Radiative Signatures of Electron Acceleration in a Fully Cavitated Laser Plasma Wakefield
In this work, the fully cavitated or bubble regime of Laser Wakefield Acceleration was experimentally studied. It provides acceleration fields in excess of 100 GeV/m while also generating uniform electric field gradients. This allows for the production of high-energy electron beams with moderate energy spread and femtosecond bunch lengths. Although quasi-monoenergetic beams have been observed in experiments, it remains unproven that these beams were generated in this fully cavitated acceleration structure. In addition, these high-energy electron beams should have temporal structures on the order of tens of femtoseconds. However, current detection techniques are still limited to the measurement of features no less than ~100 fs.A unique second harmonic radiation signature was first observed in this investigation, confirming the existence of the bubble regime. This second harmonic was generated by the interaction of the bubble's high-density electron sheath with a strong laser field. The bubble size, propagation distance, and the existence of multiple bubbles was obtained through careful characterization of this second harmonic signature. The observed strong correlation between the features of the second harmonic signature and the accelerated electrons provides a guidance to optimize the performance of Laser Wakefield Accelerators.A theoretical investigation of the space charge fields associated with a relativistic ultrashort electron beam revealed new insight into the physics of Electro-Optic detection of ultrashort electron bunches. Instead of the commonly accredited Pockels effect, it was realized that for femtosecond electron bunches a formalism fully incorporating sum and difference frequency generation was necessary. From this realization a new technique to measure such bunches, bandwidth mixing cross-correlation frequency resolved optical gating (BMX-FROG), was developed. This technique was modeled numerically for various bunch lengths and crystal thicknesses. These solutions were benchmarked against a well-established simulation code and showed excellent agreement. Finally, an experimental feasibility study of the components necessary for a BMX-FROG was made. The results of which confirmed that it is a viable device for measuring a reliable source of ultrashort electron beams.
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