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This thesis describes a buffer gas positron accumulator which has been used by the ATHENA collaboration to produce and detect the first cold antihydrogen atoms. In particular, the work presented is centred on the implementation of a so-called rotating wall electric field to compress a positron plasma in preparation for recombination with antiprotons, in addition to maximising the accumulation efficiency by the automatic optimisation of the trap applied potentials. Central results include successful compression of the positron plasma with an increase in central density by a factor of six. The use of a rotating wall during accumulation of a positron plasma has been investigated for the first time with interesting distinctions observed when compared to a typical accumulation without the use of a rotating wall. Results show that its application results in a greater number of positrons being accumulated at a given time, and that there appears to be an optimum, pressure dependent, time at which to apply the rotating wall. Results also indicate that the accumulation time at which the plasma is maximally compressed is also pressure dependent. A useful program has also been written, and successfully implemented that automatically optimises the electrode array applied potentials to maximise the trapping efficiency after the growth of new solid neon moderators.

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This thesis presents measurements of high energy ion beams accelerated from high intensity laser interactions, with underdense through to near critical density plasmas, and also presents an application of laser generated ion beams. The first experimental measurements of longitudinally accelerated ion beams from high intensity (-1020 Wcm-2 ) laser interactions with an underdense (0.04 ne) helium plasma are presented. The ion beam was found to have a maximum energy for He2+ of 40+3 _8 MeV, with the highest energy ions being collimated to a cone of less than 10ø. Two dimensional particle-in-cell simulations show that additional effects, due to the time varying magnetic field associated with the fast electron current, enhance the accelerating electric field and provides a focusing mechanism on the ions. Very low density foam targets were used to investigate proton acceleration from near to critical density plasmas. Experimental results show a decrease in acceleration efficiency just above the critical density. Simulations of the interactions show the proton acceleration is very sensitive to the ability of the laser to propagate through the plasma. The lowest density foams allow the best laser propagation, thus enabling proton beams to be accelerated to energy and numbers comparable to those from a solid target. The suitability of a laser generated proton beam for the measurement of self-generated magnetic fields in laser generated plasma has been investigated. The technique was then used to study a novel magnetic reconnection geometry lIsing two laser beams. Proton probing provides evidence for the formation of the reconnection layer and the corresponding instabilities.

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No description available.