Speaker
Description
One of the most important subjects in detached divertor plasma studies is reliable diagnostics of the electron density ($n_e$) and temperature ($T_e$). He I line-intensity-ratio-method (LIR-method), based on a visible spectroscopy and a collisional-radiative (CR) analysis, offers a non-invasive measurement of these parameters. When this method is applied to plasmas with high neutral densities, reabsorption of the emitted photons called the radiation trapping becomes problematic. A practical way to account radiation trapping is to introduce the Optical Escape Factor (OEF) in CR analysis. Typically, OEFs are introduced to the resonant transitions with ground state, i.e., $1^{1}\mathrm{S}$–$n^{1}\mathrm{P}$ series. $n$ represents the principal quantum number. Calculations of the OEFs are based on spatial profiles of $1^{1}\mathrm{S}$ atoms and $n^{1}\mathrm{P}$ atoms. So far, several models have been proposed for calculation of the OEFs [1], and these models assume spatially uniform ground state atoms. However, sometimes ground state atoms have non-uniform distribution due to neutral depletion [2] or particle transport, making above assumption no longer valid.
In this situation, we propose a new model for the OEF calculation to account for the spatial distribution of the ground state atoms. Based on the model described in Ref. [1], we have extended the formulation so that it can consider any arbitrary spatial distribution of ground-state atoms. To investigate impact of the distribution assumed in the CR analysis, recently we have conducted a preliminary experiment using RF plasma. Visible lines of wavelength at 667.8 nm ($2^{1}\mathrm{P}$-$3^{1}\mathrm{D}$), 706.5 nm ($2^{3}\mathrm{P}$-$3^{3}\mathrm{S}$), and 728.1 nm ($2^{1}\mathrm{P}$-$3^{1}\mathrm{S}$) were collected for the LIR-method. In addition, the line emission from $3^{1}\mathrm{P}$ level was also collected to calculate OEFs. A Langmuir probe was also used to obtain $n_e$ and $T_e$. Results of the LIR-method using several different distributions for ground-state atoms showed that determination of $n_e$ and $T_e$ was significantly dependent on the distribution selected. However, agreement in $n_e$ and $T_e$ was relatively poor at peripheral region of the target plasma, regardless of the distribution selected. One probable reason for this disagreement is uncertainties in the OEF for $1^{1}\mathrm{S}$-$2^{1}\mathrm{P}$ transition. Currently we are trying to evaluate more reliable OEFs by adding emission from $2^{1}\mathrm{P}$ level to the CR analysis. In the presentation, we will report impact of the spatial distribution of the ground state atoms on LIR-method, based on the improved OEF calculation.
[1] Y. Iida et al, Phys. Plasma 17, 123301 (2010).
[2] R. M. Magee et al, Phys. Plasma 20, 123511 (2013).