| Sawtooth oscillations are one of the most common and significant magnetohydrodynamic(MHD)instabilities in tokamak plasma.It can result in the periodic relaxation of the temperature and density of the core plasma.The small sawtooth oscillations are beneficial to avoid impurity accumulation in plasma core and potentially contribute to removing helium ash for fusion burning plasma.However,the large sawtooth crash may trigger other MHD instabilities,like tearing modes(TMs)or neoclassical tearing modes(NTMs)which possibly deteriorate the plasma performance or even lead to plasma disruption.Therefore,it is important to understand the evolution process of sawtooth crash and explore the experimental methods of sawtooth control for the comprehension of sawtooth oscillations,maintaining the steady-state operation and preventing discharge disruption for tokamak.The sawtooth behaviors are studied in experimental advanced superconducting tokamak(EAST)in this dissertation.First,sawtooth crashes and the excitation process of the tearing mode after sawtooth crash are studied in the condition of high-Z impurity accumulation in core plasma;Second,the effect of n=2 RMP on sawtooth instability is investigated and the active sawtooth control experiment is carried out by adjusting resonant magnetic perturbation(RMP)parameters in the low q95 operation regime in EAST.Since the upgrade with new lower W divertor in 2021,EAST has an almost allmetal-wall operating environment.It is found that medium-Z and high-Z impurities easily accumulation in plasma core during EAST operation.Triggering of m/n=3/2 tearing mode(m and n are the poloidal and toroidal mode numbers)and m/n=2/1 tearing mode by large sawtooth crash are observed in the condition of high-Z impurity accumulation in core plasma.The effect of impurity accumulation in core and sawtooth crash on the excitation of tearing mode is also studied.The result shows that when the impurity concentration in the plasma core reaches a high level,the tearing mode becomes unstable after the crash of a long-period sawtooth.Sawtooth crash leads to the particle and the heat transport outward,and then the particle transport to the rational surface,resulting in the changes in density gradient near the rational surface.The density perturbation near the rational surface occurs first,and then TMs become unstable.The results show that the excitation of tearing mode is related to both sawtooth period and the degree of impurity accumulation in the core.The large sawtooth behaviors with a period of up to 150 ms are observed at the low q95 operation regime in EAST,and the influence of RMP on sawtooth behavior is also found.During the plasma discharge with n=2 RMP,it is observed that the sawtooth period depends on the phase difference between upper and lower of the RMP coils ΔφUL,where n is the toroidal mode number of the applied RMP.By scanning the RMP phase difference ΔφUL,it is found that the sawtooth period and amplitude change significantly with the variation of the RMP phase difference.The sawtooth period and amplitude are the smallest at the RMP phase difference ΔφUL=270°.The plasma response is analyzed by using the MARS-F code for different RMP phase.The results show that when the phase difference of RMP is ΔφUL=270°,the plasma responses more strongly.Further study shows that the change of the sawtooth behavior is related to the loss of fast ion caused by RMP in the plasma.It is known that the fast ion has stabilizing effect on sawtooth behavior.The alteration of the RMP spectrum would cause changes in the plasma response,which can affect the confinement of fast ion in the plasma.The loss of fast ion can decrease the fast ion stabilizing effect on sawtooth,which eventually leads to change in the sawtooth behavior.The sawtooth control experiments are preliminarily achieved by n=2 RMP,which provides a foundation for the active control of sawtooth in EAST high βN scenario in the future.Further works will focus on the experiment optimization in order to avoid fast ion loss. |
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