Exploring the Use of Graphene as a Target Material for Laser Plasma Ion Acceleration

Sammanfattning: The interaction of a solid target with an ultra-high intensity laser pulse can result in the laser plasma acceleration of ions. The recent proposal of a new laser plasma ion acceleration scheme, named chirped standing wave acceleration, has created interest in a new class of ultra-thin solid target materials, either freestanding or as a part of novel compound targets. In this thesis, it is assessed, if it is feasible to use targets made of freestanding graphene, a carbon allotrope consisting of only one or a few atomic layers. For this purpose, a target system was developed to mount freestanding graphene in the ion acceleration experiment at the Lund Laser Centre, and commercially available graphene targets were put to a number of tests. Using the Lund Terawatt Laser, the threshold for laser induced damage of the targets was determined. Further, freestanding graphene targets were exposed to ultra-high intensity laser pulses, in order to evaluate the effect on these ultra-thin samples and the supporting structure. The conditions were similar to those in conventional laser plasma acceleration experiments. For analysis and alignment purposes, the targets were imaged using an existing on-line microscope system, and the steps required to extend the imaging system with a Raman spectroscopy setup were explored. The specially designed and constructed target mounting system was found to work reliably, and the graphene targets used in this project were found to be robust enough to be handled in the experimental environment. While the Raman spectroscopy was not fully implemented, the microscope system was extensively used and found capable to reveal occasional imperfections of the freestanding graphene samples. The damage threshold fluence was found to be approximately 0.1 J/cm^2 for the graphene targets. In the ultra-high intensity shots, small damage was inflicted to the frame supporting the graphene, and particle acceleration was observed. Accelerated ions were recorded with nuclear track detectors. They show traces of protons with energies above 1 MeV, and some signals also of heavier ions. The acceleration is attributed to a target normal sheath acceleration-like process, possibly involving the graphene-supporting copper grid, but the limited data does not allow a definite interpretation. The results of this thesis show that freestanding graphene is robust enough to be used in future studies of laser plasma interaction. Adjustments need to be made to the existing target geometry to prevent an ionisation of the graphene-supporting structure. Based on the measured damage threshold, it is concluded, that, for future studies, the temporal laser pulse contrast needs to be improved, by reducing the amplified spontaneous emission of the laser.