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Title: Development of a multi keV X-ray backlighter source based on laser irradiation of extended cluster gases
Author: Lowe, Hazel
ISNI:       0000 0004 7232 7109
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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High intensity laser-cluster interactions have been studied for over two decades and have demonstrated extremely e fficient coupling of the incident laser energy to the high local density extended cluster medium, launching a strong radiative shock of interest for laboratory astrophysics studies. Laser-cluster interactions have also shown potential as a source of K-alpha emission. In this work, a long scale length (10 mm), high density (10^19 atoms/cm^3) cluster source has been commissioned to carry out a range of new experiments investigating the use of the extended hot plasma lament generated by the laser-cluster gas interaction as an x-ray source for x-ray back-lighting motivated by the low debris, high repetition rate nature of laser-cluster gas interactions. The first high intensity, ultra-short pulse (45 fs) laser interaction experiment with an extended Kr or Xe cluster medium at densities of 1017 atoms/cm£3 was carried out on the 12 J/45 fs Astra Gemini laser at a peak laser intensity of 10^19 W/cm^2. Up to 97% of the incident laser energy was absorbed by the cluster medium, generating free electron temperatures of 2.5-6 keV measured via a Ross pair continuum x-ray spectroscopy diagnostic. The hot plasma lament was imaged using an x-ray pinhole camera, showing a hollow x-ray structure with diameter greater than the initial laser-cluster interaction region, evincing radial energy transport before the peak in continuum emission was reached. The Alameda Applied Sciences Corporation (AASC) fast gas valve was deployed for the first time on a short pulse, high power laser experiment on the 500 mJ/500 fs Cerberus laser at Imperial College London, demonstrating point projection imaging with an x-ray source size of 150 micrometres above 2 keV in the direction of peak x-ray emission which was found to be along the laser axis on the laser entry side of the cluster gas volume. Up to 94% of the incident laser energy was absorbed by the Ar, Kr or Xe cluster medium producing free electron temperatures of 1-3 keV, with a resonance in heating seen at a cluster radius of 2.4 nm, measured by means of a single photon counting diagnostic. The x-ray conversion e ffciency from incident laser energy to emitted x-ray energy above 2 keV was estimated at 0.3%, corresponding to 1.5 mJ of emitted x-ray energy from the interaction. Optical probing of the interaction revealed two spatially separate regions in which the cluster medium was ionised on the rising edge of the pulse.
Supervisor: Smith, Roland Sponsor: Engineering and Physical Sciences Research Council ; Atomic Weapons Establishment
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral