Impact of Ti/Te ratio on ion transport based on EAST H-mode plasmas

Jianwen LIU (刘建文),Qing ZANG (臧庆),Yunfeng LIANG (梁云峰),Jiale CHEN (陈佳乐),Xiaohe WU (邬潇河),2,Alexander KNIEPS,Jiahui HU (扈嘉辉),Yifei JIN (金仡飞),2,Bin ZHANG (张斌),Yuqi CHU (储宇奇),Haiqing LIU (刘海庆),Bo LYU (吕波),Yanmin DUAN (段艳敏),Miaohui LI (李妙辉),Yingjie CHEN (陈颖杰),Xianzu GONG (龚先祖) and the EAST Team

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

2 University of Science and Technology of China,Hefei 230026,People’s Republic of China

3 Forschungszentrum Jülich GmbH,Institut für Energie-und Klimaforschung– Plasmaphysik,Partner of the Trilateral Euregio Cluster (TEC),Jülich 52425,Germany

Abstract At the EAST tokamak,the ion temperature (Ti) is observed to be clamped around 1.25 keV in electron cyclotron resonance (ECR)-heated plasmas,even at core electron temperatures up to 10 keV (depending on the ECR heating power and the plasma density).This clamping results from the lack of direct ion heating and high levels of turbulence-driven transport.Turbulent transport analysis shows that trapped electron mode and electron temperature gradient-driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas.Nevertheless,recently it was found that the Ti/Te ratio can increase further with the fraction of the neutral beam injection(NBI) power,which leads to a higher core ion temperature (Ti0).In NBI heating-dominant Hmode plasmas,the ion temperature gradient-driven modes become the most unstable modes.Furthermore,a strong and broad internal transport barrier (ITB) can form at the plasma core in highpower NBI-heated H-mode plasmas when the Ti/Te ratio approaches～1,which results in steep core Te and Ti profiles,as well as a peaked ne profile.Power balance analysis shows a weaker Te profile stiffness after the formation of ITBs in the core plasma region,where Ti clamping is broken,and the core Ti can increase further above 2 keV,which is 80% higher than the value of Ti clamping in ECR-heated plasmas.This finding proposes a possible solution to the problem of Ti clamping on EAST and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

Keywords: ion temperature clamping,transport,neutral beam injection,stiffness,internal transport barrier

1.Introduction

A major goal of fusion energy research is to heat and confine ionized gases (i.e.deuterium and tritium) so that they fuse into heavier elements,producing significantly more power from fusion reactions than the power put into the plasma for heating [1].An essential condition for the realization of fusion reactions is to have a high fusion triple product(niTiτE),which is extremely dependent on the temperature of the main ions.Therefore,it is critical to raise the ion temperature effectively in the fusion devices.

In future plasmas,alpha particles will be predominantly slowed down by electrons,since their energy at birth is typically two orders of magnitude larger than the temperature of deuterium and tritium (DT) fuel ions.They will then indirectly heat ions through electron–ion collisions [2].Therefore,electron heat transport in plasmas with differentTi/Teratios is essential for the study of magnetic confinement fusion.So far,the research of electron heat transport has been performed on many tokamak and stellarator devices(NSTX [3],ASDEX Upgrade [4–8],DIII-D [9–11],JT-60U[12],JET [13–17],W7-X [18,19],T-10 [20–22],TJ-II [23]).Experimentally,ion temperature (Ti) clamping occurring atTi～ 1.5 keV independent of the magnetic configuration has been already reported in both ASDEX Upgrade (L-mode)and W7-X electron-heated plasmas [18,19],which is mainly due to the lack of direct ion heating (ion heating through equipartition only) and the high levels of turbulent heat transport.This turbulent transport is considered to be mainly produced by micro-instabilities including,but not limited to,ion temperature gradient (ITG)-driven modes,trapped electron modes (TEMs),and electron temperature gradient(ETG)-driven modes [24].This discrepancy in heating schemes can cause confinement degradation,as it was initially observed in Heliotron-E [25] by applying electron cyclotron resonance heating (ECRH) and confirmed in LHD[26,27],which revealed a criticalTe/Tiratio for the degradation of ion transport at～0.75.Research in JET has already reported a study of ITG turbulence,which can affect theTiprofile stiffness [28].Recently,TEM and ETG-driven modes have been identified as the dominant mechanisms for the turbulent transport in electron heating-dominant plasmas on EAST [29–31].Therefore,the investigation of turbulent heat transport is essential in controlling the core transport and improving the plasma confinement performance.

