Pedestal Equilibrium And Stability Analysis In Long-pulse H-mode Discharges On EAST

In tokamaks,high confinement mode(H-mode)is usually accompanied by periodic edge localized modes(ELMs),which would cause severe damage to the plasma facing components of the devices by the transient heat load.Suppression or mitigation of ELMs is a major challenge facing ITER and future fusion devices.Understanding the pedestal stability is of great significance for effective control of ELMs.The kinetic profile diagnostics recently developed on EAST make it possible to perform the kinetic equilibrium reconstruction and subsequent quantitative pedestal stability analysis.The main contents of this paper include the introduction of lithium beam emission spectroscopy(Li-BES)diagnostic on EAST and the edge electron density profile measurement based on this diagnostic,a new method for visible imaging measurement of position and displacement of the last closed flux surface,pedestal ideal linear peeling-ballooning mode stability analysis with ELITE code based on kinetic equilibrium under various discharge conditions on EAST and linear stability analysis of effects of E×B flow shear on edge low toroidal mode number kink/peeling mode.The main contents and results of this paper are summarized as follows.Li-BES diagnostic has 32x4 channels with a radial observation range of about 30 cm,covering almost the whole Li-I(2s-2p)resonance line emission region at the midplane on the low field side.It has a spatial resolution of about 1 cm and a sampling rate of 2 MHz.The edge electron density profile is reconstructed based on the collisional-radiative model,which has been compared with that provided by other diagnostics on EAST,such as TS,microwave reflectometry and reciprocating Langmuir probes and their results are shown to be consistent.The electron density profile in the SOL region is observed with this diagnostic.The evolution of temporally well resolved edge electron density profiles during fast transient events,such as the L-H transition and ELM crashes can be also achieved.The electron density profile with temporal resolution of about 10 ?s is observed to become steeper during the L-H transition process,suggesting the electron density pedestal is formed.The radially affected area of the grassy ELM is observed to be localized inside the pedestal region,while the density pedestal crashes and a front structure forms and spreads inward beyond the pedestal during the type-? ELM crash.A new method for measuring the position and displacement of the last closed flux surface(LCFS)with visible imaging diagnostics has been developed on EAST.The location of the LCFS can be determined in camera images according to the relative position relation between the LCFS and the radial relative intensity profile of Li-II spectrum in discharges with Li-coating wall.With the benefit of the regular shapes of the antennas and limiters for calibration,the positions of certain points on the LCFS in the tokamak can be calculated.The results are consistent with that given by EFIT which indicates that the method presented in this paper provides an effective and convenient approach to determine the position of the LCFS in EAST.The displacement of the LCFS in discharges with Li-coating wall is also measured and the error mainly depends on the spatial resolution of pixels which is about 3 mm.By using this method,the phenomenon that the position of the LCFS changes with the variation in RMP signals is observed.Based on the kinetic profile diagnostics recently developed on EAST,the kinetic equilibrium reconstruction has been performed as well as subsequent quantitative pedestal stability analysis with ELITE code.Pedestal stability analysis is performed for a typical long-pulse H-mode discharge#67238 on EAST.The results show that the pedestal is in the stable region close to the PBM stability boundary and dominated by toroidal mode number n around 10-15.The calculated most unstable mode structure is mainly localized in the steep gradient region and consistent with the relative fluctuation profile of the emissivity measured by MESXR diagnostic across ELMs.Compared with a typical type-I ELM discharge#71450 with larger total plasma current(Ip = 600 kA)and similar other discharge conditions,the long-pulse H-mode discharge(Ip = 450 kA)is smaller in both peak pressure gradient and peak current density and more stable in peeling-ballooning instability in pedestal region.The critical peak pedestal pressure gradient of#67238 is evaluated to be 65%of#71450 based on equilibria generated by scanning the peak pressure gradient around the experimental values and keeping the collisionality unchanged.Two important features of EAST tokamak compared with other tokamaks,including a wider pedestal correlated with poloidal pedestal beta and a smaller inverse aspect ratio,may generally help to get access to the small ELM regime in the long-pulse discharges.The effects of uncertainties in measurements on the linear stability results are analyzed,including the ne profile position,separatrix position and the Zeff value.Pedestal stability analysis is also performed for a typical grassy ELMy H-mode discharge#71096 on EAST.The results show that pedestal in the grassy ELM regime is on the kink/peeling boundary.This is different from the grassy ELM regimes in other tokamaks or type-? ELM regimes,which were all concluded to be destabilized at the high-n ballooning stability boundary.The nonlinear simulations with BOUT++indicate that the essential difference between the grassy ELM and the type-I ELM is whether the pedestal current can still drive the low-n PBMs unstable when the pressure gradient is reduced.A physical model has been established based on a new mechanism for destabilizing low-n kink/peeling modes by the ExB flow shear,which underlies the EHOs in QH mode,separately from the previously found Kelvin-Helmholtz drive.We find that the differential advection of mode vorticity by sheared ExB flows modifies the 2D pattern of mode electrostatic potential perpendicular to the magnetic field lines,which in turn causes a radial expansion of the mode structure,an increase of field line bending away from the mode rational surface,and a reduction of inertial stabilization.This enhances the kink drive as the parallel wavenumber increases significantly away from the rational surface at the plasma edge where the magnetic shear is also strong.This destabilization is also shown to be independent of the sign of the flow shear,as observed experimentally,and has not been taken into account in previous pedestal linear stability analyses.