Impact of T_(i)/T_(e )ratio on ion transport based on EAST H-mode plasmas

At the EAST tokamak, the ion temperature(T_(i)) 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 T_(i)/T_(e)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 H-mode 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 high-power NBI-heated H-mode plasmas when the T_(i)/T_(e)ratio approaches ~1, which results in steep core Teand Tiprofiles, as well as a peaked neprofile. Power balance analysis shows a weaker Teprofile stiffness after the formation of ITBs in the core plasma region, where Ticlamping is broken,and the core Tican increase further above 2 keV, which is 80% higher than the value of Ticlamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of Ticlamping 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.

Kinetic equilibrium reconstruction with internal safety factor profile constraints on EAST tokamak

Reconstruction of plasma equilibrium plays an important role in the analysis and simulation of plasma experiments. The kinetic equilibrium reconstruction with pressure and edge current constraints has been employed on EAST tokamak. However, the internal safety factor(q) profile is not accurate. This paper proposes a new way of incorporating q profile constraints into kinetic equilibrium reconstruction. The q profile is yielded from the Polarimeter Interferometer(POINT)reconstruction. Virtual probes containing information on q profile constraints are added to inputs of the kinetic equilibrium reconstruction program to obtain the final equilibrium. The new equilibrium produces a more accurate internal q profile. This improved method would help analyze EAST experiments.

Studies of beam ion confinement to enhance plasma performance on EAST

A key physics issue for achieving steady-state high-performance plasmas on EAST tokamak is to decrease beam-ion losses to improve plasma confinement during neutral beam injections(NBIs).To decrease the beam losses,previous counter-I_(p)NBI injections are upgraded to co-I_(p)injections.Analysis shows that due to the reversed direction of drift across the flux surfaces caused by the pitch angle,the beam prompt loss fraction decreases from about 49%to 3%after the upgrade.Moreover,because of the change of entire beam path,beam shine-through(ST)loss fraction for counter-I_(p)tangential and counter-I_(p)perpendicular injections is reversed to co-I_(p)tangential and co-I_(p)perpendicular injections,respectively.Due to the change in the initial trapped-confined beam ion fraction caused by the peaked pitch profiles,the losses induced by toroidal ripple field are also reversed after the upgrade.To further improve the beam-ion confinement under the present NBI layout,the amplitudes of toroidal field are increased from 1.75 to 2.20 T.Result shows that,due to the smaller orbit width and peaked pitch angle profile,the beam prompt loss power is lower with higher toroidal field.Due to the synergy of higher initial trapped-confined beam ion fraction and narrower Goldston-White-Boozer(GWB)boundary,the loss induced by ripple diffusion is higher with higher toroidal field.The combined effect of beam ST loss,prompt loss and ripple loss,contributes to the increase in beam ion density.The decrease in beam loss power enhances beam heating efficiency,especially the fraction of beam heating ions.Finally,comparison between simulation and measurement by^(235)U fission chamber(FC)indicates that the increase in neutron rate is mainly contributed by improvement of beam-ion confinement.This study can provide potential support for beam operation and high-T_(i)experiment on EAST tokamak.

Inversion techniques to obtain local rotation velocity and ion temperature profiles for the x-ray crystal spectrometer on EAST

Inversion techniques are conducted based on the tangential x-ray crystal spectroscopy(TXCS)geometry on EAST to obtain the local profiles of ion temperature(Ti)and toroidal rotation velocity(vt).Firstly,local emissivity profiles of the impurity argon are obtained using the asymmetrical Abel inversion.Then,the local vt and Ti profiles are calculated by considering the local emissivity profiles and the TXCS detailed geometry.In addition,how the changes in the vt profiles affect the accuracy in the Ti profiles is discussed in detail.It is also found that the lineintegrated Ti profiles are becoming less accurate with the increase in the radial gradient in the local vt profiles.Nonetheless,accurate Ti radial profiles are reconstructed after considering the effects of the emissivity and velocity,which are verified by comparing the inverted vt and Ti profiles with those local profile measurements from the Charge eXchange Recombination Spectroscopy(CXRS)on EAST.

EAST托卡马克上共振磁扰动对锯齿行为的影响

锯齿振荡是托卡马克装置上常见的磁流体不稳定性之一.先进实验超导托卡马克(experimental advanced superconducting Tokamak,EAST)装置放电中观察到外加共振磁扰动(resonant magnetic perturbation,RMP)会影响锯齿振荡行为.本文研究了环向模数n=2的RMP对锯齿周期的影响.通过RMP线圈相位差扫描实验发现,当RMP上下线圈(n=2)相位差ΔφUL发生变化时,扰动场的谱型发生变化,锯齿周期和崩塌幅度也随之发生明显的改变.单流体电阻磁流体力学(MARS-F)程序分析表明,在相同实验条件下,当RMP上下线圈(n=2)相位差为ΔφUL=270°时,等离子体芯部区域的响应最强,此时锯齿周期和幅度最小.锯齿行为的改变可能与RMP引起的快离子损失相关.该研究利用RMP上下线圈(n=2)实现对锯齿行为的有效调控,有利于未来EAST发展高比压放电条件下的大锯齿自动控制技术.

Experimental study of density gradient-driven micro-instabilities and the confinement degradation during H-mode in EAST

A broadband(BB)mode is observed by collective Thomson scattering diagnostics in repeatable shots of EAST and analyzed for the first time.This BB mode usually grows during L–H transitions,featuring a BB quasi-coherent mode with increasing frequency.During H-mode operations,it is characterized by steady-state BB in the high-frequency range(f~200–2000 k Hz),at the electron scale(k_(θ)ρ_(s)=1–2),mainly driven by the density gradient,and is sensitive to the value ofηein the region of interest(ρ=0.4–0.8),wherehe=(R/L_(Te))/(R/L_(ne))is the ratio of the normalized electron temperature gradient and density gradient,and the regionρ=0.4–0.8 usually has a relatively low collisionality(v_(eff)<5).The frequency of BB is found to be dependent on the electron temperature and density gradient,which is a typical feature of electron-driven turbulence.A negative correlation between the energy confinement and the intensity of the BB turbulence during H-mode has been found,which indicates a strong electron thermal transport induced by the BB turbulence.The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the region of interest(ρ=0.4–0.8),thus makingηedecrease and the BB destabilize further.These characteristics of BB are related to the theoretical density gradient-driven trapped electron mode.It should be noted that this mode is not observed by other diagnostics in EAST,and shows very different features to the coherent modes in the edge.