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Title: High-intensity laser interactions with optically levitated liquid microdroplets
Author: Price, Christopher
ISNI:       0000 0004 7232 9737
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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The interaction of sub-picosecond pulses of high intensity laser light with micron, and sub-micron scale objects is currently highly topical. These targets are in a size regime that is intermediate between atomic cluster targets ( ~100 Angstrom) and macro- scopic solids which, unlike microtargets, have been the subject of many studies over recent years. Mass-limited objects can couple very strongly to an intense laser field due to transient plasma resonances or Mie scattering processes and absorb substan- tial amounts of laser energy. These targets off er a unique geometry and size that can potentially produce significantly higher x-ray photon energies than compared to interactions with bulk materials of the same atomic number, and under the same irradiation conditions. This thesis describes, for the first time, the design, construction and character- isation of a new class of in-vacuo optical levitation trap optimised for use in high- intensity, high-energy laser interaction experiments. The optical trapping of 10 micron oil droplets in vacuum was demonstrated, over time-scales of >1 hour at extended distances of 40 mm from the final focusing optic. A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with 5 micron spatial resolution. The stability of the levitated droplet was such that it would stay in alignment with a 7 micron irradiating beam focal spot for up to 5 minutes without the need for re-adjustment. The performance of the trap was assessed in a series of trial high-intensity laser experiments using a hybrid optical parametric neodymiun:glass Chirped Pulse Am- plifi cation (CPA) laser system, with focused peak intensities in excess of 10^17 Wcm-2. X-ray knife-edge measurements, using image plate, and single-hit CCD photon energy spectroscopy demonstrated the creation of a spatially symmetric, micron scale x-ray source (< 22 microns), with a measured two electron temperature distribution of between 0.4 and 2.3 keV. These relatively low values were thought to arise from low laser- plasma coupling efficiencies with the droplet, due to the high contrast of the laser system. Initial tests also demonstrated very low integrated RF signals produced from laser-droplet interactions, x9 smaller than wire targets of comparable atomic com- position, highlighting the potential of this technique when extended to kJ, petawatt class lasers for use in probing of material science and high energy density plasma experiments.
Supervisor: Smith, Roland Sponsor: Engineering and Physical Sciences Research Council ; DTA
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral