Several applications of a few-cycle, optical probe combined with an apochromatic imaging system in Laser Wakefield Acceleration experiments on the JETI 40 TW laser system in Jena, Germany will be described. This research is motivated by the need for direct imaging of the wakefield to gain a deeper understanding into the physics underlying the formation, injection, and acceleration processes as well as the need to underpin numerical simulations with detailed experimental data.
Several experimental setups using the few-cycle probe system have been implemented in LWFA campaigns allowing for various measurements to be performed. Electron plasma density distributions were measured interferometrically using a Mach-Zehnder type interferometer, and an achromatic quadri-wave lateral shearing interferometer. These density measurements could then be compared directly to the plasma-wave’s wavelength at different stages of the wakefield’s evolution and within different parts of the plasma-wave. Shadowgraphic imaging techniques were also used to investigate differences in the wakefield between self-injection and ionization-injection regimes. Relatively smooth evolution of the wakefield with a lack of wave-breaking radiation in the ionization-injection case compared to a more turbulent evolution seen in the self-injection case may lend evidence to explaining the differences in the electron-beam’s pointing and profile seen between the two regimes.
The large spectral bandwidth of a few-cycle optical probe allows for further investigation of the pump laser’s and the wakefield’s evolution using spectral filtering of the recorded shadowgraphic images. Strong magnetic fields in the plasma in the kilotesla range caused by the relativistically intense pump laser induce a birefringence in the plasma that can be potentially used to track the evolution of the pump as it propagates. Preliminary results show a strong spectral absorption within the VIS-NIR spectrum that alters the intensity distributions seen in shadowgraphic images of the wakefield. Discerning how different mechanisms in the pump-plasma-probe interaction contribute to the shadowgraphic images continues to be a fruitful collaboration between experimental data and 3D PIC simulations.
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