In W7-X,it was recently found that wall treatment and pellet injection can increaseTitransiently in electron-heated plasmas,thereby breaking theTiclamping [19].However,stably maintaining such a high-Tiplasma remains a challenge.Hence,finding an effective method to increase the coreTiis crucial for future fusion reactions.

The experimental results presented in this manuscript focus on the EAST tokamak [32–34].EAST is a fully superconducting non-circular section tokamak device with a major radiusR≤1.9 m and a minor radiusa≤0.45 m,designed to achieve long-pulse steady-state high-performance H-mode plasmas at ITER-like configurations,primarily with radio frequency (RF) wave heating schemes.In EAST,the main auxiliary heating systems include ECRH,lower hybrid waves (LHW),ion cyclotron resonance heating (ICRH),and neutral beam injection (NBI) systems.Recently,two new ECRH systems have been developed for current drive and MHD control [35].With these upgrades,EAST has already achieved a stable long-pulse operation (more than 100 s)with a high core electron temperature (Te0＞ 10 keV) [36].

The achievement of high coreTehas been considered to be due to the strong synergistic effect between different RF waves heating on EAST,but so farTiis much lower thanTe.Previous results demonstrated that as the density ramps up,Tiincreases,and the correspondingTeprofile stiffness and turbulent transport are both affected in the electron-heated Lmode plasmas [37].AlthoughTicontinues to increase during the density ramp-up,Ticlamping does not occur as the clamping position has not yet been reached (＜ 1.25 keV).In this paper,turbulent transport analysis has been performed with differentTi/Teratio plasmas.Additionally,Ticlamping is studied in detail for the first time in electron heating-dominant plasmas on EAST,and a comparison between ECRheated plasma (on EAST,the electron-heated plasma can be obtained with applying ECRH,so-called the ECR-heated plasma) and NBI heating-dominant plasma is being discussed in detail.We observe thatTicould be effectively improved by applying NBI with a higher power fraction,where the ion transport recovers by increasing theTi/Teratio.

In previous studies,the relationship between fishbone instabilities and internal transport barriers (ITBs) on EAST was discovered [38,39].Here,we will discuss in detail the impact of ITB formation on the improvement of plasma performance and the increase in coreTiin NBI heatingdominant plasmas.For the turbulent transport and power balance analyses in this work,the transport code TRANSP[40] and the integrated modeling OMFIT [41] are utilized.

The remainder of this paper is organized as follows.The characteristics ofTiclamping on EAST are described in section 2.The effect of NBI heating on the core plasma profiles and its associated dominant micro-instability modes are discussed in section 3.The formation of strong and broad ITBs in NBI heating-dominant plasmas is shown in section 4.Finally,the discussion and conclusion are presented in section 5 and section 6,respectively.

2.Ion temperature clamping on EAST

On EAST,the ion temperature has generally been observed to be clamped at a maximum value mostly independent of the electron temperature in ECR-heated plasmas.The variation between the core ion temperature (Ti0) andTe0under different heating mechanisms is shown in figure 1.It can be seen that theTiclamping occurs at around～1.25 keV in ECRheated plasmas,while this phenomenon disappears when NBI is applied with a higher power fraction.In this section,the characteristics of ECR-heated and NBI heating-dominant plasmas will be discussed in detail.

For ECR-heated plasmas,Te0can vary widely,whereasTi0is constrained to a maximum value of around～1.25 keV,where the ion temperature is clamped.The Ohmic heating,ECR-heated L-mode and H-mode plasmas are shown in figure 1(a).Although those distributions differ slightly,the maximumTiin all cases is clamped at～1.25 keV.Both the core electron and ion temperatures are low for Ohmic heating.The main reason for the lowerTi0in ECR-heated Lmode plasmas with a highTe0is the low collisional coupling between electrons and ions in low-density L-mode plasmas[37]. In ECR-heated H-mode plasmas,Ti0increases compared to Ohmic and ECR-heated L-mode plasmas,which might be due to the improved plasma performance resulting in the formation of an edge transport barrier (ETB),further increasing theTi0(without raising it above the clamping limit).

