Eight lines in the visible wavelength range of the neutral helium were applied for diagnosing the electron density and temperature of low-pressure Large Helical Device (LHD) helium plasma instead of the conventional way that uses three lines. The collisional-radiative (CR) model of low-pressure helium plasma was revised by including the optical escape factors of spontaneous transitions from 2^1P, 3^1P, and 4^1P to the ground state 1^1S for dealing with the plasmas under optically thick conditions. An algorithm based on the revised CR model was developed to fit the number density of the eight excited states measured by optical emission spectroscopy (OES) method. The electron density, electron temperature, ground state density, and the optical escape factors above were selected as the fitting parameters. The object function was set as the summation of the difference between the number density measured in experiment and the number density calculated by the CR model. A regularization term is introduced
regarding the optical escape factors. The coefficient of the regularization term was optimized through bias and variance analysis. The results show that the error between the number density
calculated by the algorithm and measured in experiment was generally improved compare to the method that only used three lines.
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