Any correspondance related to these papers should be addressed to either one of the authors, or the surface science webmaster.

The production and study of laser-driven flyer plates

PhD Thesis, University of Cambridge, 1998
S Watson

Abstract

Various aspects of the laser-driven flyer process were studied. The work presented in this thesis involved characterising the process of launching flyers, approximately 1 mm in diameter, from targets with aluminium film thicknesses up to 6 µm. The efficiency with which laser pulse energy was coupled into targets was shown to be a function of the energy. Energy coupling differences were observed between targets ablated in the confined and unconfined geometry. The different absorption mechanisms involved were attributed to the dynamics of the plasmas.

The laser-induced plasma was shown to form during the time of laser irradiation. The rapid increase in intensity of the plasma suggested rapid heating. The expansion of the plasma produced an ablation site area greater than the area of laser irradiation. The ablation site area increased with laser pulse energy due to the increased expansion of the induced plasma. The quantity of substrate material vaporised or melted was also shown to increase with laser pulse energy, as was the extent of substrate damage. A threshold of energy was determined, above which whole flyers were launched.

Time-integrated and high-speed framing photography were used to identify the expansion of the laser-induced plasma and the subsequent reaction of ejected material from the target. Estimates were made of the thickness of aluminium totally ablated at a given laser pulse energy, by noting the thickness of film from which material was not ejected.

Streak photographs of laser-driven flyers contained a lot of data on the flyer's performance. This technique was used in conjunction with post impact analysis to characterise flyers by their velocity, planarity, integrity and dimensions, according to the film thickness, laser pulse energy and the distance traversed by the flyer. A modified kinetic energy equation was used to approximate the relationship between the flyer velocity and the laser pulse energy.

The distance within which flyers remained intact was determined by calculating the distance at which light from the plasma penetrated the flyer and was detected by the streak camera. This provided information on the threshold within which a flyer might be considered effective for a certain application. The planarity of flyers were shown to decrease with increasing distance traversed. It is suggested that this may be directly related to the intensity distribution of the laser beam spatial profile. The consequent velocity gradient across the flyer may go some way to explaining the causes of flyer break-up. Flyer area was shown to increase with laser pulse energy and was attributed to plasma expansion during launch. An estimate of flyer thickness was made using the analytical model by Lawrence and Trott (1991).

Flyer thickness was determined by the film thickness of the target from which it was launched and the mass of film ablated by the laser. It was obvious that the flyer thickness was dependent on the film thickness (the velocity of flyers was inversely proportional to the square-root of film thickness), but the ablated film thickness was less understood. The ablated thickness of film contributing to the plasma was measured by detecting tracer materials in the plasma spectrum. Experiments were conducted to test the suitability of candidate tracer metals, before choosing yttrium, silver and gold. Unique yttrium lines were identified in the spectra of plasmas in multilayer films of aluminium containing yttrium tracer layers. As the depth of the layer into the film increased, the yttrium signal diminished and then disappeared. This effect was coincident with another effect which involved the decrease in overall plasma intensity with increasing tracer layer depth. This was attributed to the tracer layer acting as a thermal barrier and changing the thermal properties of the multilayer film according to its depth. The extinction depth was the depth of tracer layer at which the tracer no longer appeared in the spectra and was therefore assumed to cease to contribute to the plasma. The influence of the tracer layer on the thermal properties of the multilayer film also ceased at this depth. These data provided information on ablation depths and it was shown that this depth increased noticeably with laser pulse energy.

Mass spectrometry was used to detect tracer materials from laser ablated targets, to complement the photo-emission spectroscopy experiments. Many difficulties were encountered, but unique tracer signals from silver were eventually detected from the ablation of multilayer targets containing it. Due to background noise levels and low intensity tracer signals, the data were poor. They showed that the ablation depth was greater than that measured for multilayer films containing yttrium using photo-emission spectroscopy. This may be due to the fact that yttrium has a very low thermal conductivity compared with aluminium and silver. The yttrium may have delayed thermal diffusion sufficient to cause the difference in ablation depths for each type of multilayer film. The photo-emission spectroscopy experiments were considered to have been the most successful, although improvements to the mass spectrometry experiments could provide a useful technique to be used in conjunction with it.

This successful characterisation of the performance of laser-driven flyers will be important in determining the effectiveness of flyers in whichever application they are intended for use. Applications involving the shock initiation of secondary explosives benefit from flyers that can provide high pressure shocks. An optimum flyer would be thin, planar, intact and travelling at high velocity. For such an application it would also be necessary to know the distance from the target within which the flyer is effective. The data presented in this thesis will be useful for determining the conditions required to produce optimum flyer characteristics. The techniques that were developed can be applied to extend the characterisation study.

Reference

R.J. Lawrence & W.M. Trott (1991) "A simple model for the motion of pulsed laser driven thin flyers" J. Phys. IV France 1 Colloq. C3 (DYMAT 91) 453-458


Thesis available upon request.

For local users: edit