Laboratoire d'Optique Appliquée


Nov 18 2022


10 h 00 min

Thesis Lucas Rovige

This PhD thesis presents experimental work on the development of a high-repetition rate (kHz) laser-wakefield accelerator using few milijoules, near-single cycle laser pulses.  We explore a large set of experimental parameters to optimize the accelerator by controlling the plasma density and profile, pulse duration, type of gas and injection mechanism used in experiments. We demonstrate significant performances improvement, notably with progress made on the long-term stability and reliability of the accelerator with continuous and stable operation of the accelerator for several hours accumulating a record of 18 million consecutive shots. We achieve this gain in stability by using a newly designed type of gas target resulting in an asymmetric hydrodynamic oblique shock enabling injection in the downward density transition of the shocked region.  Using particle-in-cell simulations, we understand in details the underlying causes leading to an optimized and stable acceleration regime. The typical electron beam energy has also been increased by a factor of two, up to 8 MeV, while a single-shot beam divergence as low as 3mrad is achieved using helium instead of nitrogen to form the plasma. We then present the results of a first application experiment in radiobiology where our accelerator is used to irradiate cancerous cells, taking advantage of the newly acquired stability.

Secondly, we study the specificities of the interaction of near-single cycle pulses with an underdense plasma that occurs in our accelerator, mainly through the effect of the carrier-envelope phase (CEP). We observe and control experimentally for the first time CEP effects in a laser-wakefield accelerator, that manifest through a dependence of the electron beam pointing to the laser initial optical phase. We also show significant (up to 30%) charge variations in some cases when changing the value of the CEP. By carrying out particle-in-cell simulations, we explain these effects by the periodic off-axis injection of several electron sub-bunches triggered by the oscillation of the asymmetry of the plasma wave in the laser polarization direction due to the CEP shifting during propagation. Finally, we discuss preliminary results on carrier-envelope phase effects on the electron energy spectrum associated with ionization injection in a helium-argon gas mixture.