Algorithm improvement for the surface morphology diagnostics based on the Gram-Schmidt orthonormalization and the least square ellipse fitting under the EAST-like vibrational environments

Abstract

To realize in situ surface morphology diagnosis of Plasma-Facing Materials (PFMs) in the Experimental Advanced Superconducting Tokamak (EAST), a diagnostic system which is called DUT-SIEP (Speckle Interferometry Experiment Platform in Dalian University of Technology) is in development. Mimicking the actual vibrational conditions in EAST, Dual-wavelength Phase-Shifting Interferometry (DPSI) should be sensitive to vibrational environment and would reduce accuracy in the morphology measurements. Therefore, an approach combining the Gram–Schmidt orthonormalization and the Least-Square Ellipse Fitting (GS-LSEF) was proposed to replace the conventional phase extraction process to improve the measurement accuracy in the EAST-like vibration conditions. The new approach only requires two interferograms at each wavelength while the conventional method needs three at least for each wavelength. This significantly reduces the phase errors in the acquisition processes. The Gram–Schmidt orthonormalization is performed to correct the phase shift errors and determine the elliptic coefficients. In the Least-Square Ellipse Fitting, the elliptic coefficients are used to compensate the phase errors from the disturbed background intensity. The approach can significantly improve the measurement efficiency and avoid the infinite loops caused by the numeric divergent. The method was verified by laser ablation on tungsten (W) material for simulating erosion-processes in EAST. Results show that the proposed approach has significantly improved in accuracy by approximately twofold comparing with those by the convenient Four-step phase-shifting approaches due to reducing the noise superposition problems in the EAST-like vibrational conditions.