It has been reported that plasma confinement in the cold region of a Uramoto‐type sheet plasma ion source is very important for negative hydrogen ion production [Rev. Sci. Instrum.66, 1035 (1995)]. In the present experiments, the stainless chamber was biased as low as 100 V relative to the anode potential in a 150 V–15 A discharge. The negative hydrogen ion current density (5 mA/cm2) was found to be larger than the positive hydrogen ion current density under the same experimental conditions. These experimental results suggest that the negative hydrogen ions are formed directly from atomic hydrogen. ©1995 American Institute of Physics.