The electron and negative ion densities and temperatures are reported for a large volume hybrid multicusp negative ion source. Based on the scaling laws an analysis is made of the plasma formation and loss processes. It is shown that the positive ions are predominantly lost to the walls, although the observed scaling law isn+∝I0.57d. However, the total plasma loss scales linearly with the discharge current, in agreement with the theoretical model. The negative ion formation and loss is also discussed. It is shown that at low pressure (1 mTorr) the negative ion wall loss becomes a significant part of the total loss. The dependence ofn−/neversus the electron temperature is reported. When the negative ion wall loss is negligible, all the data onn−/neversus the electron temperatures fit a single curve. ©1995 American Institute of Physics.