Novel diagnostic techniques for investigations into energy and matter transport in dynamic systems

Change log
El Gabbani, Nadeem 

Light-matter interactions are fundamental to many technologies used today. They are complex, non-linear processes sensitive to the properties of the source, material, mechanisms, and environment in which they interact. In order to control a process, its key features must be measurable and characterised. To date, most efforts in doing so follow either empirical or theoretical approaches, requiring existing or potential users of laser technology to invest significant time and money in their attempted use. In this work a novel diagnostics platform is theorised for the study of light-matter interactions by assessing energy transfer within a process relative to certain performance attributes e.g. ablation efficiency (kg/W), coupling momentum efficiency (N.s/W) and kinetic energy ratio (plume energy over pulse energy) with the aim of producing widely relatable results which significantly lessen the need for heavily empirical or theoretical work. The missing element of the diagnostic platform, a torsion balance, capable of measuring impulsive forces and mass loss, was designed, constructed, and calibrated to the ultra precision standards required to measure nano-scale forces including a force and mass-loss resolution of nN and µg respectively. The tool was then used to characterise the behaviour of some ‘generic’ pulsed nano-second light matter inactions with varying source parameters such as total energy input (58.6 mJ to 468.8 mJ), repetition rate (5 kHz to 40 kHz), temporal pulse shape (up to 500 ns duration and 50 kW peak power), and materials (aluminium, silicon, and PVC), as well different environments (air and vacuum) and processing strategies (scanning and single point drilling). Three dimensional processing maps were then produced using the torsion balance, laser, environment, and material data to represent the quantitative and qualitative nature of results. When convoluted or stitched together, the maps uniquely enable the optimisation of process specific performance with respect to laser, material, and environmental parameters, to which the torsion balance data was very sensitive (ablation efficiencies from 10-7 to 10 13 kg/W, coupling momentum efficiency from 10-4 to 10-9 N/W and kinetic energy ratios from 10-5 to 437 %). Regions of high or low performance, could be easily identified using the qualitative map elements, whilst being relatable to quantitative data such as total energy, repetition rate and pulse shape, which are in turn relatable to theory using conventional methods. A single data set, for example, ablation efficiency plotted against total number of pulses and repetition rate for a given pulse shape, material, and environment, could be captured and processed in 16 minutes. In this work the ability to generically optimise a laser based process or technology very easily, quickly, and efficiently was demonstrated. For the first time, this has enabled high parameter resolution, light-matter-environmental parameter sweeps to be produced within hours, instead of months or years. This is important because it could revolutionize the way light-matter interactions are approached and completely change where time, energy and financial resources are spent in academia and industry. The end result could be the production of a well-defined process map for any given laser, material, and environment.

O'Neill, William
Space, Propulsion, Force Balance, Force Measurement, Laser, Laser Processing, Laser Matter Interactions, Torsion Balance, Ultrashort Laser, Ultrafast Laser, Laser Manufacturing, Laser Propulsion, Energy Saving, LMI, Laser Ablation
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
EPSRC (1645527)