Recently,it was found that theTi/Teratio can increase further by applying NBI with a higher power fraction(PNBI/Ptotal),and once a certain threshold is exceeded,theTiclamping is broken.Figure 1(b) illustrates the centralTi0versusTe0for different NBI power fractions.Note that the heating power calculated here refers to the absorbed power.In a comparison to ECR-heated plasmas,Ti0is further increased as the fraction of NBI power increases above the threshold (～40%),with its maximum value exceeding 2 keV,which is approximately 80% higher than that in the ECRheated plasmas.A detailed comparison of plasma parameters between ECR-heated and NBI heating-dominant plasmas will be presented in section 3.Here,the decrease inTe0is mainly related to the increases in plasma density.A largeTi0can be achieved by following theTe0=Ti0line,as shown in figure 1(b),where the NBI power fraction is around 70%.Here,the star points depicted in figures 1(a) and (b) represent the experimental shots to be analyzed in detail later,and all of them are H-mode plasmas.A detailed analysis of the ITB formation will be presented in section 4.Further experimental investigation shows that the plasma performance can be significantly improved when a strong and broad internal transport barrier (ITB) forms in the core in NBI heatingdominant H-mode plasmas,where the phenomenon ofTiclamping is further broken.In brief,there is a threshold dependency between theTi/Teratio and the NBI power fraction,and a trend of raising theTi/Teratio can be observed following the increase in NBI power fraction.Therefore,after exceeding the power fraction threshold (～40%),the application of NBI power can not only effectively enhance the centralTi0and the ratio ofTi/Te,but also significantly contribute to the overall improvement of plasma confinement performance in fusion devices,just like the formation of ITBs.

Figure 1.Variation of core electron temperature (Te0) with core ion temperature (Ti0) in (a) ECR-heated plasmas and (b) NBI heatingdominant plasmas on EAST.

3.Effect of NBI heating on the core plasma profiles and its associated dominant micro-instability modes

In order to understand the underlying mechanism ofTiclamping and the predominant unstable modes involved,a comparative analysis of turbulent transport in ECR-heated and NBI heating-dominant plasmas has been performed on EAST.It should be noted that the characteristics of turbulent transport in ECR-heated L-mode plasmas on EAST were previously analyzed and published in reference [37].

3.1.Characteristics of kinetic profiles

In this comparative turbulent transport analysis,two typical H-mode plasmas were selected on EAST.One is an ECRheated plasma (#80725) with auxiliary heating consisting of～2 MW 4.6 GHz LHW and～0.5 MW on-axis ECRH.The other is an NBI heating-dominant plasma (#102407)sustained with～2 MW 4.6 GHz LHW and～2.9 MW in total of NBI power.Figure 2 shows the radial distributions of the electron and ion temperatures,electron densities (ne),and theTi/Teratios for those two shots.Here,ρ is the square root of the normalized toroidal magnetic flux.For the ECR-heated plasma,theTeprofile measured by Thomson scattering (TS)diagnostic [42] is rather peaked,while theTiprofile measured by tangential imaging x-ray crystal spectrometer[43,44] is relatively flat.The synergistic effect between LHW and ECRH has been demonstrated before on EAST [30],where a small proportion of ECRH can increase theTegradient significantly.A lowerTi/Teratio can be obtained due to a lowTiin the core plasma region.However,for EAST shot #102407,with a certain amount of NBI heating,theTiprofile becomes peaked in the core region (ρ＜ 0.5),while changing little in the outer region (ρ＞ 0.5).Ti0increases significantly (figure 1(b)) and is much higher than the value of theTiclamping (～1.25 keV) in ECR-heated plasmas.In addition,Tedecreases across the radial direction and its profile becomes less peaked.Theneprofile reconstructed from the 11-channel far-infrared laser polarimeterinterferometer (POINT) diagnostic [45] shows a flat distribution and a higher value than that of shot #80725.Moreover,in the NBI heating-dominant plasma,theTi/Teratio becomes larger and approaches 1 within the entire confined plasma radius,indicating the importance of NBI power injection for ion heating in the plasmas.

Figure 2.(a) Electron temperature profile,(b) ion temperature profile,(c) electron density profile,and (d) Ti/Te ratio profile for EAST(shot #80725 for ECR-heated plasma and shot #102407 for NBI heating-dominant plasma).

