Coulomb explosion measurements of ultra-low density ultracold plasmas

We propose to use time-of-flight spectroscopy of an ultracold neutral plasma to determine the density of very small plasmas. The plasma is created by photo-ionizing laser-cooled atoms in a magneto-optical trap. When the electron temperature is high enough, the plasma expands because of a strong space-charge effect. We will measure this expansion using a time-of-flight spectrometer. We present a model that predicts the expected flight times. This method appears to be useful for plasmas with densities below 10$^7$ cm$^{-3}$.