I think you should concentrate in the range 3 na to 3 microamps. That should encompass the largest plasma densities and the smallest, and still enable you to get some electron temperatures at the low end. I also suggest that you take at least 128 points across a -20 to +20 V bias range (range must be that great to accommodate charging, if it occurs). That gives you 1/3 volt resolution. If it is possible, I also suggest you use a cylindrical probe of dimensions at least 1.5 cm radius and 4 cm length of active region. It should be on a boom at least 20 cm long, if possible.