3.2.Comparison of turbulent transport in ECR-heated and NBI-dominated H-mode plasmas

Figure 3 shows the electron and ion energy fluxes and effective electron and ion thermal diffusivity profiles for EAST shot #80725 at 4.417 s,which are calculated by power balance analysis [46] depending on the transport code TRANSP.The electron energy flux is dominated by turbulent flux [47] and decreases gradually as the normalized radius moves toward to the plasma boundary region.The ion energy flux differs significantly from that of electrons in EAST shot #80725,as it exhibits a slow radial increase and is significantly smaller in magnitude compared to the electron energy flux.Figure 3(b) shows a flat profile of electron thermal diffusivity χein the core,with the amplitude of electron thermal diffusivity χebeing lower than 1 m2/s insideρ=0.4,while the ion thermal diffusivity χiis relatively larger.It indicates that the peakedTeprofile is strongly correlated with the high electron energy flux and reduced turbulent transport within the plasma core region.In a direct comparison to ECR-heated plasmas,the core electron energy flux decreases significantly in EAST shot #102407,while the ion energy flux increases when NBI power is injected.Furthermore,the difference between electron and ion energy fluxes is reduced (figure 4(a)).Both the core effective electron and ion thermal diffusivity profiles are flat,with a value higher than 1 m2/s for EAST shot #102407 at 4.517 s.

Figure 3.(a) Electron and ion energy flux and (b) electron and ion thermal diffusivity profiles at 4.417 s for EAST shot #80725.

Figure 4.(a) Electron and ion energy flux,(b) electron and ion thermal diffusivity profiles at 4.517 s for EAST shot #102407.

Figure 5.The normalized growth rates ((a) and (b)) and frequency spectra ((c) and (d)) with different positions (ρ～ 0.4/0.5) for EAST shots #80725 and #102407.

The TGLF [48] model is utilized to calculate the spectrum of the most unstable linear eigenmodes,and their growth rates and frequency spectra are displayed in figure 5.The system of units used iscsρs=cs/Ωs,Ωs=eB/mic.The normalized growth rate,frequency,and poloidal wave number are γ′=γ(a/cs),ω′=ω(a/cs),andky=ρskθ,respectively.The length scaleais the circular equivalent minor radius of the last closed flux surface.The analysis indicates that the most unstable modes in shot#80725 are the medium wavelength low-k(ky＜1) TEM and the short wavelength high-k(ky＞1) ETG.Both unstable modes are mainly driven by theTeandnegradients [49].Here,the positive sign of ω′represents a mode rotating in the electron diamagnetic direction.For a comparison,in EAST shot #102407,the most unstable modes are the lowk(ky＜1) ITG and the high-k(ky＞1) ETG modes (figures 5(b) and (d)),which is consistent with the increasedTigradient.After changing from a normalized radius ρ=0.4 to ρ=0.5,the most unstable modes changed from a combination of ITG and ETG modes to ITG modes only.

3.3.Power balance analysis in NBI-dominated H-mode plasmas

Additionally,power balance analysis using the transport code TRANSP combined with the equilibrium code kinetic-EFIT has been conducted to study the stiff transport [50,51].Here,theTeprofile stiffnessis defined by the electron heat flux in gyro-Bohm [11] units=qeeBR2,where ρsis the Larmor radius,qeis the electron heat flux,Ris the tokamak major radius,andBis the toroidal magnetic field) divided by the normalizedTegradientR/LTe[4,49,52].

To demonstrate the effect of NBI power injection,the power scan of NBI for EAST shot #102407 is considered.The power scan of NBI was performed at three different times (3.017 s,3.317 s,and 4.517 s) corresponding to three different NBI heating power levels (PNBI～1.3 MW,PNBI～2.2 MW,andPNBI～2.9 MW,respectively).Figure 6 shows the variation of the electron heat fluxin gyro-Bohm units against the normalizedTegradientR/LTefor EAST shot #102407.A ‘negative profile stiffness’ [19]appears at ρ=0.3 in the electron temperature channel following the increase of NBI power in EAST shot #102407,while the change of electron heat fluxin gyro-Bohm units is relatively stable or slightly increased at ρ=0.4.This indicates that theTeprofile stiffness in the core region becomes weaker as the NBI power increases.Moreover,theTeprofile stiffness becomes stronger as the normalized radius ρ moves outward,which is similar to that observed in the ECR-heated L-mode plasmas [37].Figure 7 shows the growth rates and frequency spectra of the most unstable modes at ρ=0.4 for EAST shot #102407.It is shown that the growth rates of the ITG modes increase with NBI power injection,which is considered to be due to the increasedTigradient.However,the ETG modes are not greatly affected by the change inTiprofile.This result also suggests that the ITG modes are the most unstable modes in the highTi/Teratio plasmas on EAST.

