Experimental Studies On The Effect Of On-axis ICRF Heating On The Sawtooth In EAST Device

Sawtooth instability(also called as sawtooth mode)is a common macroscopic instability in the plasma of EAST(Experimental Advanced Superconducting Tokamak)device.The small and frequent sawtooth can help eliminate the high Z impurity in the plasma core or "helium ash" in the future D-T fusion reaction,improve the plasma confinement performance,while the long sawtooth may induce the Neo-classical Tearing Mode(NTM),Or it is coupled with the Edge Localized Mode(ELM),which seriously affects the high performance constraints of the plasma.Therefore,controlling the sawtooth period is a very important content in the study of improving the plasma confinement performance.Among them,the ion cyclotron resonance frequency wave(ICRF)heating will be an important means to control the sawtooth period in the future fusion experimental reactor device.At present,the relevant experimental research on the influence of ICRF on sawtooth in EAST device is relatively lacking.In view of this,in this paper,the influence of on-axis ICRF heating on parameters such as sawtooth period was investigated.In order to obtain the sawtooth period,sawtooth amplitude,the radius of q=1 surface and the plasma pressure gradient at q=1 surface under ICRF heating,we developed an algorithm for detecting the sawtooth crash time,which mainly uses the double sliding window algorithm to process the single channel signal of soft X-ray(SXR)from the plasma core to detect the possible sawtooth crash edge,and then uses singular value decomposition(SVD)to process the SXR multi-channel signal to determine the true sawtooth crash edge.In order to test the performance of the algorithm,the algorithm was tested with a total of 8496 sawtooth crash time points under various parameters of plasma,and a high accuracy of 96.4%was obtained;Anti-noise robustness test also shows that the algorithm is robust enough.On the basis of the algorithm for identifying the time of sawtooth crash,we have realized the automatic output of sawtooth period,sawtooth amplitude,radius of q=1 surface and plasma pressure gradient at q=1 surface,which provides a powerful tool for the next analysis of sawtooth behavior under ICRF heating.Using the sawtooth-related parameter data analysis tool developed based on the SXR diagnostic data,combined with the fast ion specific pressure given by the neutron flux detection and diagnosis system and the neutron camera system,and the EFIT constrained by the diagnostic data of the polarization interferometer(POINT)giving a reliable plasma current density distribution,we analyze the influence of the ICRF power change in a single shot under the on-axis ICRF heating condition,the ICRF power under different background heating conditions,ICRF power modulation and ICRF phase change on various parameters such as sawtooth period.Our research reveals that:①the sawtooth period is positively correlated with the ICRF power,and the sawtooth amplitude and the plasma pressure gradient at the q=1 surface are consistent with the sawtooth period as a whole;② Under specific plasma parameters and heating conditions,to change the radius of q=1 surface,the ICRF power may need to exceed a certain threshold;③ Under heating conditions with a higher proportion of ICRF power,sawtooth period,q=1 surface radius and fast ion specific pressure are more sensitive to the change of ICRF power;④ When the ICRF power is modulated and the ICRF phase change from 180° to-90°,the sawtooth period will decrease as a whole;⑤Through the analysis of the above experimental phenomena,it can be seen that the changes of parameters such as the sawtooth period in the plasma of the EAST device under on-axis ICRF heating are closely related to the fast ions produced by ICRF and the change of the radius of the q=1 surface caused by ICRF heating.To sum up,we have developed an algorithm for detecting the time of sawtooth crash on the EAST platform,counted and analyzed the changes of the sawtooth period and other parameters under on-axis ICRF heating,and concluded that these changes is closely related to the fast ions produced by ICRF and the change of the radius of q=1 surface caused by ICRF heating,and discussed the feasibility of using ICRF to change the sawtooth period on the EAST device.These works can provide valuable reference value for using ICRF to control the sawtooth period in EAST or the next generation fusion device in China in the future.