Research Of Plasma Behaviors For High Density Discharges On The J-TEXT Tokamak

The plasma density is one of the most important parameters for magnetic confinement fusion.The Deuterium-Tritium(D-T)fusion produced power is proportional to the square of plasma density.For the purpose of produsing more fusion power,the higher plasma density is the better.However,magnetic confinement experiments show that the plasma operating densities always have an upper limit.Once the plasma density exceeds this limit,a series of instabilities will appear and eventually lead to a major disruption.In order to ensure the magnetic confinement fusion device operating safety and stably at enough high plasma density,it is necessary to study the behavior of high density plasmas and the physical mechanism of density limit disruption.In the study of high density plasma,the turbulent fluctuations associated with micro-instabilities is an important parameter,which reflect the information of plasma transport and confinement.In order to study the changes of plasma transport and confinement during density limit disruption,the polarimeter-interferometer system on J-TEXT tokamak has been exploited to measure far-forward collective scattering from the electron density fluctuations.In this dissertation,we perform experiments on J-TEXT tokamak to investigate the high density plasma behaviors and mechanism of density limit disruption.Firstly,we explore the density operating region of J-TEXT tokamak,and the dependence of maximum operating density on discharge paremeters.It is observed that the density operating region is extended significantly by using improved wall and plasma current decreasing-modulation,the maximum operating density of J-TEXT tokamak is increased from 0.42nG up to 1nG.By concluding a large number of density limit disruption discharges on J-TEXT,it is found that the maximum plasma density is proportional to the total plasma current and weakly depends on the toroidal magnetic,which is consistent with Greenwald scaling law.Secondly,we investigate the evolutions of plasma profiles(including density,temperature and current density)with density.It shows that the plasma profiles evolutions can be divided into two different stages,linear stage and saturation stage,which is similar to dependence of confinement on density.At the linear stage,the density profile peaks with density increasing,meanwhile the temperature and current density distributions flat.At the saturation stage,the peaking factor of density distribution stops increasing,and the electron temperature distribution overall decreases,inferrs that the plasma energy decreases and the confinement tends to saturation.Finally,the plasma behaviors during density limit disruption discharge has been systematically studied on J-TEXT.The multifaceted asymmetric radiation from the edge(MARFE)and detached plasma(DP)phenomena are observed.The MARFE on J-TEXT is charactered by the poloidally local region at high field side(HFS)edge with high density and strong radiation.By analyzing the data from POLARIS,it is found for the first time that the MARFE will lead to plasma current density locally decline at HFS edge.Combining with numerical simulation,it is proposed for the first time that the plasma current locally shrinking induced by MARFE products radial magnetic field at the q = 2 resonant surface,thus triggs the 2/1 tearing mode and leads to density limit disruption.By investigating the poloidal distribution of neutral particles and plasma parameters at scrape-off layer(SOL),a new mechanism of MARFE formation is proposed,is that:the poloidal asymmetry of edge radial transport caused by toroidal effect makes the plasma pressure at outboard much larger than inbord,which leads to the neutral particles concentrating at HFS edge,thus triggs the condensation instability and eventually MARFE occurrs.The DP results from MARFE rotating poloidally,it is charactered by the shrinkage of plasma channel,enhancement of radiation loss,increase of impurity contents and enhancement of MHD instability.Furthermore,the enhancement of MHD instability is found to be caused by the mode coupling between the 2/1 small magnetic perturbance and 1/1 internal kink mode.The great enhancement of radiation loss and MHD instability induced by the mode coupling are considered to be the direct reason of DP causing density limit disruption.Based on the observation of MARFE and DP,the dependence of density threshold of MARFE and DP on discharge parameters(such as the total plasma current,safety factor and plasma horizontal displacement)is studied experimentally.And the experimental results are explaned reasonably by using the physical mechanism proposed above.