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Jung, Daniel (2012): Ion acceleration from relativistic laser nano-target interaction. Dissertation, LMU München: Faculty of Physics
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Abstract

Laser-ion acceleration has been of particular interest over the last decade for fundamental as well as applied sciences. Remarkable progress has been made in realizing laser-driven accelerators that are cheap and very compact compared with conventional rf-accelerators. Proton and ion beams have been produced with particle energies of up to 50MeV and several MeV/u, respectively, with outstanding properties in terms of transverse emittance and current. These beams typically exhibit an exponentially decaying energy distribution, but almost all advanced applications, such as oncology, proton imaging or fast ignition, require quasimonoenergetic beams with a low energy spread. The majority of the experiments investigated ion acceleration in the target normal sheath acceleration (TNSA) regime with comparably thick targets in the μm range. In this thesis ion acceleration is investigated from nm-scaled targets, which are partially produced at the University of Munich with thickness as low as 3 nm. Experiments have been carried out at LANL’s Trident high-power and high-contrast laser (80,J, 500 fs, t=1054nm), where ion acceleration with these nano-targets occurs during the relativistic transparency of the target, in the so-called Breakout afterburner (BOA) regime. With a novel high resolution and high dispersion Thomson parabola and ion wide angle spectrometer, thickness dependencies of the ions angular distribution, particle number, average and maximum energy have been measured. Carbon C6+ energies reached 650MeV and 1GeV for unheated and heated targets, respectively, and proton energies peaked at 75MeV and 120MeV for diamond and CH2 targets. Experimental data is presented, where the conversion efficiency into carbon C6+ (protons) is investigated and found to have an up to 10fold (5fold) increase over the TNSA regime. With circularly polarized laser light, quasi-monoenergetic carbon ions have been generated from the same nm-scaled foil targets at Trident with an energy spread of as low as ±15% at a central energy of 35MeV. High resolution kinetic simulations show that the acceleration is based on the generation of ion solitons due to the circularly polarized laser. The conversion efficiency into monoenergetic ions is increased by an order of magnitude compared with previous results in the TNSA regime. The advances in ion energies and the control over the spectra mark an important basis for future research of laser-driven ion acceleration and might enable laser-based implementation of these applications in the future.