The production mechanism of negative ions has been investigated experimenta
lly in a cesium seeded volume type negative ion source. The cesium light em
ission intensity, Cs ion volume density, plasma parameters, H- current, and
work function of the plasma grid were measured. The change in plasma param
eters was small even in a high Cs ion density region(up to 10(12) CM-3) whi
ch was also confirmed by zero-dimensional simulation. Upon heating the plas
ma grid, the work function decreased and the negative ion current increased
to become three times larger than that of the low plasma grid temperature
condition. This increase was in good agreement with the decrease of the wor
k function of the plasma grid surface. Therefore, it is considered that the
Cs contribution for the negative ion production is the surface effect in t
he negative ion source.