Thesis

Comprehensive characterisation of laser plasma wakefield accelerated electrons

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Awarding institution
  • University of Strathclyde
Date of award
  • 2012
Thesis identifier
  • T13232
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Abstract
  • This thesis describes the first experimental diagnosis of all of the key parameters of mono-energetic, laser wakefield accelerated electrons from a single driving laser, such as charge, transverse emittance, energy spread and bunch length. All of the experiments utilise the 35 fs, 45 TW, TOPS laser on the ALPHA-X beam line. Electron beam energy spectra have been measured using a high resolution magnetic spectrometer. These electrons have an average peak energy of 83 1.3 MeV. The narrowest measured energy spread is sg=g = 0:8 % which is deconvoluted to 0.5 %. This deconvoluted energy spread sets an upper limit to the bunch length of 0.3 mm due to the curve of the electrostatic potential in the accelerating bubble. This short bunch length is confirmed with the use of coherent transition radiation, used to measured a bunch length of 1.6 0.8 fs after 1 m of propagation which is shown to be < 0.3 mm at the source. In addition, a sub-structure of two separate bunches was observed in both the energy and longitudinal distributions. These two measurements of sub-structure are consistent with each other. The energy separation of the two peaks provides a spatial separation due to the curve of the electrostatic potential which, when allowed to propagate 1 m from the source, agrees well with the measure longitudinal bunch structure. This demonstrates that multiple electron bunches can occur within one bubble, resulting from either a complex injection process or interactions within the bubble. The thesis also presents the first single shot transverse emittance measurement of mono-energetic LWFA electrons. The emittance was measured as low as en;x = 1:1 p mm mrad with an average value of en;x = 2:2 0:7 p mm mrad, normalised to 125 MeV. This is an upper limit to the emittance and gives a bunch radius at the source of 3 mm. To produce an electron beam with an emittance of 2:2 p mm mrad, the electrons were injected within a radius of 3:4 0:4mm at the rear of the accelerator. The energy spread and beam divergence were both found to increase with increasing charge, Q. The increase in the divergence is proportional to Q1 2 (although Q1 3 is within errors) caused by an increase in the transverse forces per unit length i.e. an increase in peak current. The increase in energy spread is therefore due to an increase in beam loading caused by the increase in peak current and not due to an increase in bunch length.
Resource Type
DOI
Date Created
  • 2012
Former identifier
  • 947971

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