Spatial distribution of negative ion density near the plasma grid

Density distributions of negative hydrogen (H^-) ions and negative deuterium (D^-) ions were measured with the laser photodetachment method in the extraction region of the negative ion source. The distribution of H^- ion density peaks at the center of the ion source, while that of the D^- ion shows a flatter profile in the direction parallel to the plasma grid. The positive ion densities of hydrogen and deuterium estimated from the positive saturation current indicate similar profiles with different amounts close to the grid. The difference in the H^- ion and D^- ion distributions can be explained by the difference in the negative ion yield and the survival probability of the ions due to the isotope effect.