We have investigated by probes and by photodetachment the plasma properties of two negative ion sources of different volume. We compared the data relevant to these two sources and identify the effects induced by the change in volume and the isotope effects. The observed scaling law for the electron temperature is in each caseTe∼I0.27d. Positive ions are predominantly lost to the walls, but due to the increasing influence of their loss by dissociative recombination when the source volume is increased, the scaling laws arene∼I0.7d, for the small source, andne∼I0.59d, for the large source. Atoms limit the production of negative ions and destroy them. Thus we see an isotope effect, with higher negative ion density in hydrogen than in deuterium, and the effect of the source volume, with the negative ion density larger in the small source than in the large one. ©1996 American Institute of Physics.