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

Chen, Xiang

Undergraduate Honors Thesis

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

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/6w924c404

Abstract

Plasma Wakefield Accelerators (PWFA) can accelerate electron beams with gradients that are hundreds of times greater than conventional RF accelerators and may provide a path toward a future energy frontier lepton collider. The ability to preserve the beam emittance depends strongly on the longitudinal density profile of the PWFA plasma source. One method of generating the plasma source is to ionize a gas such as argon or helium with a high intensity laser pulse. This plasma source quickly recombines on the order of nanoseconds, and so a new plasma source must be created for every shot. The plasma source is a filament that is tens of centimeters long, less than a millimeter wide, and has a density in the range 10^15 to 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 studies 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 basis. To accomplish this, an ultrashort, low energy laser pulse is sent through the plasma source, after which the pulse is split and focused onto multiple CCD cameras, each one imaging a different plane inside the plasma source. A phase retrieval algorithm is applied to the recorded intensity patterns, and an index profile is generated. From this, a density profile can be calculated. Fundamental aspects of the phase algorithm will be discussed, including how to optimize experimental parameters to achieve better reconstruction quality. The sensitivity and resolution of the algorithm will be assessed, and possible improvements will be described.

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