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 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.

Measurement of molybdenum ion density for L-mode and H-mode plasma discharges in the EAST tokamak

We report the measurement of total molybdenum ion density for L-mode and H-mode plasmas on EAST using spectral lines observation and calculation based on an impurity transport code.A flat-filed extreme ultraviolet spectrometer with some spatial resolution is used to obtain the radial profiles of molybdenum spectral line emissions.The absolute calibration for the extreme ultraviolet spectrometer is finished by comparing the calculated bremsstrahlung intensity with the readings of CCD detector.Molybdenum ion transport study is performed using the radial ion density profiles and one-dimensional impurity transport code STRAHL.The total molybdenum density profiles are determined from the transport analysis.The molybdenum density during L-mode and H-mode phases are obtained,which are about 3 and 4 orders of magnitude smaller than the electron density,respectively.An inward pinch is found during the H-mode phase that leads to the peaked profile of molybdenum density.

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.

Study on divertor heat flux under n=3 and n=4 resonant magnetic perturbations using infrared thermography diagnostic in EAST

Resonant magnetic perturbations(RMPs)with high toroidal mode number n are considered for controlling edge-localized modes(ELMs)and divertor heat flux in future ITER H-mode operations.In this paper,characteristics of divertor heat flux under high-nRMPs(n=3 and 4)in H-mode plasma are investigated using newly upgraded infrared thermography diagnostic in EAST.Additional splitting strike point(SSP)accompanying with ELM suppression is observed under both RMPs with n=3 and n=4,the SSP in heat flux profile agrees qualitatively with the modeled magnetic footprint.Although RMPs suppress ELMs,they increase the stationary heat flux during ELM suppression.The dependence of heat flux on q_(95)during ELM suppression is preliminarily investigated,and further splitting in the original strike point is observed at q 495=during ELM suppression.In terms of ELM pulses,the presence of RMPs shows little influence on transient heat flux distribution.

Physical design of a new set of high poloidal mode number coils in the EAST tokamak

An external resonant magnetic perturbation(RMP)field,which is an effective method to mitigate or suppress the edge localized mode(ELM),has been planned to be applied on the ELM control issue in ITER.A new set of magnetic perturbation coils,named as high m coils,has been developed for the EAST tokamak.The magnetic perturbation field of the high m coils is localized in the midplane of the low field side,with the spectral characteristic of high m and wide n,where m and n are the poloidal and toroidal mode numbers,respectively.The high m coils generate a strong localized perturbation field.Edge magnetic topology under the application of high m coils should have either a small or no stochastic region.With the combination of the high m coils and the current RMP coils in the EAST,flexible working scenarios of the magnetic perturbation field are available,which is beneficial for ELM control exploration on EAST.Numerical simulations have been carried out to characterize the high m coil system,including the magnetic spectrum and magnetic topology,which shows a great flexibility of magnetic perturbation variation as a tool to investigate the interaction between ELM and external magnetic perturbation.

Realization of T_(e0)>10 keV long pulse operation over 100 s on EAST

In 2021,EAST realized a steady-state long pulse with a duration over 100 s and a core electron temperature over 10 keV.This is an integrated operation that resolves several key issues,including active control of wall conditioning,long-lasting fully noninductive current and divertor heat/particle flux.The fully noninductive current is driven by pure radio frequency(RF)waves with a lower hybrid current drive power of 2.5 MW and electron cyclotron resonance heating of 1.4 MW.This is an excellent experimental platform on the timescale of hundreds of seconds for studying multiscale instabilities,electron-dominant transport and particle recycling(plasma-wall interactions)under weak collisionality.

The Experimental Advanced Superconducting Tokamak

1.Introduction New sustainable energy is urgently needed to meet the fastgrowing requirement for clean energy in this century.Nearly 80%of the world’s energy is still generated by burning fossil fuels,resulting in pollution and climate change.To realize long-term sustainable development,it is necessary to explore large-scale new energy sources that do not produce carbon dioxide(CO_(2)),within the next few decades.

Long Pulse H-Mode Scenarios Sustained by RF Heating on EAST

Abstract EAST has demonstrated its capability of long pulse operation using RF heating （LHCD and ICRF） in past experiments. One key issue to realize the long pulse H-mode expert- meats is to sustain the plasma current for steady state operation. Based on the calculations of the transport code ONETWO and its coupled RF code GENRAY, two scenarios have been proposed to achieve the 10 s H-mode plasma with Ip=400 kA, 〈 ne 〉=4.5×1019 m-a, βN=2, and the 100 s H-mode plasma with Ip=280 kA, 〈 ne 〉=3.5×1019 m-a, βN=1.8 recently. The current drive of lower hybrid wave is an important issue in the two scenarios. An experimental result on lower hybrid current drive in H-mode plasmas on EAST is also presented.

Observation of Pedestal Plasma Turbulence on EAST Tokamak

Pedestal plasma turbulence was experimentally studied by microwave reflectometry on EAST tokamak. The characteristics of edge pedestal turbulence during dithering L-H transition, ELM-free H-mode phase and inter-ELM phase have recently been studied on EAST. An edge spatial structure of density fluctuation and its dithering temporal evolution is observed for the first time on the EAST tokamak during the L-H transition phase. A coherent mode usually appears during the ELM-free phase prior to the first ELM on EAST tokamak. The mode frequency gradually decreases as the pedestal evolves. Analysis shows that the coherent mode is in the pedestal region inside the separatrix. In plasma with type-III ELMs, a precursor mode before ELM is usually observed. The frequency of the precursor was initially about 150 kHz and gradually decreased till the next ELM. The mode amplitude increases or shows saturation before ELM. In the plasma with compound ELMs composed of high and low frequency ELMs, the precursor was also observed before the high frequency ELM while the harmonic oscillations with frequencies of 20 kHz, 40 kHz and 60 kHz appear before the low frequency ELM.

