Electron Distribution of an Ultracold Neutral Plasma

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.~ In addition, short electric field pulses can dump a fraction of plasma electrons without affecting the ion expansion.~ Evaporation ceases for several microseconds before quickly refilling to match the shape of the unperturbed signal.~ The dynamics of this process should yield insight into the actual electron distribution, especially for high-energy electrons, where deviations from a Boltzmann distribution should be most pronounced. This work is partially supported by the NSF.