Negative hydrogen ions are extracted from a sheet plasma and extracted H−currents are optimized for several parameters; a location of the extraction electrodes, a bias voltage, a filling gas pressure and a discharge current. There is an optimum position of the electrodes near the plasma periphery and an optimum pressure for the H−production. The profiles of the electron density and temperature are measured and the relation between the profiles and the H−yield is investigated. It is found that the H−yield is optimized when the electron temperature in front of the extraction grids decreases below 1 eV and that this optimum condition can be achieved by changing the position of the electrodes and/or by changing the filling gas pressure.