Effects of cesium vapor injection onH−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. Here, the results with a focus on electron temperature and gas pressure dependence onH−volume production are presented and discussed. ConsideringH−surface production due to H atoms and positive hydrogen ions, enhancement ofH−production and pressure dependence ofH−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 ofH−production, so-called electron cooling is not very effective if plasma parameters are initially optimized with the use of a magnetic filter. ©1998 American Institute of Physics.