Xianzu GONG (龔先祖),Liqing XU (徐立清),*,Jinping QIAN (錢金平),Juan HUANG(黃娟),Rui DING(丁銳),A M GAROFALO,Annika EKEDAHL,Long ZENG (曾龍),Erzhong LI (李二眾),Bin ZHANG (張斌),Shiyao LIN (林士耀),Biao SHEN (沈飆),Mao WANG (王茂),Handong XU (徐旵東),Xinjun ZHANG (張新軍),Miaohui LI (李妙輝),Guizhong ZUO (左桂忠),Qing ZANG (臧慶),Haiqing LIU (劉海慶),Bo LYU (呂波),Liang WANG (王亮),Youwen SUN (孫有文),Guosheng XU (徐國盛),Jiansheng HU(胡建生),Damao YAO (姚達(dá)毛),Yu WU(武玉),Liqun HU(胡立群),Bingjia XIAO(肖炳甲),Nong XIANG(項(xiàng)農(nóng)),Kun LU (陸坤),Yuntao SONG(宋云濤),Baonian WAN(萬寶年),Jiangang LI(李建剛) and the EAST Team

1 Institute of Plasma Physics,Chinese Academy of Sciences,Hefei 230031,People’s Republic of China

2 General Atomics,San Diego 92186-5608,United States of America

3 CEA,Institute for Magnetic Fusion Research (IRFM),Saint-Paul-lez-Durance F-13108,France

Abstract 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.

Keywords: EAST tokamak,high Te,integrated operation scenario

1.Introduction

The development of a high core electron temperatureTe0of over 8.6 keV(～100 million degrees Celsius)has been set up as one of the EAST main goals since the campaign in 2018,and the plan is to extend the duration of the scenario to the thousandsecond timescale in the future [1-3].This is a platform for investigating electron thermal transport under weak collisionality,which is close to ITER dimensionless parameters[4].The study of electron-dominant heating is a crucial issue because the sustaining of fusion power at a steady output requires controlling the electron thermal transport in low collisionality,and thus heating fueling ions for self-consistent burning [5-7].The collisionality parameterνe*～0.02in EAST in the high-Te0scenario is close to the dimensionless value in ITER [1,3].This is also a platform to investigate the particle balance [8-11],which is an integrated issue related to plasma-wall interaction and radio frequency(RF)wave heating.

In 2018,EAST realized this scenario of a normalized collisionality parameterνe*～0.02under pure RF heating with a pulse length oft=10 s[1].In the 2019 campaign,the EAST group made great progress in developing the scenario and the underlying physics,especially in studying multiscale instabilities and interactions.The discharge pulse was extended to 20 s,which is ten times as long as the plasma current diffusion timescale[3].In the 2021 campaign,the RFdriven fully noninductive current andTe0＞10 keV discharge have been extended to 105 s with several engineering upgrades and novel techniques.First,a newly upgraded lower divertor is equipped with an excellent water-cooling system for heat flux and particle exhaust control.Second,a novel technique of injecting a lithium (Li) aerosol into the edge plasma is developed for density control [8].

2.Experiment setup

EAST is a medium-size non-circular tokamak with a major radiusR0= 1.80 ±0.05 m and a typical minor radiusa=0.45 m [12].The maximum torodial axial magnetic field isB0=2.5 T.In 2021,the electron cyclotron resonance heating (ECRH) was upgraded,allowing a total power of up to 1.4 MW for the central electron heating.The infrared radiation (IR) camera system was also upgraded in 2021,which covers all RF antennas and over 70% of divertor targets,to monitor the surface temperature of key plasma-facing materials.The integrated optical system provides an identical view of IR and visible light to ensure safe operation.An algorithm processing method is used in deducting and correcting linear drift of the integrator for magnetic diagnostic data in the EAST plasma control system.

3.Results

3.1.Te0 ＞10 keV over 100 s

3.2.Key parameter control in long duration

Figure 2(a) shows the new ITER-like lower tungsten divertor with flat structure,which enhanced particle and power exhaust capability during 100 s highTe0operation.A careful operational optimization between the plasma shape (e.g.,X-point)and the outer gap in lower single null(LSN)configuration was carried out for the maintenance of the high-RF power coupling and for the avoidance of the formation of hot spots on the 4.6 GHz LHCD antenna with a power up to 2 MW.The LSN magnetic configuration uses the iso-flux control [14]scheme based on the high performance of the newly upgraded ITERlike tungsten lower divertor in heat flux control.Figure 2(b)shows the LSN magnetic configuration and excellent shaping control(which matches very well with the shape control points at all of 20 s,40 s and 60 s) during long-pulse operation.

