Speaker
Description
0ne of the major challenges on the pathway to fusion power plants is safe control of large plasma heat load to the strike points. To keep the plasma heat load below acceptable limits, fusion reactors are expected to operate in detached divertor regime. The detached divertor is facilitated by electron-ion recombination and molecular activated recombination (MAR). Although experimental studies on the MAR processes under realistic divertor plasma conditions have been limited, they have been attracting considerable attention in recent years, owing to improvements in plasma diagnostic systems and analysis techniques.
To understand contribution of MAR reactions to the plasma detachment, reaction rates of the entire reaction chains must be evaluated. Such analyses require various plasma parameters, including ro-vibrational temperatures of hydrogen molecules in ground electronic state, densities and temperatures of electrons and ions, fractions of atomic and molecular ions. Electron temperature ($T_\mathrm{e}$) and electron density ($n_\mathrm{e}$) are easily obtained than other parameters by using a Langmuir probe (LP). However, presence of molecular ions ($\mathrm{H}_{2}^{+} $ and $\mathrm{H}_{3}^{+}$) and negative ions ($\mathrm{H}^{-}$) is usually ignored in LP analyses, introducing uncertainties in the evaluated values, particularly when MARs are strongly facilitated.
In this work, we propose a diagnostic method for determining $n_\mathrm{e}$ and $T_\mathrm{e}$ based on a collisional-radiative analysis of atomic and molecular hydrogen emissions. $n_\mathrm{e}$ can be inferred from line intensity ratio of two Balmer lines, without assuming fractions of molecular and negative ions. To incorporate molecular reactions into the analysis, we employ a collisional-radiative code developed for hydrogen molecules. In addition, the population escape factor is introduced to account for the radiation trapping effect.
As a preliminary experiment, we have tested the applicability of this technique for $n_\mathrm{e}$ determination using hydrogen ionizing plasma, and good agreements have been confirmed with measurements from a Langmuir probe. Currently we are extending the technique to $T_\mathrm{e}$ determination using intensity ratio of the Balmer series and the hydrogen Fulcher-$\alpha$ band. We plan to test the applicability of the $T_\mathrm{e}$ determination for a wide range of plasma conditions.
In the presentation, we will introduce details of our method, present experimental results, and discuss applicability of the method. Impacts of the radiation trapping effect and selection of the Balmer lines on electron density determination will also be addressed.
This work is supported by JSPS KAKENHI grant number JP24K00607.