show that enough Ne seeding is essential to effectively reduce the electron temperature and thus to suppress W sputtering.Moreover,ELM suppression is observed when Ne and W impurities enter the core plasma,which could be correlated to the enhanced turbulence transport in the pedestal.

Feasibility of applying the lower cut-off frequency for the density radial coverage extension in EAST reflectometry measurement

The extraordinary mode(X-mode)lower cut-off frequency is proposed for use in the reflectometry diagnostic on ITER for the electron density profile measurement,which is a trade-off between extreme plasma parameters and the accessible probing frequency.In contemporary experiments,the lower cutoff frequency can be identified at the probing frequency below the electron cyclotron frequency(f_(ce)) under certain plasma conditions.We provide here,for the first time,the experimental validation of the use of the lower cut-off frequency for the density profiles via the reflectometry measurement on EAST.The corresponding group delay of the lower cut-off frequency evolves continuously with the upper one,revealing a reasonable radial coverage extension of reflectometry measurement toward the plasma core.It is concluded that the lower cut-off frequency can be used as a supplement to the upper one in the density profile inversion process,which is of particular interest in the high magnetic field and/or density discharge to extend the radial coverage of reflectometry measurement.

Experimental study of double tearing mode on EAST tokamak

The double tearing mode(DTM)in a high βN(βN>1.5)discharge with internal transport barrier on EAST was investigated.A 15 kHz tearing mode(TM)(m≥3,n=3)appears at outer q=2 surface at first,which is stable in the highβN phase.Then a 2 kHz TM(m=2,n=1)occurs at inner q=2 surface.Soon after,high βN collapsed with the crash of ELM,and DTM formed during the collapse of βN.The positions of the two islands of the DTM are consistent with the q=2 surface.The temperature fluctuations are the strongest between the two magnetic islands.A statistical analysis of high βN discharges operating with the reversed magnetic shear configuration in the EAST 2015-2018 campaign revealed the existence of the DTM in many discharges.During the DTM phase,all βN are reduced by 10%-30% within 0.1 s.However,there are two different βN behaviors thereafter-with and without βN recovery.Studying the physical mechanism of βN recovery during the DTM phase will benefit steady-state operation with reversed shear configurations in the future.

Experimental study of core and edge fluctuations by reflectometry on EAST tokamak

An eight-channel poloidal correlation reflectometer(PCR)with O-mode polarization has been installed in the EAST tokamak to measure the fluctuations from core to edge.The PCR launches eight different frequency microwaves(20.4,24.8,33,40,42.4,48,52.6,57.2 GHz)into the plasma from the low field side and two poloidally separated antennae are used to receive the reflected waves.As a result,the diagnostic can measure fluctuations in eight(radial)×two(poloidal)spatial positions.The diagnostic has been applied to study the core and edge pedestal fluctuations during an inter-ELM phase in H-mode plasma.This inter-ELM phase can be divided into two stages.In the first stage,a low frequency(<50 kHz)broadband fluctuation dominates in the pedestal gradient region.In the second stage,this fluctuation is strongly suppressed and quasi-coherent fluctuations(QCFs)appear.The QCF’s amplitude increases with the pedestal density gradient,implying density gradient driven instabilities.But the core fluctuations inside the pedestal show no evident changes during the inter-ELM phase.