Figure 7.The normalized growth rates (a) and frequency spectra(b) at ρ=0.4 for EAST shot #102407.

4.Formation of ITB in NBI-dominated H-mode plasmas

In order to compare with the NBI heating-dominant H-mode plasmas,in this section,the impact of the ITB formation on the plasma performance with NBI power injection will be discussed in detail.

4.1.Formation of ITB in Te,Ti,and ne profiles

Figure 8 shows an overview of the main plasma parameters for this H-mode plasma (EAST shot #56933).During the plasma current flat-top phase (430 kA),the magnetic fieldBTis 1.6 T,the auxiliary heating includes 4.6 GHz LHW(PLHW≈1 MW) and NBI,where the NBI power is applied at 2.4 s and then gradually increases in four steps,and finally,the total injection power of NBI reaches 3.7 MW att=3.52 s.The plasma enters H-mode at aboutt=2.45 s,and the formation of strong and broad ITBs in theTe,Ti,andneprofiles appears att=～3.59 s with the increase in NBI power fraction.After the formation of ITBs,the core lineaveraged density ＜ne＞ and the stored energy increase rapidly,as shown in figures 8(b) and (d).Meanwhile,the energy confinement enhancement factorH98also increases when the ITBs are formed.

Figure 8.Time evolutions of (a) plasma current Ip,(b) line-averaged density,(c) auxiliary heating power of LHW and NBI,(d)stored energy,and (e) confinement enhancement factor H98y2 for EAST shot #56933.The dashed lines represent three different times analyzed later.

Figure 9.(a) Electron temperature profiles,(b) ion temperature profiles,(c) electron density profiles,and (d) Ti/Te ratio profiles for EAST shot #56933.

Figure 10.(a) Electron,(b) ion,and (c) total (electron and ion) thermal diffusivity profiles for EAST shot #56933.

Comparisons of theTe,Ti,ne,andTi/Teprofiles measured at three times with different NBI heating power fractions are presented in figure 9.All of them are H-mode plasmas with a strong ETB.These selected time instants are indicated by the dashed lines in figure 8.The core kinetic profiles show that both theTeprofile and theTiprofile become much steeper following the NBI power increase,where a strong and broad ITB is formed in the core plasma region after the last NBI power injection.The formation of the broad electron and ion ITBs is accompanied by a peakedneprofile,where an electron density ITB is also formed,as shown in the line-averaged density (figure 8(b)).Here,theneprofile was measured by TS diagnostic.Ti0does not exceed the clamping value ofTiatt=2.75 s.However,the centralTi0increases significantly with the increasing NBI power fraction,and thenTi0increases further above 2 keV att=3.7 s,which is much higher than the value ofTiclamping in the ECR-heated plasmas.Although strong ITBs are formed in both the electron and ion channels,Tiis a little higher than theTein the core plasma region.Moreover,theTi/Teratio in the core increases slightly as the NBI power fraction increases.Here,the experimental data are marked as the star points in figure 1(b).The ITB foot can be observed at about ρ=0.5,and the temperature and density profiles out the ITB region does not change significantly.The thermal diffusivities corresponding to these three time instants are shown in figure 10.Both the electron and ion thermal diffusivities in the core are low under different NBI power fractions,and a further reduction in the total (electron and ion) thermal diffusivity can be observed in the core region (ρ～0.4) when the strong and broad ITBs are formed.

