Investigation of atomic and molecular processes in H_α emission through modelling of measured H_α emissivity profile using DEGAS2 in the ADITYA tokamak

In order to understand the atomic and molecular processes involved in the emission of the hydrogen Balmer alpha (Hα) spectral line from tokamak plasmas, the measured radial profile of Hα emissivity in ADITYA tokamak discharges has been modeled using the DEGAS2 neutral particle transport code. The radial profile of emissivity has been measured using a 1.0 m multi-track spectrometer and with PMT array based space-resolved visible spectroscopic diagnostics involving interference filters. The radial profile of the neutral hydrogen density has been obtained using the DEGAS2 code by reproducing the experimentally observed Hα emissivity profile. It has been found that the neutral density falls by 80-200 times at the plasma center compared to its maximum value at the plasma edge near the limiter (α = 0.92). Detailed investigation of the contributions of atomic and molecular processes involved in Hα emission reveals the significant presence of hydrogen molecules and molecular ions within ?4 cm of the limiter radius. The contributions from processes involving H+2 ions are found to be higher than those from the molecular hydrogen dissociation with H+2 dissociative recombination dominating over H+2 dissociation at the extreme edge of the plasma.