Langmuir Probe Techniques for a Sub-Millimeter Plasma Filament Used in Plasma Wakefield Acceleration.*

Plasma wakefield acceleration (PWFA) is an advanced linear particle acceleration technique that may be instrumental in the design of a future lepton collider.  Its high acceleration gradients are observed in the nonlinear wave structure that exists in the wake of an electron beam moving through a plasma.  A major challenge with a PWFA device is the preservation of beam emittance. The main solution to this problem consists of establishing a density ramp up regime where the electron beam enters the plasma.  To achieve this, there must be a reliable method to profile the longitudinal plasma density. This is a challenge for many traditional plasma diagnostics due to the rapid decay rate (tens of nanoseconds) of the plasma, the relatively low initial density of 10^15-17 cm^-3, and the narrow,  sub-millimeter width of the plasma filament. In this poster we present measurements obtained from the use of double and triple Langmuir probes to measure the density and temperature profile of a laser-ionized Ar or He PWFA plasma source. We compare the results to a model that includes the initial plasma density and temperature profile expected from the laser ionization process as well as the decay process due to diffusion and recombination.