Relativistic electron-cyclotron resonances in laser Wakefield acceleration

In this thesis, the magnetized, relativistic plasma that overlaps the pump laser in Laser Wakefield Acceleration (LWFA) was investigated. The Jeti 40 laser was used to drive the plasma wave and a transverse, few-cycle probe pulse in the visible to near-infrared spectrum was implemented to image the laser-plasma interaction. The recorded shadowgrams were sorted depending on the properties of the accelerated electron bunches, and subsequently stitched together based on the timing delay between the pump and probe beams. The resulting data showed two signatures unique to the relativistic, magnetized plasma near the pump pulse. Firstly, a significant change in the brightness modulation of the shadowgrams, coinciding with the location of the pump pulse, shows a strong dependence on the pump’s propagation length and the probe’s spectrum and polarization. Secondly, after ~1.5 mm of propagation in the plasma, polarization-dependent diffraction rings appear in front of the plasma wave. A mathematical model using relativistic corrections to the Appleton-Hartree equation was developed to explain these signals. By combining the model with data from 2D Particle-in-Cell (PIC) simulations using the VSim code, the plasma’s birefringent refractive index distribution was investigated. Simulated shadowgrams of a 3D PIC simulation using the EPOCH code were also analyzed with respect to the aforementioned signals. The results of the study present a compelling description of the pump-plasma interaction. The previously unknown signals arise from relativistic, electron-cyclotron motion originating in the 10s of kilotesla strong magnetic fields of the pump pulse. Advantageously, a VIS-NIR probe is resonant with the cyclotron frequencies at the peak of the pump. With further refinement, the measurement of this phenomenon could allow for the non-invasive experimental visualization of the pump laser’s spatio-temporal energy distribution and evolution during propagation through the plasma.