  We present a study of the expansion of an ultracold neutral plasma (UCNP)
with an initial density distribution that decays exponentially in space,
created by photoionizing atoms shortly after their release from a quadrupole
(or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is
evident in the evolution of the plasma size and velocity, indicating the
dynamics are reasonably well described by a model of hydrodynamic expansion of
a quasi-neutral plasma. However, for low plasma density and high initial
electron temperature, excess ion kinetic energy in the vicinity of the central
density peak suggests significant local non-neutrality at early times.
Observations are compared to the well-understood self-similar expansion of an
UCNP with an initial Gaussian density distribution, and a similar scaling law
describes the evolution of plasma size for both cases.
