Statistical Analysis Of Plasma Disruptions And Experimental Study Of Minor Disruptions On J-TEXT Tokamak

Disruption may occur due to loss of confinement of tokamak plasmas.Plasma disruption usually releases thermal and electromagnetic energy in fast time scale,resulting in large thermal load on in-vessel components and strong electromagnetic force on the surrounding components,and it can cause considerable damages to tokamak,especially large tokamaks.Therefore,to preserve the integrity of the device,disruption should be mitigated or prevented.Though Magnetohydrodynamics（MHD）instability plays an important role in disruptions,the mechanism of disruption is unclear.So,in order to mitigate or prevent disruption,and protect the tokamak,it is necessary to study the statistics and mechanism of plasma disruptions.There are several identified causes of disruptions.Operation region for steady operation are imposed by both low safety factor（q）disruptions and density limit disruptions.The operation region is analyzed for the J-TEXT Ohmic discharges in this dissertation.The operation region is described by the Hugill diagram,which combines low-q and high density limits.It is found that the operation region is expanded by using wall material and other control technique over years on J-TEXT.In detail,the high-density limit is increased from less than 0.5n_G to 0.7n_G and the low-q limit is lowered from 2.8 to2.2,due to the reduced impurity content by coating graphite on the wall;the operation region is further expanded to 0.85n_G and q_a².0,by suppressing the disruptive MHD precursor by using externally applied resonant magnetic perturbations.Here,n_G and q_a are the Greenwald density limit and edge safety factor,respectively.Based on the data shown in the Hugill diagram,the parameter region of MHD modes are presented.It is found that m/n=2/1 tearing mode（TM）appears for a wide parameters region with 2.4<q_a<4 and n_e<3?10¹⁹m^-3,here m and n are the poloidal and toroidal mode numbers.While for other lonely MHD modes such as m/n=5/2,3/1,4/1 and 7/2,they appear only when their rational surfaces are close to plasma edge（or m/n^q_a）,and these MHD modes may transit to 2/1 TM when changing the plasma parameters.In addition,correlation analysis between the amplitude and frequency of the 2/1 TM for different plasma conditions reveals that there is a threshold between normal discharges and density limit discharges,which would be a reference to predict density limit disruptions.Similarly,the growth rate of 2/1 TM for different plasma conditions also shows that the growth rate of low q and density limit disruptions MHD precursors are bigger.And there is a threshold of the density limit disruption precursor when thermal quench is triggered.And the threshold depends on temperature,density and so on.It is found that the direct causes are mainly tearing mode locking and too low rotation of MHD.According to the dischange control,disruption happens mainly by human factors like gas injection by MGI/SMBI,penetration or mode locking by RMP and so on,but they are not the direct physics causes.Besides,minor disruptions after field penetration by static m/n=2/1 resonant magnetic perturbations（RMPs）are also studied in this dissertation.It has been found that minor disruptions characterized by fast heat crash and density pump-out but without plasma current crash occurs after the field penetration of RMPs.Based on the time sequence of minor disruption,it is characterized by a fast-crash stage followed by a slow recovery process.In the fast-crash stage,heat and particle transport collapse at a time scale of less than 1 ms,which is similar to that of major disruption but is less harmful.To understand the cause of the minor disruption,further experiments are performed.Results show that during the mode locking,a strong oscillation in both temperature and density occurs only when the magnetic perturbation is stronger than a critical value,the edge safety factor q_a>3,and the core line-averaged electron density n_e is lower than a critical value.Power spectrum analysis reveals that the oscillation is due to the m/n=3/1 mode.During the recovery process,an unlocking m/n=3/2 mode is also observed in addition to2/1 and 3/1 locked modes.All these findings indicate that multiple modes coupling may be the primary cause of fast heat and particle crash,and that factors including strong magnetic perturbation leading to a large island,high q_a for the 3/1 locked mode,and low density for easy field penetration can advance the occurrence of minor disruptions.m/n=2/1 and 3/2 is also important dring the minor disruption in density limit and low q limit.