Studies On Low-Z Impurity Behavior In J-TEXT Tokamak

Impurity in the tokamak plasma will increase the radiation loss and influence the plasma energy balance and particle balance,thus the temperature of the plasma decrease and plasma confinement deteriorate.Moreover,it leads to plasma disruption.With impurity into the plasma,it decreases the fusion power in fusion devices.Therefore,how to control the impurity behavior efficiently in high-parameter and steady-state fusion reactor becomes one of the crucial problems.Many problems about the impurity behavior are not understood clearly and completely so far.It is difficult to explain the observed experimental results based on the analysis of the neo-classical impurity transport theory due to the existence of an anomalous transport related to turbulence closely.Generally speaking,the focus of anomalous transport is on the investigation of the radial transport behavior across the magnetic field lines(It can be regarded as vertical transport in suitable situations).However,under certain conditions,the contribution of parallel transport makes the impurity distribution more complicated.However,parallel transport makes the impurity distribution more complicated under certain circumstances.Therefore,it is necessary to take the effects of both vertical and parallel transport on the impurity distribution into consideration.In this paper,a series of experiments are carried out to research the behavior of low-Z impurity in plasma based on the J-TEXT Tokamak.In order to study the impurity behavior,a set of diagnostic systems for radiation and impurity spectra are set up:1)Absolute eXtreme Vacuum ultraViolet(AXUV)bolometer diagnosis.It includes poloidal arrays and toroidal arrays.Based on this diagnosis,the radiation profile of plasma and the toroidal symmetry information of radiation can be obtained by inversing the measurements.2)The photodiode array(PDA)C Ⅲ radiation diagnostics.Its principle is measurements of the C Ⅲ spectral line(464.7nm)by using the filter and lens imaging to provide the C Ⅲ radiation information in the position of high/low field side and limiters.It is mainly applied to analyze the behavior of C Ⅲ ion.3)Monochromatic imaging diagnosis.It can provide specific wavelength impurity radiation profile in the position of high and low field side in order to analyze the specified impurity ions(e.g.C V)behavior.In this paper,the main contents are as follows:(1)Studies of impurity radiation.The STRAHL code is employed to simulate the radiation loss in J-TEXT tokamak.Combined with transport coefficient which calculated by the STRAHL code and the correction of the AXUV detectors’ responsivity,the results of AXUV diagnosis can be well reproduced.The observation result of impurity radiation behavior under different density discharge shows that edge cooling phenomenon appears before density limit disruption and multifaceted asymmetric radiation from the edge(MARFE)phenomenon appears along with impurity radiation of carbon has a significant enhancement.The radiation loss power is only about 30%of the total input power in this process.There is a threshold between the radiation loss and density limit by impurity injection to change radiation loss.Different kinds of impurity can change the behavior of MARFE by affecting the radiation loss of the edge plasma and maybe the reason is the change of the radiative cooling function.Meanwhile,the asymmetry of C III radiation has been observed in the edge plasma even though MARFE not occurs and the asymmetry is more obvious with the increase of density.The experimental results show that the asymmetry is closely related to carbon source,parallel impurity transport and vertical impurity transport.(2)Studies of the impurity transport.First,the carbon impurity transport coefficients in the position of high/low field side are obtained by the STRAHL code simulation.The results show that the diffusion coefficient of high field side is smaller than low field side,and the impurity diffusion coefficient in LFS decreases along with the increase of density.Second,in order to explain the asymmetric phenomenon of impurity radiation,a set of experiments are carried out in terms of carbon source,ion-impurity friction force and centrifugal force.The effect of the asymmetry of impurity distribution with local carbon source has been studied by the change of the position of limiters and impurity injection.The results show that the carbon source has a significant effect on the toroidal asymmetry of C III radiation,but it has little effect on the poloidal asymmetry.It is confirmed by the STRAHL simulation and experimental results of different positions of impurity injection.In addition to the carbon source,the ion-impurity friction force also plays an important role in the poloidal asymmetry of C III radiation.The results show that the ion density gradient and the ion temperature gradient have influence upon the direction and magnitude of the asymmetry.The influence of ion-impurity centrifugal force can be reflected only in some special discharges(such as locked modes)because of it is less affected by the plasma rotation.