| lectrons 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. By treating the electrons as a zero-temperature fluid, we have calculated their spatial distribution given a fixed Gaussian ion density and applied field. The zero-temperature approximation gives the maximum number of electrons that can be held in the plasma in the absence of evaporation. Performing this calculation at all times allows us to predict the flux of cold electrons from the plasma, which is in good agreement with our measured 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. We discuss the possibility of using this and other measurements to probe the thermal distribution of electrons. |
|