Design of line array detection scheme for Polarimeter-Interferometer system on EAST

Abstract

A multi-channel POlarimeter-INTerferometer (POINT) system has been constructed in the core region of Experimental Advanced Superconducting Tokamak (EAST) plasma for electron density profile and current density profile measurements, with a spatial resolution of 8.5 cm. To explore the small spatial scale density fluctuation phenomena in the core plasma induced by MHD instabilities effects, a higher spatial resolution and higher responsivity detection is required.A novel line array detection scheme of POINT is proposed. By replacing the single Schottky mixer with a highly integrated line array HEMT detector at the central channel position, the existing single channel can be extended to 5 channels for spatial continuous measurement in the core plasma, which has a significantly simplified structure design compared with the discrete upgrade scheme. Related optical design, component selection, and bench tests have been carried out to validate the feasibility and reliability of the novel scheme.The IF signals are detected by an AlGaN/GaN line array high electron mobility transistors (HEMT) detector based on five silicon lenses integrated. The average responsivity of the line array detector elements is 40 kV/W. Compared with the existing planar diode Schottky mixer with a sensitivity of 750 V/W, the line array detector can also meet the essential requirements of coherent signal measurement. Benefiting from the highly integrated array detector, the spatial resolution will be improved to 4.5 mm in the novel scheme. Additionally, a circular Plano cylindrical lens is designed as a beam-shaping component to compress the Gaussian beam into a stripe. Bench test results displayed the excellent performance of the cylindrical lens, which is consistent with the simulation results. Furthermore, normalized responsivity coefficients of array elements are determined by test results.