Determination of electron temperature and density at plasma edge in the Large Helical Device with opacity-incorporated helium collisional-radiative model

- The reabsorption effect is included in the helium collisional-radiative model.
- Electron temperature and density are derived for the Large Helical Device (LHD).
- Line emission location is found to be little changed during the discharge.
- This measurement method can be used to determine the position of effective plasma boundary.

Spectra of neutral helium in the visible wavelength range are measured for a discharge in the Large Helical Device (LHD). The electron temperature (Te) and density (ne) are derived from the intensity distribution of helium emission lines. For that purpose, a collisional-radiative model developed by Sawada et al. [Plasma and Fusion Res. 2010;5:001] which takes the reabsorption effect into account is used. It is found that incorporation of the reabsorption effect is necessary to obtain a set of Te and ne giving consistent line intensity distribution with the measurement, and that those parameters obtained vary as the line-averaged ne changes in the course of time. The position where the helium line emission dominantly takes place is located with the help of Te and ne profiles measured by the Thomson scattering system. The result indicates that the emission position is almost fixed at the place where the connection length of the magnetic field lines to the divertor plate leaps beyond 10 m. Because intense neutral atom line emission suggests the vigorous ionization of neutral atoms, the helium line emission location determined here can be regarded as the effective boundary of the plasma.