4.2.Turbulent transport analysis with the formation of ITB

Figure 11 shows the variation of electron heat fluxin gyro-Bohm units with the normalizedTegradientR/LTefor EAST shot #56933.Here,we choose the location inside the ITB (ρ=0.4) and near the ITB foot (ρ=0.5) for the analysis.Compared to the previous experiments [37],theTeprofile stiffness becomes much weaker when the strong and broad ITBs are formed,with thedecreasing andR/LTeincreasing noticeably.Meanwhile,a negative profile stiffness appears in the electron temperature channel,indicating that the formation of ITBs can effectively reduce theTeprofile stiffness.Because the confinement on the heat and particle channels becomes better with the formation of ITBs[39] (which is mainly due to the reduction of turbulent transport associated with the ITG modes [53]),it results in a drop in the electron heat fluxin gyro-Bohm units and an increase in theTegradient in the strong and broad ITB region.Both the changes in the electron heat fluxand normalizedTegradientR/LTeshow a weakTeprofile stiffness.Furthermore,the coreTiincreased by 65% compared to the clamping value ofTiin ECR-heated plasmas,demonstrating a great positive impact of ITB formation on weakening theTeprofile stiffness and improving the plasma performance.

Figure 11.Variation of electron heat flux in gyro-Bohm units with normalized Te gradient R/LTe.The gray circles represent the case with ITBs.

In order to better understand the experimental results,the variations of theTi0/Te0ratio with ＜ne＞/nGWand the growth rate of most unstable modes with theTi0/Te0ratio on EAST are summarized in figures 12 and 13,respectively.Note that the signals in figure 13 represent the maximum value of the growth rate for each unstable mode.Figure 12(a) shows that the maximum value of theTi0/Te0ratio for ECR-heated plasmas is less than 0.5.A bifurcation can be observed as the plasma transitions from L-mode dominance to H-mode dominance following the ＜ne＞/nGWincrease.This demonstrates that the density plays a crucial role in the formation of H-mode plasmas.Notably,the most unstable modes are TEMs when theTi0/Te0ratio is small (figure 13),which is mainly due to the high coreTeand the largeTegradient.However,the TEM and ETG modes coexist as theTi0/Te0ratio increases,which is consistent with the increase in ＜ne＞/nGW.Figure 12(b) shows the variation of theTi0/Te0ratio with ＜ne＞/nGWfor NBI heating-dominant plasmas.Quantitatively,theTi0/Te0ratio increases with the fraction of the NBI power to the total power.For plasmas withPNBI/Ptotal=10%-40%,theTi0/Te0ratio increases slightly compared to the case of ECR-heated plasmas (figure 12(a)).Furthermore,theTi0/Te0ratio decreases slightly as＜ne＞/nGWincreases,where TEMs and ETGs are the most unstable modes,as shown in figure 13.As the fraction of NBI power increases (PNBI/Ptotal=40%-70%),theTi0/Te0ratio increases,resulting in a change in the most unstable modes.The appearance of ITG modes is observed as theTi0/Te0ratio continues to increase,and their growth rate increases following theTi0/Te0ratio increase,while the ETG modes remain at a low level.In addition,theTi0/Te0ratio increases slightly following ＜ne＞/nGWincreases.As the ratio of NBI to the total injection power exceeds 0.7,theTi0/Te0ratio increases significantly.The maximumTi0/Te0ratio can reach around～1.4 at moderate density and with the formation of ITBs,as shown in figure 12(b).Here,the star points with various colors represent different total heating powers.However,it can be observed that the total heating power has little effect on theTi0/Te0ratio.For a high NBI power fraction,the most unstable modes are ITGs,which is consistent with the findings described in section 3.These results suggest that TEMs and ETGs are the most unstable modes for smallTi0/Te0ratio plasmas,while ITGs become dominant with the increasing NBI power fraction,which also has a positive correlation with the growth rate of the ITG modes.This is considered to be due to the increase in theTiandTigradient during this process.

Figure 12.Variation of Ti0/Te0 ratio with ＜ne ＞/nGW for (a) ECRheated plasmas and (b) NBI heating-dominant plasmas on EAST.

Figure 13.The growth rate of most unstable modes (ITG,ETG,and TEM) as a function of Ti0/Te0 ratio at ρ=0.4 on EAST.

Figure 14.Variations of Ti0/Te0 ratio with ion thermal diffusivity and χe/χi ratio at ρ=0.3 ((a) and (b)),and ρ=0.4 ((b) and (d)) on EAST.

