Study The Effect Of Electrode Biasing On 2/1 Tearing Mode On J-TEXT Tokamak

In tokamak fusion device,the avoidance of major disruption is a crucial research topic,and the control of tearing mode is a crucial part of this topic.At present,there are many methods to control the tearing mode,but there are some limitations,so it is necessary to explore more reliable means to control the tearing mode.Based on the Electrode Biasing(EB)system of JTEXT tokamak,this paper explores the response of the 2/1 tearing mode to the EB current,and controlling the 2/1 mode locking process,and successfully unlocking the locked tearing mode with the application of EB.Firstly,the response of the 2/1 tearing mode to the EB current is studied systematically.It is found that the response can be divided into two processes: rapid response process and slow response process.Among them,the slow response process is related to the momentum transport process of the plasma,during which the variation of the frequency and amplitude of the tearing mode is related to the rotation/rotational shear change of the plasma around the magnetic island.In the fast response stage,the rapid change of the tearing mode frequency is almost synchronized with the increasing of the EB current.The results show that the rapid change of the tearing mode frequency is related to the EB current value and the ramping time of the EB current.The more detailed analyses show that the rapid change of tearing mode frequency is delayed by the change of EB current by about 13 μs,which further indicated that the rapid response is independent of the momentum transport of the plasma.Combining with the previous experimental results,a new mechanism is proposed: the EB current changes the boundary rotation of the plasma,thus directly affecting the tearing mode frequency through mode toroidal coupling.Secondly,the EB is used to control the locking process of 2/1 tearing mode and to unlock the locked 2/1 tearing mode successfully for the first time.The experimental results show that the application of negative/positive EB current can increase/decrease the frequency of the tearing mode,decrease/increase the amplitude of the tearing mode,stabilize/unstabilize the tearing mode,and make the mode locking more difficult/easy to occur.Application the strong enough negative EB current can even completely avoid the occurrence of 2/1 mode locking.Even with the existence of the 2/1 locked mode,enough negative EB current can be used to unlock the locked tearing mode and make the locked magnetic island re-rotation.The results show that increasing of the negative EB current can significantly accelerate the unlocking process,while the increase of mode amplitude will slow down the unlocking process.The analysis shows that the unlocking process of EB is related to the rapid response of the tearing mode: If the EB current is sufficiently large,the local plasma as well as the 3/1 mode is forced to rotate,which will in turn drive the 2/1 mode rotation via mode coupling.Once the phase of the m/n = 2/1 magnetic island is changed to a phase different from that of SRMP,the locked mode amplitude will decline,resulting in a further change of the locked mode phase and finally the unlocking of the locked mode.