Refractive effects in phase objects and associated phenomena.

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.