الفهرس 
CHAPTER 1: INTRODUCTION, THEORETICAL BACKGROUNDOnly 14 pages are availabe for public view
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Abstract
ne of the most important topics in both theoretical and experimental physics is the study of
spherical thin shells harmonic vibrations. Many crucial scientific, industrial and military applications
depends on the mechanical stability of hollow spherical structures, in numerous fields such as, nanometric
fullerene molecules, sonoluminascences, spherical containers, naval mines, sonars, geoseismology and
stellar dynamics. In this thesis, a method for exciting mechanical and acoustics resonance of thin hallow
metallic sphere samples; using laser induced plasma shocks was discussed.
Four main experimental tests groups were implemented, where in the first group of tests, laser
induced breakdown spectroscopy (LIBS) for AISI 316L stainless steel was performed. The analysis of the
plasma emission spectrum using Boltzmann plot for 3 emission lines, estimated a plasma temperature in
order of 105 K and a plasma pressure in order of 108 Pa. The average temperatures of the laser first strike
spot were obtained by thermography which revealed a linear dependence between laser power and average
maximum temperature. Synchronized photography of plasma plumes obtained the Nd:YAG laser principal,
second and third harmonic generation wavelength at constant fluence showed a larger plume in case of
1064nm irradiation. Optical profilometry for the ablated first strike spots showed a crater diameter of about
6 mm wide, and a depths of 10 μm, 35 μm and 250 μm for a corresponding number of laser pulses of
10, 100 and 200 respectively at fluence of 1.6 J/cm2.
The second group of tests is based on detecting the laser-induced surface acoustic waves (SAW)
inside a non-Lambertian integrating sphere made of AISI 316L stainless steel. A powerful 6 ns, 450 mJ Q-
switched Nd:YAG laser beam is introduced inside the sphere through a tiny hole, and then absorbed after a
series of specular reflections on its reflective inner surface. These reflections induced a similar photoacoutic
(PA) signal propagated through the spheres wall. A contact piezoelectric detector fixed outside the sphere is
used to detect the resultant thermo-elastic vibration pattern. The temporal response of the PA signals
intensity leads to the measurement of laser power. The ratio between the acoustic time constant and the
optical time constant of the PA integrating sphere is found to be ≈ 107 which makes fast and ultra-fast laser
Summary
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power measurement very much easier. The frequency response of the PA signal obtained by fast Fourier
transforms leads to complete SAW mechanical spectroscopy. The comparison between the vibrating
spherical cavity due to hammer impulse test and Nd:YAG laser pulse excitation revealed three types of
vibrations: the natural flexure, the Helmholtz resonance, the breathing and transverse mode vibration
harmonics. A complete theoretical modeling, design, test and calibration procedure of a novel power meter
are inclusive. This design is suitable for checking the output power of pulsed high power laser sources
directly without splitting or damping.
The third group of tests investigated the acoustic response of the system, in which the sound pressure
level (SPL) is recorded in the middle of a full anechoic chamber, using a non-contacting microphone, placed
in front of the beam entrance hole. Relaying on the fact that the confined laser induced plasma (LIP) shock
inside the cavity, the hollow sphere is considered as a neck-less Helmholtz resonator that produces an
acoustic burst of temporal standing waves, characterized by a large harmonic bandwidth. Tests revealed the
generation of few milliseconds width, high SPL harmonic acoustic impulses in range of 60-110 dB,
shockwaves. It was found that the generated SPL depends on the laser wavelength, the cavity diameter, the
laser ablation threshold fluence of the cavity material, the plasma temperature and the plasma pressure. It
was found also, that the frequency response of the cavities was depending on the cavities diameter and laser
wavelength. In this context, constant laser fluence of 5 J/cm2, an odd /even acoustic harmonic multiplicity
dependence on the Nd:YAG laser principal, second and third harmonic generation wavelength was revealed.
The SPL contour plot around the resonators seemed to be quasi spherical due to the contribution of cavity
breathing modes in sound generation. The dependence of Helmholtz harmonics on mode coupling
phenomena is believed to be governing the acoustic frequency response. The prospective application of this
result is to append dodecahedron loudspeakers in some standard acou