Blob Behaviors In The Scrape-Off Layer In No-ELMs H-mode Discharges On EAST

Blobs are widely observed in the tokamak plasmas,they are intermittent coherent structures that extend along the magnetic field line,and their densities and temperature are much higher than the background plasmas.It is generally acknowledged that the blob is one of the main contributors to the turbulence transport in the edge,especially in the scrape-off layer(SOL)region.In addition,the cross-field heat transports caused by blobs can change the heat distribution in the SOL,so they are closely related to the plasma wall interactions(PWI).At present,there are relatively few studies on blob behaviors in high-confinement mode(H-mode)on each tokamak,but the H-mode,especially the no-ELMs H-mode is a promising operation in fusion reactors.Therefore,it is a critical issue to investigate the blob behaviors in no-ELMs H-mode,and the study contributes to understanding and predicting the blob behaviors and their impacts in future fusion devices such as ITER and CFETR.In this paper,the blob behaviors in noELMs H-mode,their influencing factors and the physical mechanisms involved are investigated,based on the Experimental Advanced Superconducting Tokamak(EAST).Lithium beam emission spectroscopy(Li-BES)is the main diagnostic used in this paper,and we have improved and upgraded it.By installing a new narrower band interference filter to the optical system,the background emission of the system is significantly suppressed and the signal-to-noise ratio of the system is improved.Sodium beam emission spectroscopy(Na-BES)has a higher signal-to-noise ratio and longer lifetime than Li-BES,and Li-BES is being upgraded to Na-BES on EAST.A highbrightness and stable sodium(Na)ion source is developed,and the performance of Na ion beam optics is simulated by the COMSOL Multiphysics taking the space charge effect into account in the injector of the Na-BES.The maximum current of the Na-beam can reach 9 mA and the lifetime of the developed Na ion source is evaluated to be 63 mAh,which are both superior to the performance of Li ion sources.The system parameters required to get a well-focused ion beam were determined through the simulation,and it was clarified that the Einzel lens has a good focusing property on NaBES,providing a basis for the design and installation of Na-BES on EAST.The no-ELMs H-mode can be achieved in two ways:developing spontaneous stable ELM-free H-mode scenarios and using external means to suppress ELMs.The blob properties in spontaneous ELM-free H-mode plasmas and their comparisons to those in low-confinement mode(L-mode)and improved confinement mode(I-mode)have been investigated firstly.The blob properties in L-mode and I-mode are similar,and those in ELM-absent H-mode are different from them.The blob detection rate(or frequency)Nb is smaller in H-mode.The experimental blob size δb and the velocity scalings vr,b-δb both show a good agreement with the theoretical models.The variation of blob properties during the L-I and H-L transitions,and their relations to the edge turbulence,SOL density and collisionality,Er×B and so on are discussed.The variation of the edge turbulence,SOL temperature and Er×B shear are inconsistent with the changes in blob,while the blob detection rate shows a correlation with the SOL density.In addition,the blob detection rate is found to increase with the divertor collisionalityΛdiv,indicating a dependence of blob behaviors on Λdiv.The possible mechanism is that the high collisionality yields higher effective resistivity for blob circuit in the parallel direction,and thus the blobs become weakly electrical connected to the divertor target sheath.As a result,the sheath dissipation decreases,which contributes to the larger blob detection rate Nb.The L-mode-like particle confinement in I-mode would lead to little variation of SOL density during the L-I transition,while the high-particle confinement in the H-mode would lead to a decrease in SOL density during the L-H transition.The differences in SOL density and collisionality in the three operation regimes may be the cause of the differences in blob behaviors among them.In high density conditions,blobs experience an increase in size,lifetime and transport,known as the blob high-density transition(HDT)phenomenon.The blob HDT in spontaneous stable ELM-free H-mode and its comparison to that of L-mode have been investigated on EAST.The blob HDT has been observed in both spontaneous ELM-free H-mode and L-mode.When the plasma density normalized to the Greenwald density limit exceeds a certain threshold,the blob size,lifetime and transport increase by 2-3 times.The density threshold of blob HDT in H-mode and Lmode is similar,both near the normalized average density fGW=0.5.By means of density ramp-up(or the plasma current ramp-down)to change the SOL collisionality Λ and impurity injection only to change the divertor collisionality Λdiv,the physical mechanisms of blob HDT are studied.It is found that the potential mechanism of blob HDT between H-mode and L-mode is similar,the variation of the SOL collisionality determined the blobs transition but not the divertor collisionality.The possible physical mechanism is that the high collisionality leads to the blobs becoming electrically disconnected to the divertor target sheath,and then the blob converts from the "sheath limited"(SL)to the "inertial"(IN)regime.As a result,the sheath dissipation decreases significantly,which increases the blob size and transport.Resonant magnetic perturbation(RMP)is one of the effective external means to suppress ELMs.The impacts of rotating RMP and static RMP field on blob behaviors during the ELM suppression phase in H-mode discharge have been investigated in EAST.It was found that during the ELM suppression phase with the rotating RMP,the plasma boundary is corrugated by the rotating RMP field,and there is an RMP-induced radial displacement in the edge plasma.The edge density,density gradient and SOL low-frequency turbulence are all modulated by the rotating RMP,showing similar periodic changes to the rotating RMP coil current.This periodic change indicates that RMP causes a non-axisymmetric three-dimensional plasma structure in the edge.The low-frequency turbulence suppression accompanies with the outward plasma displacement and the increase of local density,and its enhancement accompanies with the inward plasma displacement and the decrease of local density.Further analysis shows that the low-frequency turbulence in the SOL exists in the form of a blob structure.The blob parameters including size,lifetime,detection rate and radial velocity,as well as the transports induced by blobs,are all modulated by the RMP field.The modification of low-frequency turbulence may mainly result from the blob modulation by RMP.The relations between the variation of the SOL blob and the broadband turbulence in the blob generation zone,and the SOL plasma parameters,as well as the local Er×B shear are discussed for understanding the potential mechanisms of blob modulation by RMP.It was shown that the edge broadband turbulence,SOL electron temperature Te,parallel connection length L‖ and collisionality Λ are not the cause of the variation of the blob.For the blob suppression phase ErxB shear is higher than that without RMP while ErxB shear for the blob enhancement phase is smaller than that without RMP.These results suggest that ErxB shear in the blob detection zone may be the potential mechanism of the blob modulation during the application of RMP.In addition,the impacts of static RMP on blob behavior during ELM suppression H-mode discharge and L-mode discharge are studied.It was observed that the static RMP had different effects on the blob during these two discharge processes,which may be related to the different toroidal non-asymmetry caused by RMP under different conditions.Similar to the results of rotating RMP,SOL electron temperature Te,parallel connection length L‖ and collisionality Λ are not the cause of the variation of blob,Er×B shear may be the main mechanism of the blob variation caused by static RMP.