Laser-driven ion acceleration from carbon nano-targets with Ti:Sa laser systems

Over the past few decades, the generation of high energetic ion beams by relativistic intense laser pulses has attracted great attentions. Starting from the pioneering endeavors around 2000, several groups have demonstrated muliti-MeV (up to 58 MeV for proton by then) ion beams along with low transverse emittance and ps-scale pulse duration emitted from solid targets. Owing to those superior characteristics, laser driven ion beam is ideally suitable for many applications. However, the laser driven ion beam typically exhibits a large angular spread as well as a broad energy spectrum which for many applications is disadvantageous. The utilization of nano-targets as ion source provides a number of advantages over micrometer thick foils. The presented PhD work was intended to investigate laser driven ion acceleration from carbon nano-targets and demonstrate the potential feasibility for biological studies. Two novel nano-targets are employed: nm thin diamond-like-carbon (DLC) foil and carbon nanotubes foam (CNF). Both are self-produced in the technological laboratory at Ludwig-Maximilians-Universitat Munchen. Well-collimated proton beams with extremely small divergence (half angle) of 2 degrees are observed from DLC foils, one order of magnitude lower as compared to micrometer thick targets. Two-dimensional particle-in-cellsimulations indicate a strong influence from the electron density distribution on the divergence of protons. This interpretation is supported by an analytical model. In the same studies, the highest maximum proton energy was observed with a moderate laser intensity as low as 5*10^18W/cm^2. Parallel measurements of laser transmission and reflection are used to determine laser absorption in the nano-plasma, showing a strong correlation to the maximum proton energy. This observation indicates significance of absorbed laser energy rather than incident laser intensity and is supported by an analytical model. The ion energy also depends on pulse duration, a reduced optimum pulse duration is found as compared to micrometer thick targets. This behavior is attributed to a reduction of transverse electron spread due to the reduction of thickness from micrometer to nanometer. These remarkable proton bunch characteristics enabled irradiating living cells with a single shot dose of up to 7 Gray in one nanosecond, utilizing the Advanced Titanium: sapphire LASer (ATLAS)system at Max-Planck-Institut of Quantum Optics (MPQ). The experiments represent the first feasibility demonstration of a very compact laser driven nanosecond proton source for radiobiological studies by using a table-top laser system and advanced nano-targets. For the purpose of providing better ion sources for practical application, particularly in terms of energy increase, subsequent experiments were performed with the Astra Gemini laser system in the UK. The experiments demonstrate for the first time that ion acceleration can be enhanced by exploiting relativistic nonlinearities enabled by micrometer-thick CNF targets. When the CNF is attached to a nm-thick DLC foil, a significant increase of maximum carbon energy (up to threefold) is observed with circularly polarized laser pulses. A preferable enhancement of the carbon energy is observed with non-exponential spectral shape, indicating a strong contribution of the radiation pressure to the overall acceleration. In contrast, the linear polarization give rise to a more prominent proton acceleration. Proton energies could be increased by a factor of 2.4, inline with a stronger accelerating potential due to higher electron temperatures. Three-dimensional (3D) particle-in-cell (PIC) simulations reveal that the improved performance of the double-layer targets (CNF+DLC) can be attributed to relativistic self-focusing in near-critical density plasma. Interestingly, the nature of relativistic non-linearities, that plays a major role in laserwakefield-acceleration of electrons, can also apply to the benefit of laser driven ion acceleration.