High contrast laser driven monoenergetic proton beams and near critical density plasma diagnosis In this dissertation the experiments are conducted at the Jenaer Titanium: Sapphire 200 Terawatt Laser System (JETi200) located in Jena Germany. With its excellent temporal contrast the few nanometer freestanding target can remain in a solid state for a few picoseconds before the main pulse arrives greatly reducing the pre expansion of the target. The resulting proton beams exhibit distinctive features in terms of cut off energy and energy spectrum distribution. The proton beams in the presented experiment show a more than 30 MeV monoenergetic peak under the circularly polarized laser and the highest peak particle kinetic energy per Joule of laser energy is around 20MeV J. As opposed to the circularly polarized driving light the cut off energy shows weak dependence on the target thickness when irradiated with linearly polarized light. Moreover the implementation of a transmission light diagnostic in the experiment indicates that the transmission light of the main pulse is significantly weaker than that in other similar experiments. The energy and energy spectrum of the protons provide the potential to conduct in vivo research and proton skin therapy using the Terawatt level laser system. Laser contrast significantly impacts laser driven ion acceleration as low contrast can lead to premature expansion of thin targets. The evolution of premature expansion caused by pre pulses is primarily based on numerical calculations research. However in this paper I conduct a comprehensive experimental study of pre pulse induced pre plasma evolution including the measurement of pre plasma evolution time and comparison with a previous numerical model. This investigation is especially beneficial for the latest generation of laser ion accelerators as it enables the precise quantification of temporal contrast requirements in the Petawatt laser driver era.