The functions of a biased plasma grid of a negative hydrogen (H−) ion source for both pure volume and Cs seeded operations are reexamined. Proper control of the plasma grid bias in pure volume sources yields: enhancement of the extracted negative ion current, reduction of the co-extracted electron current, flattening of the spatial distribution of plasma potential across the filter magnetic field, change in recycling from hydrogen atomic/molecular ions to atomic/molecular neutrals, and enhanced concentration of H− ions near the plasma grid. These functions are maintained in the sources seeded with Cs with additional direct emission of negative ions under positive ion and neutral hydrogen bombardment onto the plasma electrode.