Figure 1.Time histories of EAST high-Te0 long-pulse discharge#98958.(a) Plasma current Ip and loop voltageVl oop ,(b) lineintegrated electron density and the feedback signal of SMBI,(c) central electron temperature Te 0, H98 and β p ,(d) radio frequency wave power PLH and PEC and lower-outer flat type divertor temperature measured with an infrared (IR) camera.

Figure 2.(a) The new lower divertor picture with flat structure,(b) LSN magnetic configuration and red star control points,(c) clear green color in the CCD frame at =t 40 s,showing the injection of the Li aerosol into the plasma edge,traces of the position of X-point (d) and recycling parameter (e).

Figure 3.(a)Te, Ti and q profiles at 6 s,(b)Ne profile at 6 s,(c)the HXR profile with energy range of 20-40 keV,(d)modeled current profiles.The vertical dashed line indicates the location of the q =1 surface.

Because the RF coupling is very sensitive to the plasma density,the density control in long-pulse plasma operation is very important [15].The EAST group made a big effort to achieve plasma density control with a timescale over 100 s.By means of the novel technique of injecting a Li aerosol into the edge plasma continuously and in real time combined with SMBI feedback [9,16],the plasma density is well controlled in the long-pulse operation,as shown in figure 1.The green color in figure 2(c) indicates that a Li aerosol is injected into the edge plasma.

An excellent plasma shape control within a few millimeters is important not only to the RF coupling but also to the constant density control.Figure 2(d) indicates that the major radius of X-point is controlled within a range of a few millimeters.The recycling parameter below unity (figure 2(e))during 100 s reflects a good maintenance of the plasma wall condition.The global recycling coefficientRglobalis evaluated by equation [17]:whereNeis the electron density,τpis particle confinement time,fis the fueling efficiency of external gas injection andqQinjectionis the external gas injection rate.

Figure 4.An m/n = 1/1 helical mode is observed in the core of a high-Te0 discharge with a duration over 100 s.(a) Windowed frequency spectrum of soft x-ray measurement,(b) competition of m/n = 1/1 mode and small-scale turbulence measured by Doppler backscattered diagnostic.The growing of m/n = 1/1 mode decreases the intensity of turbulence.

3.3.Power balance analysis

3.4.Multiscale interaction and discussion

Previous observations with EAST suggest that the existence of them/n= 1/1 mode plays a key role in sustaining stationary high-Te0long-pulse plasmas [1].The sawtooth replaced by a saturated MHD mode [22]is important to high performance plasma with highTe0.Because the presence of sawtooth instabilities will reduce performance and might trigger deleterious instabilities.In agreement with the observations in 2018 and 2019,the helicalm/n=1/1 mode is also present in the plasma core in long-pulse high-Te0discharge#98958 in 2021,as shown in figure 4.As shown in figure 4(b),them/n=1/1 mode can interact with small-scale turbulence,which is driven by electron temperature gradient,hence a decrease the turbulence intensity.In[3],a turbulence driven current has been confirmed,and it could be a candidate explanation of multiscale interaction between turbulence andm/n= 1/1 mode.Saturated interchange mode around the magnetic axis of plasma drives a near-helical flow pattern[22]and then mitigates the core turbulence level,which could be another reason for multiscale interaction.Multiscale interaction provided a new mechanism for turbulence mitigation under low torque.

4.Conclusion and future plan

A steady-state long-pulse discharge with high-Te0,lasting longer than 100 s,which is aided by an ITER-like tungsten divertor,has been obtained with a fully noninductive plasma current driven by LHCD and ECRH.This newly achieved steady-state long-pulse high-Te0scenario demonstrates the progress of physics related to multiscale instabilities under weak collisionality.EAST plans to demonstrate the steadystate high confinement(H mode)operation withTe0＞8.5 keV and maintain it over 1000 s (wall-plasma equilibrium time) in the future.Furthermore,a newly developed novelVloopfeedback control technology will be applied for MHD control in future.

Acknowledgments

The authors would like to acknowledge all of the EAST contributors and collaborators both domestic and international.The list of names can be found in the appendix at https://iopscience.iop.org/article/10.1088/1741-4326/ac2993.This work was supported by the National Key R&D Program of China (No.2022YFE03010003) and National Natural Science Foundation of China (No.12275309).