Experimental Investigations Of Sawtooth Oscillations On J-TEXT

Sawtooth oscillations are one of most important magnetohydrodynamics(MHD)instabilities in tokamak plasmas.Sawtooth crash can trigger seed islands of neoclassic tearing modes(NTMs),potentially leading to major disruptions,which is deleterious hardly for devices,while sawtooth crash can also sweep out the He ash after nuclear fusion reactions in International Thermonuclear Experimental Reactor(ITER).In recent decades,experiments and theories on sawtooth have been performed in quantity but there are no consistent understandings of sawtooth oscillations.To study the sawtooth and m=1 mode in plasma core,soft X-ray diagnostic system(SXR)is upgraded in J-TEXT,and relevant codes for data processing are developed,which are utilized for study of sawtooth oscillations and m=1 mode in J-TEXT.In this paper,five soft X-ray arrays are established in J-TEXT,three of which with radial resolution of 1.5cm detect the radiation with energy of 0.5keV-10keV from plasma region within r<23 cm(minor radius a=25-27cm)and the others with radial resolution of 0.5cm are utilized for measurements of MHD in plasma core and boundary,respectively.Tomographic reconstruction for SXR has been applied for studying evolution of sawtooth and 1/1 mode,two-point correlation is applied for analysis of phase in plasma core from the weak mode,the bispectrum analysis can provide the evidence that 1/1 and 2/1 modes interacts,and stochastic subspace identification can separate different mode and identify their mode number.The sawtooth oscillations in J-TEXT are widely studied in this paper.The period of sawteeth is 1-6 ms and the crash time is 20-120 μs and the frequency of precursor and post 1/1 modes is 5-10 kHz and 3-8 kHz,respectively.Double sawteeth are also found,which reveals existing incomplete reconnection.The tomographic reconstructions reveal that the "hot core" and "cold island" exist in phase of sawtooth crashes during sawtooth oscillations,which confirms that the incomplete magnetic reconnection is coincident with the sawtooth on J-TEXT.In this paper explorative study of effects of resonant magnetic perturbations(RMPs)on sawtooth oscillations and m=1 modes has been performed,and it is found that m/n =2/1 and 2/2 RMPs can affect the sawteeth while effects of 1/1 RMPs are not obvious.1)When penetration of m/n=2/1 RMPs,sawteeth disappear and the m=1 mode is triggered.The m=1 mode can couple with the rotating 2/1 mode and the mode frequency is 1.3-3.2kHz,increasing with incremental electron density.The m=1 mode can exist with locked 2/1 mode and also interact with rotating 2/1 mode.2)Effects of 2/2 RMPs on sawteeth are studied.It is found that 2/2 RMPs can enlarge the sawtooth and suppress the m/n=1/1 and 2/2 modes while the 1/1 RMPs only suppress the 1/1 mode slightly on J-TEXT,probably because the RMPs is not large enough.