Multi-Object-Plane Phase Retrieval for Plasma Density Profile Reconstruction

Plasma Wakefield Accelerators (PWFA) can accelerate electron beams with gradients that are hundreds of times greater than conventional RF accelerators. The ability to preserve the beam emittance depends strongly on the longitudinal density profile of the plasma source. One method of generating the plasma source is to ionize a gas (argon or helium) with a high intensity laser pulse. This plasma source recombines on the order of nanoseconds, and is a filament that is tens of centimeters long, less than a millimeter wide, which has a density in the range 10^15 – 10^17 cm^-3. In order to measure the shot-to-shot variation in the plasma source, an ultrafast diagnostic is needed. Here, I will present simulations and measurements of a laser phase retrieval diagnostic that utilizes the density-dependent index properties of the plasma to reconstruct the plasma density profile on a single shot . An ultrashort, low energy laser pulse is sent through the plasma source, after the pulse is split and focused onto multiple CCD, each one imaging a different plane inside plasma. A phase retrieval algorithm is applied to intensity patterns, and an index profile is generated. Then, a density profile can be calculated. Sensitivity and resolution of the diagnostic will be assessed.