Investigation Of Helical Current Induced By Electrode Biasing In The SOL Of J-TEXT Tokamak And Their Impact On Plasma

Nuclear fusion is one of the most promising solutions to the energy crisis.Among the many types of fusion devices,tokamak is the device with the highest operating parameters and the most promising controllable nuclear fusion.In tokamak plasma,macroscopic magnetic fluid instability such as edge local mode(ELM)and tear mode(TM)will reduce plasma confinement performance.If not controlled effectively,it can even cause plasma disruption.The external resonant magnetic perturbation field(RMP)is widely used to control ELM and TM.However,the RMP coils in the fusion reactor need to be installed behind the cladding material and hence will be exposed to a large number of high-energy neutrons.This coils will be difficult to repair once damaged.Therefore,there need a flexible and efficient way of providing RMP for future fusion device.The EAST experiment found that lower hybrid wave can drive the helical current filaments(HCFs)in the scrape-off layer(SOL).This HCFs can change the edge magnetic topology and effectively suppress the ELM.Theoretical studies have found that sufficient helical current can be provided to control the ELM by applying a bias to the ITER divertor.At present,there is a lack of experimental research on actively driving and controlling the HCFs in SOL to generate RMP.Therefore,it is necessary to confirm the existence of the helical current of the SOL and to investigate whether it has a prospect of providing RMP for the plasma.In this paper,we use the biased electrode system of J-TEXT tokamak to induce the helical current in SOL.By analyzing the magnetic field calculated by the model and the experimentally measured,it is found that the helical current from the electrode flows in two directions parallel to the magnetic field lines and finally connected to the limiters.Therefore,the path of the helical current depends on the safety factor at the electrode and the relative position of the electrode to the limiter.The experiment found that the amplitude of the helical current is positively correlated with electrode size and plasma current.The amplitude of the helical current increases as the electrode approaches the last closed magnetic surface(LCFS)from the far SOL.However,as the electrode spans the LCFS,the magnitude of the helical current will decrease.Detailed studies have shown that the spatial path and amplitude of the bias-driven helical current can be actively controlled by changing the plasma parameters and electrode parameters.In order to better understand the characteristics of the helical SOL current,modeling analysis was adopted.The model traces magnetic field lines from various areas of the electrode surface,and specifies that the direction of the current is parallel to those magnetic field lines.The results show that the space paths of helical current are different,if they originated in different regions of the electrode surface.Setting the amplitude of helical current in the model is proportional to the effective area and inversely proportional to the length of the magnetic field line connection length.The calculated disturbance magnetic field coincides with the experimentally measured,indicating that the current distribution in the model reveals the three-dimensional spatial distribution of the spiral current in the experiment.When +200 V bias voltage is applied to the electrode,helical current of +150 A can be driven,which could generate the m/n = 3/1 magnetic field component of up to 11 Gauss/kA.Since the path of the helical current is adaptive to boundary magnetic field,and hence the resonance magnetic field component produced by the helical current is much more efficient than the conventional RMP coil.Both the experimental results and the calculation results show that the helical current can act as a new method of providing RMP for plasma.