Study On Correlation Between X-point Displacement And The Edge Localized Modes Control Via Using 3-D Magnetic Perturbed Field In Tokamak

Edge localized modes（ELMs）as the common magnetohydrodynamic（MHD）instability in advanced tokamak devices operating in high-confinement mode（H-mode）.ELMs will burst periodically in the edge plasma region.During each ELM burst,a large number of particles taking high energy will be ejected from the plasma to the plasma facing components（PFCs）,such as the divertor.Excessive thermal deposition will greatly reduce the lifetime of the plasma facing components,and even affect the steady state operation of the tokamak,especially for the future large tokamak devices,such as ITER.At present,there are many methods could effectively achieve ELMs mitigation or suppression,among which the application of three-dimensional resonant magnetic perturbation（RMP）fields is the most mature technology.On several tokamak devices,the type-I ELMs have been successfully mitigated into type-III ELMs or even been completely suppressed by the RMP fields.Although it has been widely used in experiments,the physical understanding of the RMP fields effectively controlling ELMs is still limited.Both experimental and theoretical investigations show that the plasma response plays an important role in the ELMs control via utilizing RMP fields.However,most of the previous theoretical studies of the plasma response focus on individual tokamak devices,comparisons between different devices should be taken to decide whether the linear response theory can be extrapolated to future large tokamak devices,such as ITER.In this work,the simulation results are compared with those based on experimental equilibrium.According to the linear plasma response model,two criterions can be used to investigate the plasma response to the RMP fields.（i）One is the amplitude of the pitch resonant component of the total radial magnetic field including the plasma response near the plasma edge b_res~1;（ii）another is the normal displacement near X-point on the plasma boundaryξ_xpt.The optimal RMP fields predicted by these two parameters are the same.Based on these two figures of merit,MARS-F code and the linear plasma response model are used to systematically study the correlation between plasma response and ELMs control.MARS-F code is calculated in the full toroidal geometry,single-fluid,resistive MHD approximation.First,according to the experimental equilibrium on ASDEX-Upgrade,DIII-D,EAST and MAST tokamak devices which equipped with RMP coils,we computed the plasma response induced by the external RMP field,assuming that the coil configurations are n=1,2,3,4 and the toroidal current phasing of the upper and lower coilsΔΦchanges from-180°to 180°.It is found that the physical parameters related to plasma response include plasma rotation,aspect ratio and edge safety factor,when conduct statistical analysis of(7~1_res andξ_xpt under the optimal coil phasing.Furthermore,in order to investigate the effect of the location of zero-crossing on the plasma response to the RMP fields,according to the experimental equilibrium in the discharge 31131 of ASDEX-Upgrade tokamak,we assumed a family of plasma toroidal rotation profiles with different braking positions（zero-crossings）in the pedestal region.We perform the study for n=1andΔΦ=0°RMP fields scenario.The simulation results show that when the location of zero-crossing cross the resonant surface in the pedestal region,b_res~1 andξ_xpt jump,which means the plasma response jumps while the screening effect of the RMP fields is reduced,due to the presence of a chain of magnetic islands,and the radial transport of particles and energy is enhanced,which provides theoretical guidance for the control of ELMs by RMP fields in large tokamak in the future.In addition,according to the experimental equilibrium of 31128 discharge in ASDEX-Upgrade tokamak,two sets of new equilibrium are constructed for different plasma major radius R₀ and minor radius a,respectively.The magnetic perturbations of n=1,ΔΦ=0°and 180°coil configurations are calculated.The results show that vacuum fields change monotonically with major radius R₀ and minor radius a,while the total fields that including plasma response show a non-monotonic variation relationship.Therefore,the vacuum theory cannot be extrapolated to future large tokamak devices,such as ITER.It is necessary to find the optimal major radius and minor radius of plasma within the plasma response theory.