Researches On Impurity Mixing Efficiency And The Effect Of Electrode Biasing On Runaway Current In J-TEXT Tokamak

The heat load,electromagnetic force load,and high-energy runaway electrons generated by the plasma disruption in tokamaks can cause serious damage to the device and endanger the safe operation of the device.At present,the mainstream idea for disruption mitigation is massive impurity injection and massive gas injection(MGI)was one of the candidate solutions.MGI can well mitigate the harm of heat and electromagnetic force load.But for runaway electrons mitigation,because MGI cannot inject a large amount of impurities in a short time and the degree of ionization during the interaction between impurities and plasma is low,the electron density increased by MGI can only reach one fifth of the theoretical density required to completely suppress the runaway electrons.Therefore,this article focuses on the study of the transport process and the mixing efficiency of different impurities under different conditions during the MGI rapid shutdown,and also analyzes their disruption mitigation effects,and the advantages and disadvantages of different impurities.In addition,this article pioneers the suppression effect of the electrode biasing on the runaway current.And the results show that the attenuation effect of the positive electrode voltage on the runaway current,and even the complete suppression at higher voltages.First of all,this article is based on J-TEXT tokamak to carry out experimental research on the disruption mitigation of MGI.By changing the valve voltage and impurity species,the impurity transport process and mixing efficiency are studied.The process of MGI rapid shutdown can be divided into three stages: vacuum transit phase,Pre-TQ and TQ phase,and CQ phase.During the vacuum transit phase,the experimental impurity motion basically conforms to the motion model based on the Euler equation,and the transit velocity is close to the theoretical velocity.In the second stage,the experiment finds that after reaching the lower boundary of plasma,the impurities could not directly diffuse into the plasma core.Instead,they deposit on the boundary of the plasma(mainly on the high field side)in Pre-TQ phase and in the TQ phase,they diffuse rapidly to the plasma core.And the higher the valve voltage is,the deeper impurities deposits.In addition,the analysis of helium impurity finds that the direction of the poloidal movement of impurities in the Pre-TQ phase is affected by the m/n = 2/1 tearing mode.The energy radiated by impurities at this stage accounts for 3% to 10% of the initial thermal energy of the plasma,and the radiation level is pretty low.In the third phase,it was found that even the plasma has been cooled,the subsequently injected impurities can still interact with it,and the radiation energy in the CQ phase is still affected by the valve voltage.As for the mixing efficiency,relying on the unique 17-channel polarimeter-interferometer,this article defines the mixing efficiency of impurities.And the results show that the mixing efficiency is only of order 5% to 8% for Ar impurity,20% to 40% for He and 90%He&Ar impurities,and 12% to 20% for Ne impurity.The mixing efficiency gradually decreases as the valve voltage increases.Experiments finds that for a single impurity,the total number of injected particles at 80%CQ has the most significant effect on mixing efficiency,and the mixing efficiency gradually decreases as the number increases.Furthermore,the longer duration of 80%CQ causes that impurity has more time to ionize and the mixing efficiency can get higher.In addition,the vacuum transit velocity of the impurity also has a certain effect on the mixing efficiency.And by comparing the effect on disruption mitigation and mixing efficiency of four species,experiments find that 90%He&Ar impurity has the most significant disruption mitigation effect.Finally,under the experimental conditions that a few particles of MGI injection trigger the disruption to generate the runaway current,the application of electrode biasing with different voltages causes the obvious effects on runaway current.It is found that the runaway current appears to increase significantly under the negative electrode voltage;positive electrode voltage has a significant suppression effect on the runaway current,and the runaway current can be completely suppressed at the voltage of about 300 V.By analyzing the transport process of injected impurity,experiments find that voltages from negative to positive can gradually increase the transit velocity of the cold front during Pre-TQ phase,making TQ appear earlier.In addition,experiments find that electrode biasing can significantly change the number of electrons in the range of 0.5 to 4.5 Me V during disruption.The negative voltage can increase the number of electrons in this range,while positive voltage can clearly reduce the number of electrons.Preliminary analysis of the experiment finds that the positive electrode voltage suppresses the runaway current by suppressing the Dreicer mechanism.