Study Of Loss Of Energetic Ions And Related Mechanisms Under Resonant Magnetic Perturbations In EAST Tokamak

The adequate confinement of energetic ions is an issue of major importance for both the high performance of present-day tokamak devices and the success of future fusion reactors.The resonant magnetic perturbation(RMP)has been a new concern for fast ion confinement.On the other hand,RMP has also been shown as an actuator to exert targeted control over the EP profile experimentally.An numerical investigation on how resonant magnetic perturbations affect the loss of energetic ions in tokamkas is carried out,which not only provides a better understanding of the underlying physics,but also helps to avoid the detrimental effects on confinement and to achieve the phase-space engineering in the future.The contributions of this work are listed in the following aspects.Firstly,the significant role of sideband resonance in formation of drift islands is revealed.Previously,it's generally believed that the broken magnetic topology play the key role in passing ion confinement.Our results suggest this intuitive interpretation may be incorrect.Due to the non-negligible cross-field drift,the structure of drift islands is essentially different from the magnetic topology.By tailoring the Fourier spectrum of RMP,the primary contribution from resonant components and the sideband contri-bution from non-resonant components are compared in detail in an equilibrium from EAST experiments.The sideband contribution is found to be more dominant as an is-land chain is closer to the plasma boundary.In the presence of plasma resposne,the role of sideband contribution can be even more dominant.In particular,when the ?N is high enough,the widths of peripheral island chains under response RMP can still be comparable to those under vacuum RMP,which helps to explain the significant loss fractions.Despite the fact that inclusion of plasma response heals the magnetic topol-ogy,the significant sideband resonance may be still a concern in future discharges.Secondly,the role of nonlinear resonances in the loss of trapped ions is revealed.By the phase-space analysis,it is revealed that the dominant loss mechanism under n=2 RMP is essentially different from that under n=1 RMP.The former arises from the near-boundary resonance with ?p/?b=1/2,while the latter arises from orbital stochasticity near the trapped-passing boundary.In particular,nonlinear resonances are shown to play the key role in promoting the stochasticity.The loss rate under n=2 RMP shows a linear dependence on the perturbation amplitude,while the loss rate under n=1 shows a quadratic dependence.The results here highlight the importance of nonlinearresonances in fast ion confinement under RMP,in addition to that of linear resonances frequently emphasized before.Thirdly,a new full orbit code,as well as many other useful modules,has been developed.Although the ORBIT/ORBIT-RF code we already have is helpful to physics analyses,it's limited by the flux coordinates and the guiding-center approximation.The new code calculates real particle orbits in cylindrical coordinates,and its simulation results are more persuasive in terms of evaluation of the loss fraction.The code has been benckmarked.Such a code can be helpful for design and analysis of experiments.Fourthly,the new full-orbit Monte-Carlo code has been used to simulate the loss of NBI ions under n=1 and n=2 RMPs.The initial distribution of beam ions are calculated with the NBI module,which takes into account the geometry of NBI.The simulation can take into account the first wall and provide a more realistic evaluation of the losses than previous results.It is found that the prompt loss and the resonant loss are the two loss channels of concern.The former is mainly related to the ion source,while the latter is closely related to the RMP spectrum.To our surprise,the inclusion of plasma response does not necessarily mean a better confinement,though the shielding effect indeed heals the magnetic topology.The further analysis reveals that passing ions are responsible for the large loss fractions in the response cases.This may arise from the significant sideband resonance emphasized above.Numerical advances in this work provide a solid basis for study of RMP-induced fast ion losses in EAST.From theoretical perspective,this work provides a better under-standing of dominant loss mechanisms under RMP.More importantly,the phase-space selective resonant behaviors reveal the possibility of engineering on EP profile by RMP.From practical perspective,the developed tools can be very useful for the design and analysis in the future.