Evaluation of hydrogen atom density in the plasma core region based on the Balmer-𝛂 line profile

The Balmer-α line profile is measured with high wavelength resolution for a discharge in the Large Helical Device. The line profile is regarded as a superposition of continuously varying Doppler broadened components and is expressed as the Laplace transform. Numerical Laplace inversion of the measured line profile gives the distribution function of line emissivity in terms of atom temperature. The temperature dependence of the line emissivity is interpreted as spatial dependence so that the ionization rate and atom density of neutral hydrogen are determined. The temperature range of the detected atoms extends beyond 2 keV which corresponds to a penetration depth of about 1 m in the plasma, or the location at ρ ∼ 0.3, where ρ is the normalized minor radius. The atom density of approximately 10 13 m −3 is derived in the plasma core region which is more than four orders smaller than that at the plasma boundary. Calculation of neutral transport with a Monte-Carlo simulation code gives satisfactory consistency with the experimental results.