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Title: Experimental aspects of plasma wakefields driven in the linear regime
Author: Thornton, Christopher Richard
ISNI:       0000 0004 7234 1648
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2016
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This thesis describes two main pieces of experimental work and a piece of development work that aims to advance laser wakefield accelerators that use lower intensity, high repetition rate lasers. The first piece of experimental work looked at developing further the capillary discharge waveguide by changing the key parameters that form the plasma channel and measuring the electron density profile by transverse interferometry. The first change was from a capacitor discharge circuit to the Compact Marx Bank system. This achieved a reduction in electrical noise but also delivered a driving current pulse which was better matched to the plasma channel and so had less current bounce. The circuit delivered a guiding window of around 200 ns and created an electron density channel profile which supported a laser spot size of 35 Âμ m over 15 mm. A further change was the use of helium gas rather than hydrogen gas; it was found that helium also formed an electron density profile that could support a focal spot size of 35 μm over 15 mm. Finally, the material of the capillary was changed from sapphire to diamond and whilst it was not possible to measure the plasma channel formed in this material, it was found that even after 10 000 plasma discharges there was no noticeable effect on the material, whereas sapphire under the same conditions had 0.7 μm of material ablated. The developmental work focussed on the design of the targets to be used in laser wakefield experiments. The capillary discharge waveguide and housing were re-designed for the specific geometry of an experiment on the ASTRA-Gemini laser at the Rutherford Appleton Laboratory (RAL) and to accommodate a gas cell in the housing of the capillary discharge waveguide. It also raised questions concerning the gas density profile present in the capillary discharge waveguide and so this was modelled using OpenFOAM. It was found that the modifications had no effect on the initial gas density profile in the simulations. The combined knowledge of the OpenFOAM software and the capillary discharge waveguide for Gemini were used to create a gas cell for the multi-pulse ASTRA experiment. This gas cell maintained a flat gas density profile ideal for creating the interaction region required in this experiment. The robust design and manufacture of this target proved itself during the experiment in which it was at times taken apart daily but could always be reassembled easily. The second experiment undertaken was the driving of a multi-pulse wakefield using the ASTRA laser at RAL. Here, a low amplitude wakefield was driven by a 55 fs duration, 2.6 x 10 21 Wm -2 peak intensity single pulse; a train of seven pulses, 50 fs each in duration and peak intensity of 0.3 x 10 21 Wm -2 and by two fully compressed pulses each 65 fs in duration and with a peak intensity of 1.3 × 10 21 Wm -2. These wakefields were measured using Frequency Domain Holography (FDH) and Temporarily Encoded Spectral Shifting (TESS) to deduce wakefield amplitudes of 1% variation of the plasma density. As well as driving a wakefield, the double pulse experiment suggests the possibility of energy recovery from the wakefield structure.
Supervisor: Hooker, Simon Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available