The Experimental Study Of Strong Radial Velocity Shear Induced Inward Flux And Magnetic Component Of Geodesic Acoustic Mode

Plasma transport is the focus of research on magnetic confined plasmas due to the fluctuations induced transport has been compared with the equilibrium transport in large size devices.The anomalous transport leaded by plasma turbulence has become the research hotspot in recent yearsand the fluctuations induced inward flux was mainly studied in stellarators and less in tokamak.On the stellarator plasmas,it has been widely found that the strong radial electric field shear will cause the inward transportation.The threshold relation between radial electric field shear and inward transport is also given,but the mechanism of inward transport is not given further.Transport in tokamak research in this area of research is rarely involved.Recently,it has been observed that inward transport is triggered by strong radial electric field shear in the pedestal region of HL-2A tokamak,suggesting the inward transport may be related with the reconstruction of pedestal.From the frequency resolved expression of fluctuation induced flux Γr,fluc,the cross phase term cos anE between electric field fluctuation Eθ and pressure fluctua-tion ne is the main term that changes the direction of turbulence transport flux,which is different from the zonal flow to suppress turbulence by reducing the amplitude of fluctuation level.In our experiment conducted on the HL-2A tokamak plasmas,the inward flux induced by the Long-Lived Mode(LLM)has been firstly observed in spon-taneous L-H transition by using specially constructed Langmuir probe array.The LLM is a kind of energetic particle modes(EPMs)excited by the resonance between internal kink mode and precessional motion of energetic trapped ions or excited by the energetic electron at q=1 surface in the core region of HL-2A tokamak.In the edge region with strong shear,the radial flux induced by LLM is reversed to radial inwardly because the cross phase term cos αnE becomes negative.By using the long-range correlation in ra-dial direction between Langmuir probe and core soft x-ray signal,we find the poloidal electric field fluctuation Eθ phase reversal during the I-phase,which is the main cause that leading to the inward transportation.The other probe settled in the poloidal distance of 80 mm also observes the same inward flux phenomenon,which indicates the inward flux has a global characteristic.In the common transport model,the formation of the transport barrier makes a very strong E × B shear to achieve the H-mode,which will interrupt the radial correlation length of the turbulence to lower the radial transport.From the generation of poloidal flow from the Reynolds stress theory,the turbulence accumulation in the radial direc-tion will lead to the poloidal acceleration.So the transport barrier could be treated as a solid wall to change the radial flux to the poloidal direction.In this phenomenological opinion,the transport barrier could also reflect radial flux to the radial inwardly direc-tion.In our experiments,not only the LLM induced transport flux is strongly related to the radial velocity gradient in the radial direction dVr/dr,but also the same relations between inward flux induced by turbulence in the low NBI power heated L mode or in ECRH heated L mode and radial velocity gradient dVr/dr.This may suggests that a narrow ’reversal transport barrier’ for the LLM and a particular frequency band of turbulence is generated locally during the I-phase at a given radial position which cause the LLM and turbulence induced flux change their transport direction.Furthermore the inward flux will lead the density gradient become steeper which may related to suppress the turbulence and improve the confinement of turbulence transport,and it will great-ly help us to understand the establishment of transport barrier and the generation of H mode state.As the high-frequency branch of zonal flows,Geodesic Acoustic Mode(GAM)has attracted amount of interest through its involvement in tokamak fusion research.It has been proved to be effective in regulating the amplitude of turbulence and suppressing the radial transport.It is known that GAM manifests as electrostatic oscillations and is not obvious in magnetic field signals in low beta plasma.However in high beta plasma,it will be significant and therefore potentially important in future plasmas.Geodesic acoustic mode(GAM)magnetic field oscillations have been investigated by using elec-tromagetic probe system which is consists of three-dimension magnetic probe and Lang-muir probe arrays in the edge of J-TEXT tokamak.The electromagetic probe system has been separated toroidally settled on the two top windows of J-TEXT tokamak.Inside the LCFS,GAM shows apparent oscillations in floating potential.In contrast,GAM magnetic field oscillations are not significant in raw magnetic fields signals.By using the toroidal correlation and magnetic-potential correlation technique,the GAM mag-netic field oscillations can be distinguished from ambient magnetic field oscillations,especially the strong MHD activity signals.The amplitudes of three dimension GAM magnetic field fluctuations amplitudes,as well as the dependence with local plasma parameters such as safety factor and plasma beta,are coincident with theoretical pre-dictions within experimental error.Toroidal symmetry and poloidal mode structure of GAM poloidal magnetic field Bθ oscillations have been identified,i.e.n≤1 and m=2.Furthermore,the GAM current sheet,in which GAM return current oscillations,which may help us to make a good understanding of the GAM radial structure and its gener-ating mechanism.