Design Of Data Acquisition And Real-time Processing System For Polarimeter-interferometer On J-TEXT Tokamak

Polarimeter-interferometer is an important diagnostic device in J-TEXT tokamak. It can realize a direct measurement of Faraday rotation angle and plasma line-integrated electron density. With the two basic physics parameters, plasma current density can be calculated by inversion. A necessary part of polarimeter-interferometer is data acquisition and real-time processing system. Data acquisition is one of the bases for polarimeter-interferometer operation. It can provide valuable data for researchers. Real-time processing can process data during acquisition, so that it can make more effective use of the raw data. The work of this paper is to design and implement a data acquisition and real-time processing sysytem with the following features: high reliability, high maintainability, high scalability, good real-time performance and friendly operation. It aims to meet the needs of polarimeter-interferometer operation both in the current and future.In the first part, this paper designs the framework of hardware and software according to the system requirements, following the principle of modularity. In hardware, we choose NI Flex RIO as the hardware platform to meet the needs of high sampling rate and good real-time performance. In software, we take fully consideration of the software performance and the difficulty to maintain and extend the system. With Lab VIEW programing language, three main function: data acquisition and storage, the real-time calculating of plasma density, plasma density feedback, are integrated into the software framework.The function is much different between the host side and the FPGA side in data acquisition and storage subsystem. In host side, we use producer/consumer programming model to realize the following main functions: configuring the system, reading data from FPGA side, and storing the data. With Lab VIEW FPGA programing module, the FPGA program uses a state machine structure to realize reliable data sampling and logic processing.On the basis of data acquisition, real-time data processing function can be developed with the same hardware. The key to calculate plasma line-integrated electron density is the calculation of phase difference. According to the signal characteristics of polarimeter-Interferometer in J-TEXT tokamak, we tested three common algorithms of phase difference: zero-crossing detection, digital correlation and FFT. Then we choose FFT as the algorithm to calculate phase difference in real-time. With Lab VIEW FPGA programing, we designed and developed a reliable real-time processing software with an accurate phase reverse algorithm. The practical running results show that the calculation results of plasma density between online and offline are consistent. Therefore, it can be used in theoretical analysis or plasma density feedback control in the future work.