Analysis of electron temperature,impurity transport and MHD activity with multi-energy soft x-ray diagnostic in EAST tokamak

A new edge tangential multi-energy soft x-ray（ME-SXR） diagnostic with high temporal（≤ 0.1 ms） and spatial（~1 cm） resolution has been developed for a variety of physics topics studies in the EAST tokamak plasma. The fast edge electron temperature profile（approximately from r a~ 0.6 to the scrape-off layer） is investigated using ME-SXR diagnostic system. The data process was performed by the ideal ‘multi-foil＇ technique, with no priori assumptions of plasma profiles. Reconstructed ME-SXR emissivity profiles for a variety of EAST experimental scenarios are presented here for the first time. The applications of the ME-SXR for study of the effects of resonant magnetic perturbation on edge localized modes and the first time neon radiating divertor experiment in EAST are also presented in this work. It has been found that neon impurity can suppress the 2/1 tearing mode and trigger a 3/1 MHD mode.

Multi-channel retarding field analyzer for EAST

A multi-channel retarding field analyzer（MC-RFA） including two RFA modules and two Langmuir probes to measure the ion and electron temperature profiles within the scrape-off layer was developed for investigations of the interplay between magnetic topology and plasma transport at the plasma boundary.The MC-RFA probe for the stellarator W7-X and first measurements at the tokamak EAST was designed.The probe head allows simultaneous multichannel ion temperature as well as for electron temperature measurements.The usability for radial correlation measurements of the measured ion currents is also given.

Radiative Divertor Plasma Behavior in L- and H-Mode Discharges with Argon Injection in EAST

Introducing strong radiative impurities as an important way to mitigate the peak heat load will be employed in EAST for high power long pulse experiments were explored under both low （L） and the first time in EAST, with the injection of argon into divertor plasmas has been considered at the divertor target plate for ITER, and operations. To this end, radiative divertor high （H） - mode confinement regimes, for and its mixture （25% Ar in D2）. The Ar injection greatly reduced particle and heat fluxes to the divertor in L-mode discharges, achieving nearly complete detached divertor plasma regimes for both single null （SN） and double null （DN） configurations, without increasing the core impurity content. In particular, the peak heat flux was reduced by a factor of ~6~ significantly reducing the intrinsic in-out divertor asymmetry for DN, as seen by both the new infra-red camera and the Langmuir probes at the divertor target. Promising results have also been obtained in the H-modes with argon seeding, demonstrating a significant increase in the frequency and decrease in the amplitude of the edge localized modes （ELMs）, thus reducing both particle and heat loads caused by the ELMs. This will be further explored in the next experimental campaign with increasing heating power for long pulse operations.

Excitation of RSAEs during Sawteeth-Like Oscillation in EAST

The excitation condition of reversed shear Alfven eigenmodes(RSAEs)has been investigated during sawtoothlike oscillation in the EAST tokamak.The sawtooth-like phenomena can be reproduced in the configuration of reversed magnetic shear,and the threshold gradient of electron temperature is formed accordingly,together with the increasing of the confinement of thermal particles.The distribution function of energetic ions density is altered dramatically when the neutral beam is switched from NBI1 L(tangent)to NBI1 R(perpendicular),which can be captured by the measurement of radial neutron camera.The RSAEs are excited easily in the vicinity of q_(min)(1.99 m≤R≤2.06 m)for the injection of neutral beam with perpendicular direction,which should be excited by the steep gradient of energetic ions density.Furthermore,the excitation of RSAEs and the formation of threshold gradient of electron temperature can take place concurrently,which means that the neutral beam with perpendicular injection is beneficial for the establishment of internal transport barrier.

Electron Cyclotron Emission Imaging Observations of m/n=1/1 and Higher Harmonic Modes during Sawtooth Oscillation in ICRF Heating Plasma on EAST

The m/n = 1/1 and its higher harmonic modes are observed in sawtooth oscillations by using the novel high- resolution 2D ECE imaging system on the experimental advanced superconducting Tokamak （EAST）. Higher harmonic modes are appearing for a short time during the crash phase of sawtooth oscillation in lower βp plasma, which is not the preferabie position in the poloidal cross section. These modes generate sharp pressure points on the inversion radius during the crash phase of sawtooth oscillation. Furthermore, reconnection events proceed in two distinctive phases. In the first phase, a small amount of heat is expelled through the weak reconnection while in the second phase the remaining large quantity of heat and particles emerged rapidly from the hot core to the peripheral region of the inversion radius. In addition, these harmonic modes are only found before and after the ICRF pulse, while in the ICRF pulse only the （1,1） mode exists in the sawtooth oscillation.

Q-Band X-Mode Reflectometry and Density Profile Reconstruction

By installing an X-mode polarized Q-band（32-56 GHz） reflectometry at the low field side on EAST,the zero density cutoff layer was determined and the edge density profile was measured in normally operating plasmas.A Monte Carlo procedure has been developed to analyze the density profiles by considering the error of time delay measured by reflectometry.By combining this Q-band and previously developed V- and W-band reflectometries,the density profiles from edge to core can be measured in most EAST experiments.The line integrated densities deduced from density profiles measured by reflectometry are consistent with those directly measured by a horizontal interferometer.The density pedestal measured by reflectometry shows a clear crash during an ELM（edge localized mode） event,after which the pedestal gradually increases and recovers in 10 ms and then remains little changed up to the next ELM.