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Laser driven ion acceleration : source optimisation and optical control

Carroll, David (2008) Laser driven ion acceleration : source optimisation and optical control. PhD thesis, University Of Strathclyde.

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Abstract

This thesis reports on experimental investigations into ion acceleration driven by high power laser pulses. Recent developments in high power, ultrashort pulse laser systems enable laser intensities beyond 1021 Wcm-2 to be achieved. When focused onto thin foil targets, plasmas with extremely high field gradients (>TV/m) are produced, resulting in the acceleration of ions to multi-MeV/nucleon energies over very short distances (microns). Results from an investigation of multiply-charged ion acceleration from heated foils, irradiated by high intensity ultrashort laser pulses, are reported. Ions with up to multi-MeV/nucleon energies are detected and the scaling of the maximum ion energy with laser parameters and ion charge distribution are measured. With the aid of PIC simulations, it is concluded that the initial charge state population distribution has little effect on the maximum energy of the highest charge state ions and that the maximum energy of lower charge state ions is strongly affected by screening of the acceleration field by higher charged ions. Results from an investigation in which spatially resolved ion emission from foil targets irradiated with high intensity ultrashort laser pulses is used to spatially resolve the acceleration field resulting from lateral transport of electrons within the targets are presented. It is found that lateral electron transport occurs over periods much longer than the pulse duration of the laser. Finally, results from experiments aimed at dynamic control and enhancement of ion acceleration using multiple laser pulses are presented. The effects of optically controlled pre-plasma expansion on proton acceleration from foil targets are investigated. Enhancement of the maximum proton energy, proton flux and beam uniformity is observed for optimum pre-plasma density scale lengths. In a separate experiment, optically controlled deformation of a target, usinga separate laser pulse initiated low temperature shock wave, is shown to change the direction of laser-driven proton beams.