Experimental Study On Divertor Plasma Detachment In EAST

For tokamaks capable of sustaining burning plasma,the divertor target is expected to face an ultra-high steady-state heat flux of～100 MWm-2.The excessive heat flux not only damages the divertor components,but also sputters impurities that will degrade plasma confinement and even lead to discharge disruption,which is a huge problem that tokamak will face.In parallel to the engineering and material developments,in physics,the divertor detachment is a very promising means to solve the problem.Divertor detachment is to increase the energy dissipation by ramping up or impurity seeding so that the energy of the plasma reaching the target plate is significantly reduced.The paper aims to further understand the detachment physics and to explore the detachment operation scheme to solve the problem through experimental studies on EAST.Divertor Langmuir probe system is the most straightforward diagnostic for detachment studies on EAST.There are 98 sets of triple-probes made up of 7 arrays in the inner and outer target plates of the upper and lower divertor on EAST.The spatial resolution is about 12.5 mm and the time resolution is 0.02 ms.The collection area of the probe tip cannot be accurately obtained due to the ablation during the long-term long-pulse discharges.An analytical model of the collection area and the accumulated injection energy is introduced to correct the data.The cross-validation with the data measured by the brand-new probe and other diagnostics proves that the method can greatly improve the accuracy of the data.The development of the diagnostic and the data calibration method laid the foundation for the detachment study in the paper.The natural detachment by high density has been achieved on EAST with the ITER-like tungsten divertor.After detachment,the divertor electron temperature drops to～5 eV,and the peak particle flux decreases with the density ramping up.The ion-electron recombination and the divertor energy radiation are both enhanced,while the surface temperature of the target plate is reduced.It has been demonstrated that the more closed divertor has a lower detachment density threshold.The detachment density threshold will also be reduced when the strike point moves towards the divertor corner.Both are attributed to the fact that the increased divertor closure results in higher neutral particle density,which in turn increases energy dissipation.The experimental results show that the absolute detachment density is slightly increased,while the normalized density threshold is lower at high plasma current.This is related to the high Greenwald density limit caused by higher plasma current.In addition,the study shows that the detachment threshold increases with the heating power.The detachment threshold of the upper divertor for the ohmic plasma is lower than 0.4nG,0.5nG-0.7nG for L-mode plasma,and higher than 0.7nG for H-mode plasma.It is difficult to achieve natural detachment when the heating power exceeds-4 MW.For L-mode plasma,the inner target is always detached before the outer target.It is independent of the plasma configuration,the toroidal field,the plasma current and the heating power.For H-mode plasma,the detachment asymmetry between inner and outer targets is related to the direction of the toroidal field.In fusion reactors,it is difficult to achieve natural detachment due to the high power,while impurity seeding will be a necessary method.The stable detachment operation with feedback control has been successfully achieved by impurity seeding in EAST.After detachment,the divertor electron temperature,heat flux,pressure and the surface temperature are all significantly reduced,and both the plasma stored energy and the energy confinement are maintained excellently.In the experiment,the relevant plasma parameters and the impurity seeding position are optimized to alleviate the heat flux with high-confinement core plasma.The research shows that impurity seeding downstream of the scrape-off layer(SOL)is more conducive to the detachment compared with in the SOL upstream because the radiation is mainly concentrated in the divertor when impurity seeding in the SOL downstream.The particles are mostly concentrated near the strike point when D2 is fueling in the SOL downstream,while they are evenly distributed on the divertor target when D2 is fueling in the SOL upstream.In addition,it is also observed that seeding close to the strike point is more conducive to impurity ionization and energy radiation.The radiation when impurity seeding in the private flux region is weaker than that in the SOL side of the divertor.The entire divertor can be detached by the single impurity seeding in the toroidal direction under the current parameter condition on EAST.Further experiments also show that D2 fueling with an ultra-high rate results in the asymmetric distribution in the toroidal direction.In conclusion,impurity seeding at SOL downstream and close to the strike point is the most favorable for detachment operation,and only a single gas-puff is required in the toroidal direction.It is necessary to accurately control the impurity amount in long-pulse detachment operations.Therefore,many detachment feedback control methods have been developed based on the divertor probe diagnostic on EAST.The rate of impurity seeding is adjusted in real time to achieve stable detachment operation by the plasma control system.After a lot of experimental exploration,a stable detached discharge with 30 s long-pulse is successfully obtained in the EAST 2021 experimental campaign,which verifies the feasibility of the divertor detachment on a long timescale.In other experiments,it is also observed that the edge-localized modes are suppressed during detachment by impurity seeding.The steady-state and the transient heat fluxes are alleviated simultaneously.Furthermore,the divertor detachment has also been achieved in helium plasmas by high density and impurity seeding.In helium plasma,the detachment characteristics are basically the same as those in deuterium plasma,while the detachment threshold is significantly higher.In the paper,the change in the collection area caused by probe ablation is quantitatively analyzed and an effective correction method is proposed,which greatly improve the accuracy of the diagnostic data.The effects of divertor closure,plasma current,heating power,and fuel particle types on detachment are clarified and the mechanisms are revealed on EAST.The influences of impurity seeding location on detachment and core compatibility are systematically studied and the optimal position for impurity seeding is determined.It is confirmed that a single gas-puff can achieve global detachment under the current parameters of EAST.The study provides data support for detachment physics and operation in tokamaks.