Furthermore,the influence of theTi0/Te0ratio on the electron and ion transport channels on EAST is also summarized here.Figures 14(a) and (c) demonstrate the variation between ion thermal diffusivity and theTi0/Te0ratio at different normalized radii.It can be observed that the ion thermal diffusivity decreases significantly when theTi0/Te0ratio approaches～1,leading to an improvement in ion channel confinement and a subsequent rise in the centralTi0,which is also beneficial for the formation of strong and broad ITBs.Additionally,it can be found that a jump in ion thermal diffusivity appears atTi0/Te0～0.4,especially at the normalized radius position ρ=0.3,where the ion thermal diffusivity first decreases and then rises slowly.This position exactly corresponds to the region where theTiclamping is broken and coincides with an NBI power fraction of～40%.This result demonstrates that the confinement of the ion channel is improved during the process of breaking theTiclamping.Figures 14(b) and (d) show the variation of the ratio between electron and ion thermal diffusivity (χe/χi)with respect to theTi0/Te0ratio,indicating a similar result to figures 14(a) and (c),which shows that the χe/χiratio gradually increases and surpasses～1 as theTi0/Te0ratio approaches～1.

5.Discussion

In this article,we have discussed the phenomenon ofTiclamping in ECR-heated plasmas (which could greatly limit the plasma performance in future fusion devices) and its difference from NBI heating-dominant plasmas on EAST.In NBI-dominated H-mode plasmas,Tican reach values up to 80% higher than the usual clamped ion temperatures in ECRheated plasmas with a resulting highTi/Teratio.This might be attributed to the direct ion heating by applying NBI with a high power fraction and the recovery of ion transport by the increasedTi/Teratio.The improved ion channel transport is associated with the increasingTi/Teratio in NBI heatingdominant plasmas,resulting in an increase in the centralTi0,despite the growth rate of the ITG mode (the predominant unstable mode) gradually increasing during this period.

Furthermore,the turbulent transport analysis in electron heating-dominant plasmas has been investigated on EAST.It was found that TEM and ETG are the most unstable modes in ECR-heated plasmas.However,the dominant unstable modes with a lowTi/Teratio plasma are different from what has been observed in W7-X [19,54],and the specific reasons for this discrepancy require further analysis.Therefore,the suppression of turbulent transport is also critical for alleviatingTiclamping.

6.Conclusion

The phenomenon ofTiclamping has been observed for the first time in ECR-heated plasmas on EAST.For ECR-heated plasmas,Te0can be varied widely,from 1 keV to 10 keV,mainly depending on the heating power and plasma density.However,Ti0is small and clamped at～1.25 keV,which is mainly due to the lack of direct ion heating and high levels of turbulent heat transport.Turbulent transport analysis shows that the most unstable modes are medium-wavelength,low-k(ky＜1) TEM modes and short-wavelength,high-k(ky＞1) ETG modes,which are mainly driven by theTeandnegradients.By comparing with ECR-heated plasmas,there is a threshold for the NBI power fraction (～40%)above which theTiclamping phenomenon is broken,coinciding with a change in ion transport.In this work,differentTi/Teratio plasmas with NBI power injection have been discussed.The coreTiincreases significantly with the NBI power injection.Turbulent transport analysis shows that the most unstable modes are the ITG and ETG modes in NBI heating-dominant plasmas,where the growth rate of ITG modes increases following theTi0/Te0ratio increase.Notably,theTi0/Te0ratio can become substantially higher when the fraction of NBI power exceeds 70%.Meanwhile,as theTi0/Te0ratio increases,there is a noticeable improvement in the confinement of the ion channel when theTi0/Te0ratio approaches～1.

In addition,as the total NBI heating power and NBI power fraction are further increased,theTeandTiprofiles become much steeper,accompanied by a peakedneprofile,after the strong and broad ITB is formed in the core plasma region.Power balance analysis shows that theTeprofile stiffness reaches its weakest value after the formation of ITBs.This indicates that the formation of ITBs can not only improve the plasma confinement,but also greatly reduce theTeprofile stiffness.This finding proposes a possible solution to the problem ofTiclamping and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

Acknowledgments

This work is supported by National Natural Science Foundation of China (No.12135015),the Users with Excellence Program of Hefei Science Center,CAS (No.2021HSCUE012),the National Key R&D Program of China (No.2022Y FE03010003),the Major Science and Technology Infrastructure Maintenance and Reconstruction Projects of the Chinese Academy of Sciences 2021,the Special Funds for Improving Conditions for Scientific Research in National Scientific Institutions 2022,and the China Scholarship Council.The numerical calculations in this paper were performed on the ShenMa High Performance Computing Cluster in the Institute of Plasma Physics,Chinese Academy of Sciences.