# Refractive effects in phase objects and associated phenomena.

## Date

1994

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## Abstract

The effect of the refraction of a laser beam propagating through three different
phase objects, i.e. a laser produced plasma and two different gas media,
is investigated in this thesis. It is shown that these effects have useful applications.
As an introduction to the work performed, a basic discussion of the
theory of light is given.
In the first experimental study, the accuracy of using the Refractive Fringe
Diagnostic, as a tool to determine the electron density profiles of laser produced
plasmas, is investigated [Buccellato et al. (1992)]. A comparative
study is performed between an established method of determining the electron
density profiles of laser produced plasmas, i.e. Nomarski interferometry,
and the Refractive Fringe Diagnostic, by comparing experimental data obtained
from the same laser shot. For the electron density profiles investigated,
it is shown that the Refractive Fringe Diagnostic over-estimates the electron
density by an order of magnitude. It is suggested that the electron density
errors are due to the inherent assumptions of the Refractive Fringe Diagnostic.
To verify this, a numerical simulation into the accuracy of the RFD is
performed on a mathematically modelled plasma. The discrepancy in the
numerical results are consistent with those of the experimental results and
these can be attributed to the assumptions made by the Refractive Fringe
Diagnostic.
Laser light refracted by a gas medium, with a specific density profile, may
produce a near diffraction limited focal spot. The remaining two experimental
investigations deal with two novel gas lenses: the Pulsed Gas Lens and
the Colliding Shock Lens.
A radially expanding cylinder of gas produces a suitable density structure
to focus laser light. A design of a gas lens, the Pulsed Gas Lens, using this
principle is proposed as a final focusing lens for a laser fusion power station
[Buccellato et al. (1993a)]. To establish the feasibility of such a lens a proof-of-
principle design for the lens is given. A numerical simulation of this lens is
performed by modelling the gas flow from the lens and raytracing through the
determined density profiles inside the lens. It is found that this lens can be
used as a focusing element. To establish certain practical aspects of the proof-of-
principle design, a beam deflection device was constructed and tested. This
beam deflection device models the lensing principle of the proposed lens.
The laser beam deflection observed did not match the computed deflection.
The opening mechanism for the proof-of-principle design did not produce an
instantaneous opening of the chamber as was assumed in the simulation. The
opening mechanism must be modified to decrease the opening time.
Diverging spherical shock waves, produced by pairs of opposing electrodes
evenly spaced on a circumference, produce a converging cylindrically symmetric
shock wave. After convergence a suitable density structure exists for
near diffraction li.mited focusing to occur. It is found that the Colliding
Shock Lens is a varifocal lens: the focal length and lens diameter increase
with time [Buccellato et al. (1993b)]. A numerical simulation is performed
to model the operation of the Colliding Shock Lens. The numerical results
compare favourably with the experimental results. From the simulation it is
established that the lens diameter can be scaled up by increasing the physical
size of the lens and the input energy to the lens. Potential applications of
the colliding shock lens are discussed.
To conclude this thesis, the results of the separate investigations are summarised.

## Description

Thesis (Ph.D.)-University of Natal, 1994.

## Keywords

Theses--Physics., Laser-Plasma interactions., Gas lenses., Plasma diagnostics., Laser beams., Laser plasmas., Gas lasers., Lasers in Ppasma diagnostics.