Laser ion-acceleration from tailored micro-plasmas

Gebhard, Johannes Thomas (2022): Laser ion-acceleration from tailored micro-plasmas. Dissertation, LMU München: Fakultät für Physik

Abstract

Levitating, isolated microspheres have gained interest as target in high intensity laser plasma experiments, in particular for laserdriven ion acceleration. Within this thesis, our unique Paul-trap based target positioning system was significantly improved and employed for a novel study with relativistic high power laser pulses. Among others, the Paul-trap system was equipped with full remote control capabilities, increasing its reliability and enabling operation at a rate of one shot every ten minutes. This enabled extended multi-shot studies using sub-focus sized targets for the first time. This capability was demonstrated during an experimental campaign at the JETi laser, where more than 200 shots on spherical targets were conducted. The scientific novelty of this study in laser-ion acceleration was that a weaker laser pulse ignited the microsphere at controlled times before the interaction with the main laser pulse. Therefore, the relativistic laser pulse effectively interacted with a microscopic plasma at reduced density. In these cases, accelerated proton yield increased by up to a factor of 19 as compared to non-expanded spherical targets and indicate a forward-directed beam. We find a distinct, high energetic proton component with maximum energies of up to 27 MeV, which is more than 2 times higher compared to planar foil targets irradiated with laser pulse in the same setup. The laser energy to proton maximum energy conversion efficiency is 18 MeV/J. This is also a factor of 2 larger compared to the optimum of 10 MeV/J that is commonly stated for foil targets. The experimental results are supported by 3D3V-PIC simulations that reproduce experimental proton and carbon spectra over a wide range of target densities and acceleration mechanisms well. The most energetic protons that we observed for expanded targets can be attributed to a plasma density range that is just above the relativistic plasma density. Here, we identify an efficient holeboring phase in the upramp of the density profile during the intensity rise of the laser pulse. Under ideal conditions, the pre-accelerated protons undergo post acceleration in the plasma fields where they nearly double their energy. Scaling the simulations to larger laser pulse energies predicts an increase of the proton energies with the square root of the laser energy, in particular 4 J seem sufficient for reaching more than 100 MeV protons, meeting the relevant window for biomedical applications.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Plasma, Paul Trap, Laser-Ion-Acceleration
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 7. September 2022
• 1. Bericht­erstatter:in: Schreiber, Jörg