Laser-driven ion acceleration from ultrathin solid targets

Résumé

The interaction of high-power (terawatt to petawatt) laser pulses with matter creates a plasma in extreme conditions, capable of sustaining huge electric fields (TV/m). These electric fields may be employed to accelerate charged particles (electrons, protons, ions) to high energies ($>$ GeV for electrons, tens to hundreds of MeV/u for protons and ions). In general these ultra-intense (10^19 Wcm-2) laser pulses interact with a target, which could be gaseous, liquid (electron acceleration) or solid (proton and ion acceleration). The presented thesis investigates experimentally and numerically the acceleration of protons and ions from ultrathin solid targets. Laser-driven ion acceleration has become a vivid field of research in the past two decades. In the well-understood Target Normal Sheath Acceleration (TNSA) scheme, ions are accelerated through a large charge-separation field at the rear side of the overdense plasma. These TNSA-generated ion beams compare favourably with conventional acceleration techniques in terms of emittance, brightness and pulse duration. Yet there are many obstacles such as higher cut-off energies and flux rates that have to be overcome to make those laser-driven particle beams useful for applications such as fast ignition or hadron therapy. Beside the TNSA scheme, other promising ion acceleration regimes have emerged. Radiation Pressure Acceleration (RPA), based on the momentum transfer from photons to electrons, offers a much better scaling and produces tens of MeV ions. Other schemes at the onset of relativistically induced transparency like the Breakout Afterburner (BOA), where the interaction changes from surface-dominated to volumetric, have also emerged. While typical TNSA acceleration occurs in micrometre-thick solid targets, both RPA and BOA dominate for ultrathin overdense targets in the range of nanometres. In the presented thesis, laser-driven ion acceleration from ultrathin targets is explored. Experimental and numerical results from the interaction of single and double laser pulse set-ups with ultrathin targets are presented. The acceleration of ions in the RPA regime with single pulse interaction is investigated, as well as the charged particle dynamics in the relativistic transparency regime. Ion acceleration in a pre-expanded plasma with two linearly polarised pulses and a novel scheme of collisionless shock acceleration with double laser pulses of mixed polarisation is demonstrated. In the latter case a first linearly polarised pulse expands the nanometre overdense plasma, and the second circularly polarised pulse drives a strong collisionless shock into the plasma. This results in an efficient heavy ion acceleration up to 100 MeV/u with relatively moderate laser intensities (10^20 -10^21 Wcm-2). This scheme has been explored in the picosecond and femtosecond regime and is shown to be highly sensitive to the target and laser parameters. Finally, a parameter study for the VEGA Petawatt laser system of the Pulsed Laser Center (CLPU) in Salamanca is presented. The VEGA laser of the CLPU will produce 30 J/30 fs laser pulses with an ultrahigh contrast and a peak power of 1 Petawatt. The obtained results on single and double pulse plasma interaction are used to explore the capability of the VEGA laser systems to efficiently produce multi-MeV ion beams.