Radiographic applications and control of TNSA proton beams
The interaction of a high intensity laser pulse with a thin solid target can drive the acceleration of protons and other heavier ions to multi MeV energies. The resulting ion beam has unique properties such as low transverse emittance, short pulse duration, high particle flux, large beam divergence and a broad energy spectrum. For applications, some of these properties are highly desirable and better than those which can be achieved with conventional accelerator technology. Others however, are detrimental for applications and require improvement. This thesis presents work aimed at exploiting these properties for radiographic purposes and exerting control over the ion beam properties for other applications. A divergent and broadband laser driven proton beam was used in the proton radiography technique to investigate the late stages of the interaction of an ablated plasma created by a ∼ 1014 W cm−2 laser with a solid target, expanding into a low density background plasma. The high temporal and spatial resolution obtained with proton radiography, combined with the large temporal window achieved, allowed the detection of the late stages of the evolution of electrostatic collisionless shocks. The evolution of the measured electrostatic potential associated with the shock front, and the qualitative observations seen in the Radio Chromic Film reveals the splitting of the shock front into two separate forward and reverse shocks in the shock frame, in agreement with simulations. Additionally, experimental measurements and comparison to simulations show that the shock potential is high enough to reflect some of the incoming upstream ions, stabilising the electrostatic shock. These shock reflected ions are also observed to be the cause of the ion-ion instability which is seen to develop at later times. This is seen to propagate through the shock features, destroying them in the processes, due to the similarity of the associated electric fields of the shock front and the instabilities. The use of curved targets was investigated to reduce the beam divergence at higher laser intensities and thus higher proton energies than has been previously studied. Several combinations of target and laser parameters were investigated over a series of experiments, with iterative improvements made in between campaigns. The produced ion beams from these targets have been used to produce warm dense matter states, and the properties of the ion beams diagnosed. Substantial improvements in the maximum temperature attainable in heated samples have been made, along with general improvements in the quality of equation of state (EoS) data, due to optimisations of the target design. Highly curved targets, when irradiated with an intense but large laser spot, have been seen to produce high energy, collimated ion beams that focus far from the interaction, with a chromatic nature to this focal position.
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