Comparison of He I line intensity ratio method and electrostatic probe for electron density and temperature measurements in NAGDIS-II

Abstract

The electron density and temperature obtained from the line intensity ratio method of HeI (λ=667.8, 706.5, and 728.1nm) are compared to the probe method in a divertor simulator. When a collisional radiative model that does not include the effect of the radiation transport was used for the analysis, ne obtained from the spectroscopic method was significantly higher than that from the electrostatic probe method. The discrepancy between the two methods increases with the gas pressure; in other words, it increases with the optical thickness. In the case that the effect of the radiation trapping is taken into consideration using optical escape factor, the discrepancy becomes moderate. And then, the parameters obtained from the line intensity ratio method agree with the probe method within a factor of 2 in the case that the radiation trapping was introduced with R=0.05m, which corresponds to the column radius of the spatial profile of the excited population density. In recombining plasmas, however, it was shown that the line intensity ratios might not be appropriate because the recombining component broke the monotonic increase/decrease dependences of the line intensity ratios on ne and Te. A measurement of another HeI line intensity of 447.1nm (2P3←4D3) is proposed for solving the problem.