Development of Zeeman Split-Assisted Tomography for Helium Line Emission Distribution in LHD

Abstract

We have measured the emission line spectra of neutral helium (2¹P - 3¹D, 667.8 nm) in a poloidal cross-section of the plasma in the Large Helical Device (LHD). The measurement has been made from two different observation ports with 40 lines-of-sight each. A tomographic analysis was done to derive the line intensity distribution, using the Hopfield Neural Network method with the Phillips-Tikhonov regularization term. The algorithm was expanded to incorporate Zeeman splittings, a feature prominently evident in the measured spectra, providing information on the position of the emission. The results revealed a concentrated radiation distribution near the X-points and on the divertor legs, specifically those connected to the inboard side divertor plates. We have confirmed that reconstructed images are available even when the analysis is performed solely with data from one of the two observation ports. This suggests the potential applicability of our approach under highly limited options for observation port arrangements. By reconstructing the line emission distribution only using the π peak signals, we show that our method can estimate the vertical emission positions from the Zeeman effect. The robustness of our new method was also checked.