Modeling of neutrals in the Linac4 H^- ion source plasma: Hydrogen atom production density profile and H_α intensity by collisional radiative model

To control the H⁰ atom production profile in the H^- ion sources is one of the important issues for the efficient and uniform surface H^- production. The purpose of this study is to construct a collisional radiative (CR) model to calculate the effective production rate of H⁰ atoms from H₂ molecules in the model geometry of the radio-frequency (RF) H^- ion source for Linac4 accelerator. In order to validate the CR model by comparison with the experimental results from the optical emission spectroscopy, it is also necessary for the model to calculate Balmer photon emission rate in the source. As a basic test of the model, the time evolutions of H⁰ production and the Balmer H_α photon emission rate are calculated for given electron energy distribution functions in the Linac4 RF H^- ion source. Reasonable test results are obtained and basis for the detailed comparisons with experimental results have been established.