Relations between light emission and electron density and temperature fluctuations in a helium plasma

The relations between three atomic lines, He I 667.8 nm (3¹D → 2¹P), 706.5 nm (3³S → 2³P), and 728.1 nm (3¹S → 2¹P), and the underlying fluctuations in a helium plasma are investigated for the quantitative interpretation of optical observations in plasma fluctuation measurements. Frequency dependent fluctuation amplitude ratios and phase delays between the line emission fluctuation and the electron density and temperature fluctuations are calculated based on a quasi-static collisional-radiative model and a linear approximation technique. For frequencies up to the upper limit of practical interest (1 MHz), the fluctuation amplitude ratios and phase delays are similar to those directly evaluated by the quasi-static model. It is found that the difference between the results from the linear approximation technique and from the quasi-static model is due to the absence of metastable fluctuations. Contributions of the 2 ¹S and 2³S metastable fluctuations to the three helium line emission fluctuations are analyzed. The linearity between fluctuations in the line emission and in the electron density and temperature is valid for fluctuation levels higher than 10.