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Effects of cesium vapor injection on H- production in a tandem volume source are studied numerically as a function of plasma parameters. Model calculation is performed by solving a set of particle balance equations for steady-state hydrogen discharge plasmas. The results with a focus on electron temperature and gas pressure dependence on H- volume production are presented and discussed. Considering H- surface production due to H atoms and positive hydrogen ions, enhancement of H- production and pressure dependence of H- production observed experimentally are qualitatively well reproduced in the model calculation, where stripping loss in the extraction and acceleration regions is taken into account. For enhancement of H$production, so-called electron cooling in the source region, as well as in the extraction region, is not so effective if plasma parameters are initially optimized with the use of a magnetic filter.