Inversion Of Electron Density Distribution On EAST Tokamak

For nuclear fusion plasma physics experiments, the electron density is one of the most important parameters, its distribution and amplitute can reflect the capabilities of an experimental device, and is also very important for understanding plasma equilibrium, characteristics of confinement and MHD (magnetohydrodynamic) instability.The far-infrared laser interferometer and microwave reflectometer are two common tools to measure the plasma line integrated density and the electron density profile, respecitvely. They have been successfully applied to many large-scale tokamak devices. The former one is based on a different refractive index when the electromagnetic wave propagating in vacuum and in the plasma. After propagating the same optical path in vacuum and in plasma, the phase difference can be measured and compared, and then the line integrated density value can be calculated. The latter one is based on the principle of radar, in non-uniform plasma, by measuring the phase delay of the reflected microwave. The density of the plasma at any location can be calculated, and then the density profile is obtained.We can get the line integrated density value at different location by far-infrared laser interferometer. From microwave reflectometer, only boundary density profile can be obtained cause of its limitations. In order to get the whole plasma density profile, appropriate mathematical calculations combined with related diagnostic data are needed, that is the dubbed density distribution inversion. For the usual density inversion method, such as Abel inversion, Separation-of-variables, Slice-and-stack, Singular-value-decomposition-inversion and Park-matrix method etc., the degree of complexity and density diagnostics of them are not the same, and also the application. In this paper, taking into account the diagnostic systems of the EAST superconducting tokamak, microwave reflectometer system and far-infrared laser polarimeter/interferometer system (POINT) are combined, and then the data of the two systems are calibrated and unifed. In this process, a method for converting microwave reflectometer data into interferometer data at different positions is explored, then, the line integrated density values of the region which interferometer systems haven’t covered are obtained. Based on assumption that the density value between two adjacent magnetic surfaces is a constant, separate the plasma into two parts from the mid-plane, then combine surface coordinates provided by EFIT(equilibrium fit), the density values can be calculated from the outmost magnetic surface to the innermost one by one, then the density distribution is obtained. By this progress, the density profiles of different shapes of EAST tokamak are inverted successfully.For the obtained density profile, a progress of error analysis is applied based on the five-channel POINT data which are the input variables and the data from three-channel vertical HCN interferometer system, respectively. The density value of the innermost magnetic surface will be calculated twice, compare this two values and make sure the error is in 3% by adjusting the magnetic surfaces. Then integrating the inverted density as this two diagnostic coordinates and comparing the calculated values with the measured values, if the error is in 5%, the density profile is believed, otherwise, the magnetic surfaces should be adjusted. It can be found that the overall error of each diagnostic system can be controlled within 5% or ever smaller through small adjustments of the magnetic surfaces, which has achieve the desired inversion accuracy.