Studies Of Limiter Biasing On J-TEXT Tokamak

Many fusion devices have tried to reduce the turbulent transport at the boundary by biasing,which affects the confinement of the core plasma.Many experiments on electrode biasing have been carried out on the J-TEXT tokamak device,and it is found that the electrode biasing have a good effect in improving plasma confinement and controlling instability.However,when the electrode biasing works,the external graphite electrode needs to be extended into the plasma,which will inevitably introduce additional impurity sources.Therefore,it is necessary to explore the ability to control the boundary electric field without increasing the impurity source.Controlling the boundary electric field to suppress the plasma boundary transport and improve the confinement ability.During the plasma discharge process,the contact area between the plasma and the limiter target is constant,and no additional source of impurities will be introduced.Therefore,the paper introduces experiment of limiter biasing on the J-TEXT device,mainly exploring the influence of the biasing of different polarities on the J-TEXT device’s on the overall confinement state and boundary transport of the plasma.In order to meet the basic requirements of the experiment,limiter biasing system has been established on the J-TEXT tokamak device.The system includes power system,adata acquisition system,and alimiter target.The data acquisition system is mainly used to measure the voltage and current signals on the limiter.The experiment of limiter biasing in this article was carried out under plasma current I_p=133k A and I_p=180k A.There are some common phenomena under the two plasma currents.Compared with the 0V bias voltage,the particle and energy confinement ability of the plasma is enhanced under positive and negative biasing.The main source of carbon impurities is the wall of the vacuum chamber under positive biasing,the main source of carbon impurities is limiter target under negative biasing..Interestingly,when I_p=180k A,after adding negative biasing,the₍（6） radiation intensity and the carbon impurity radiation intensity on the high and low field sides decrease instantaneously.The main reason may be related to the influence of the biasing on the boundary plasma under the high and low ohmic heating power.₍（6） radiation intensity,core electron temperature,carbon impurity radiation intensity,and thermal radiation intensity are all closely related to plasma density,so the data is statistically analyzed under the same density.Under the same plasma density,the increase in thermal radiation loss under positive bias is mainly caused by the increase in electron temperature;while the thermal radiation loss under negative bias does not increase significantly,the main reason may be that the impurity level is reduced and the effective charge number is reduced.The research on the influence of the limiter biasing on the plasma boundary transport is mainly carried out by using the boundary electrostatic probe diagnostic system.Experiments have found that the suspension potential changes instantaneously under positive and negative biasing.In particular,strong low-frequency electrostatic fluctuations were excited under negative biasing,including a 17k Hz geodetic acoustic mode;while under positive biasing,the low-frequency mode was not found.Other turbulent components will also have a process of gradually changing from high frequency to low frequency.In the analysis of turbulence at different positions at 180k A,It is found that the fluctuation amplitude of the ion saturation flow at a small radius of r=250mm decreases instantaneously under negative biasing,resulting in a significant reduction in the turbulent flux of boundary particles,which may be one of the reasons for the substantial increase in the core electron density.In the analysis of the heat flow distribution of the limiter target plate,it is found that the heat flow decreases on the surface of the limiter target under the positive and negative biasing.In addition,when I_p=180k A,the boundary electrostatic fluctuations are obviously suppressed,and the particle turbulent flux has a significant decrease under positive and negative biasing.Under positive biasing,the drop in particle turbulent flux is mainly due to the suppression of boundary electrostatic fluctuations;under negative bias,the decrease of particle turbulent flux is not only related to the decrease in the amplitude of electrostatic fluctuations,but also related to the decrease of the correlation between electrostatic fluctuations.