  The expansion of ultracold neutral plasmas (UCP) is driven primarily by the
thermal pressure of the electron component and is therefore sensitive to the
electron temperature. At lower densities (less than 10$^8$ /cm$^3$),
evaporative cooling has a significant influence on the UCP expansion rate. We
studied the effect of electron evaporation in this density range. Owing to the
low density, the effects of three-body recombination were negligible. We
modeled the expansion by taking into account the change in electron temperature
owing to evaporation as well as adiabatic expansion and found good agreement
with our data. We also developed a simple model for initial evaporation over a
range of ultracold plasma densities, sizes, and electron temperatures to
determine over what parameter range electron evaporation is expected to have a
significant effect. We also report on a signal calibration technique, which
relates the signal at our detector to the total number of ions and electrons in
the ultracold plasma.
