The plasma current density profile plays a key role in the development of a high poloidal beta scenario, which is essential for long-pulse and high-performance plasma operation on a tokamak. Based on the polarimetry technique, a Motional Stark Effect (MSE) diagnostic has been built on the Experimental Advanced Superconducting Tokamak. To be prepared for real-time (RT) feedback control of the plasma current density profile in the future, a RT signal processing system has been developed. The RT signal processing system is composed of three functional modules: analog-to-digital conversion (ADC) module, polarization information extraction module, and digital-to-analog conversion (DAC) module. The final objective of this system is to acquire the polarization information of the MSE. Based on the field-programmable gate array unit, fast Fourier transformation is adopted to process the Photoelastic Modulator (PEM) digital signal, which was converted from a PEM signal via the ADC module. By means of frequency spectrum separation, the components around double modulating frequencies are restored through inverse fast Fourier transformation. Furthermore, the two amplitudes of their corresponding components can be obtained through a digital harmonic analyzer technique. Afterward, the ratio of the two amplitudes is calculated by arc tangent so that the polarization angle is obtained. Finally, the information of this polarization angle is converted into a voltage signal by the DAC module and then output in RT. The test results based on the RT signal processing system are in good agreement with those based on the phase lock-in amplifiers. The working cycle of this system is shorter than 10 ms, which meets the requirements of the MSE diagnostic as a RT controller. The algorithm of RT signal processing and the relevant technology applied for building this system are presented in the main body of this paper in detail.

The motional Stark effect (MSE) diagnostic is applied to measure the safety factor q and current density profile of a tokamak device, which are important parameters in realizing the high-performance and long-pulse steady state of a tokamak. A single-channel MSE diagnostic based on dual photoelastic modulators, whose sightline meets with the neutral beam injection at a major radius of R = 2.12 m, has been built for the D window of the Experimental Advanced Superconducting Tokamak (EAST). According to the requirements of MSE diagnostic polarimetric calibration, a high-precision four-dimensional calibration turntable, driven by four stepping motors and controlled by software running on the computer, was designed for EAST. The turntable allows us to rapidly calibrate the MSE diagnostic in a series of positions and angles during EAST maintenance. The turntable can move in four dimensions of translation, yaw, pitch, and roll of the polarizer and can create linearly polarized light at any given angle with accuracy of ∼0.05° for the MSE system offline calibration. The experimental results of the MSE diagnostic calibration in the laboratory show that the turntable has the advantages of high positioning accuracy, flexible spatial movement, and convenient control and fully meets the calibration requirements of an MSE diagnosis system.