Experimental Research Of Plasma Response To External Magnetic Perturbations With Low Toroidal Mode Numbers In Tokamaks

The control of the edge localized mode(ELM)is one key issue to be solved for the safe and reliable operation of the ITER and the future nuclear fusion reactor.The externally applied magnetic perturbations(MPs)have been widely proved to be capable of controlling the ELMs.Compared to the high toroidal mode number n MPs,the MPs with low n could be generated by saddle coils located outside the vacuum vessel,and these out-vessel coils are much more suitable than the in-vessel ones for the reactor,since they are more convenience and reliable for installation,operation and maintenance.But,when applying the low n MPs for ELM control,those low poloidal mode number m resonant components can have significant effects on the core magnetohydrodynamic(MHD)instabilities,such as induce locked magnetic islands in the core and hence degrade the confinement or even lead to major disruptions.The study of the plasma response to low n MPs is of great importance,for both applying the low n MPs to robustly control ELM and coordinating the error field correction and the avoidance of those bad effects of low n MPs on core instabilities.In this thesis,the plasma response to the low n MPs have been studied experimentally on the J-TEXT and the ASDEX Upgrade tokamak.In the Ohmic heating plasmas on J-TEXT which was stable to the tearing mode,two types of plasma responses to the m/n = 2/1 resonant magnetic perturbation(RMP)have been identified,i.e.the screening effect and the penetration process.With small RMP amplitude,the phases of magnetic responses were opposite to those of the RMPs,while their amplitudes increased linearly with the RMP amplitudes,hence indicating the screening of the RMP by plasmas at the resonant surface.However,the mangetic response could grow dramatically once the amplitude of RMP exceeded a certain threshold,meanwhile the phase of the response would change to almost in phase with the RMP.This showed that the RMP penetrated inside the resonant surface and excited a large locked magnetic island.During the penetration,the toroidal rotation varied fast,the sawtooth disappeared,and the core density and the core temperature declined significantly.Statistically,the penetration threshold of the RMP increased linearly with the square root of the core line-averaged electron density.With the presence of a 2/1 locked magnetic island,the plasma response to 2/1 rotating RMP exhibited a standing wave like feature,with the nodes located around the o-and x-points of the 2/1 locked island.The electron temperature responses were zero or minimal inside the locked island or near o-/x-points.These features is significantly different from that of the screening response for a plasma without tearing mode.A phenomenological model has been proposed that the rotating RMP drives the phase oscillation of the locked island by applying a periodical electromagnetic troque.This model indicates that the standing wave like response is due to both the phase oscillation of the locked island and the poloidal gradient of the plasma parameters,hence explains those experimental observations.The nonlinear numerically modelling confirmed the forced oscillation of the locked mode,and qualitatively verified the amplitude and phase dependences among the plasma response,the phase oscillation of locked island and the rotating RMP which have been predicted by the model.In ASDEX Upgrade,the high confinement plasma responsed to the external MPs in two different time scales.By using the rotating n-2 MPs,the slow scale magnetic response has been observed during the inter-ELM phase.This response had dominate toroidal mode number with n = 2,and its amplitude was affected by the poloidal spectrum of the MPs.The responses were observed to be different between low and high field side,indicating the presence of multi-plasma response.Besides,it's found that the high field side response might be more relevant to the ELM mitigation.Applying the n = 1 MPs,the fast time scale plasma response during the ELM crashes have been observed,i.e.the first observation of the significant impact of the MPs on the ELM induced mode structures.Without the MPs,the phases of the ELM induced mode structures were highly random.However,when an n = 1 MP was applied,the phases of the ELM induced mode structures were locked to a fixed value depending on the phase of the applied n = 1MP.The impact of the n = 1 MP on the toroidal asymmetry of the edge AXUV radiation was also observed during the ELM crashes.