  Electrons in an expanding ultracold plasma are expected to be in
quasi-equilibrium, since the collision times are short compared to the plasma
lifetime, yet we observe electrons evaporating out as the ion density decreases
during expansion. We observe that a small electric field that shifts the
electron cloud with respect to the ions increases the evaporation rate. We have
calculated the spatial distribution of a zero-temperature electron cloud as a
function of applied field and ion density, which is assumed to be Gaussian at
all times. This calculation allows us to predict the flux of cold electrons
from the plasma at all times, and is in good agreement with our observed
electron signal.
