Toroidal Plasma Response Based Resonant Magnetic Perturbation Coil Optimalization

The edge localized modes(ELMs),appearing as periodic bursting plasma instabilities,are a ubiquitous feature of tokamak H-mode discharges.Type I ELMs are to be expected due to the very low pedestal collisionality foreseen for future fusion devices.The large scale,low frequency type-I ELMs may pose serious risks to machine components.Type Ⅰ ELM control thus appears to be a critical design issue for future large fusion devices.Among several candidate methods for ELM control,application of the externally applied resonant magnetic perturbation(RMP)fields seems to be a promising choice.Toroidal computations show that both of the plasma response based figures of merit-one is the pitch resonant radial field component near the plasma edge bres and the other is the plasma displacement near the X-point of the separatrix ζX-consistently yield the same prediction for the optimal]coil configuration.According to the two criteria,systematic investigation is carried out by the MARS-F code,for understanding the effect of varying the coil geometry and current configurations on ELMs control for ITER as well as for EU DEMO.MARS-F code is based on the full toroidal,resistive,linear plasma response model.In chapter 3.We performed the RMP coil geometry configuration optimization study for an ITER 15MA Q=10 inductive scenario,including the plasma response to the applied RMP fields.Four toroidal modes are exploited,assuming n=1,2,3,4 coil configurations separately.With both the upper and lower rows of coils powered,we considered three representative choices for the coil current phasing,i.e△ΨUL= 0°(even parity).180°(odd parity)or 90°.With couple of exceptions,we find that the poloidal location of the coils in the present ITER design is close to the optimum according to the plasma response based criteria.This holds for different coil current configurations with n=2,3,4,as well as different coil phasing between the upper and lower rows.Note that the n=1 resonant field is subject to the strongest screening.The coils poloidal width from the present design,on the other hand,is sub-optimal for the upper and lower rows.Modelling also finds that the plasma response amplitude sharply decreases by moving the middle row RMP coils of ITER from the designed radial location outwards.The decay rate is affected by the middle row coils’ poloidal coverage and toroidal mode.The poloidal coverage of the middle coils in the present ITER design is close to the optimum.In chapter 4.based on a EU DEMO1 reference scenario,control coil optimalization design study is carried out,for the purpose of mitigating or suppressing the ELMs in the EU DEMO.With a single midplane row of coils,a coil size covering about 30°-500° poloidal angle of the torus is found to be optimal for ELM control using the n>2 resonant magnetic perturbation(RMP)field.For off-midplane coils,the coils’ poloidal location,as well as the relative toroidal phase between the upper and lower rows of coils,also sensitively affects the ELM control according to the specified criteria.Assuming that the optimal coil phasing can always be straightforwardly implemented,following a simple analytic model derived from toroidal computationsr,it is better to place the two off-midplane rows of coils near the midplane.in order to maximize the resonant field ampl itude and to have larger effects on ELMs.With the same coil current,the ex-vessel coils can be made as effective as the in-vessel coils,at the expense of increasing the ex-vessel coils’ size.This is however possible only for low-n(n=1-3)RMP fields.In chapter 5.in order to achieve ELM control in ELI DEMO,we assume a 10 mm X-point displacement as the conservative indicator,which allows us to estimate the requirement for the coil current amplitude.With these low-n fields(n=1-3).and assuming 300 kAt maximal coil current,the computed plasma displacement near the X-point can meet the]0 mm level.With high-n fields(n=3-6),more than 300 kAt current is needed to meet the 10 mm criterion.The vacuum vessel port openings in EU DEMO sign constrain the possible poloidal location and size of the ELM control coils in the engineering.Despite this limitation,we find that proper coil configuration can be specified,that leads to the matching of the 10 mm X-point displacement criterion.The risk of partial control coil failure in EU DEMO is also assessed based on toroidal modeling,indicating that the large n=1 sideband due to coil failure may need to be corrected,if the nominal n>1 coil configurations are used for ELM control in EU DEMO.