Observation of a density spike in the ion distribution of an ultracold plasma

Expanding ultracold neutral plasmas are predicted to have a broad spike in the ion density distribution at the leading edge of the plasma. This feature forms as a result of a slight positive charge imbalance created by the prompt loss of electrons at plasma creation. We monitor the flux of ions passing through a grid in our vacuum chamber during the plasma expansion and the observed signal is fit well by a model of a spherical Gaussian cloud ballistically expanding through a circular planar grid. The shape of the initial flux indicates that the leading edge of the plasma has a sharp density cutoff rather than a smooth Gaussian tail. This feature is accentuated by applying a magnetic field perpendicular to the detection axis, slowing down the expansion of the neutral plasma relative to the ion spike. Supported by NSF PHY- 1004242.