| dc.description.abstract | It is widely recognised that the plasmas either
in nature or in laboratory are bounded in one way or the
other. In many situations, these plasmas are subjected
to external oscillating fields such as laser?produced
plasma and radio?frequency heating experiments. In the
present work the author examines the effect of external
hf field on bounded and unbounded plasmas, especially
in the frequency regime of the external field that
excludes the possibility of parametric coupling.
The thesis starts with the presentation of the
motivation and summary of the work, in the introductory
chapter. In the second chapter, the effect of
the external high?frequency (hf) fields on homogeneous
infinite plasmas is considered. In addition to elucidating
the derivation of dispersion relation, the
spectra and damping of the electron and ion Bernstein
modes are obtained and compared with the no?external
hf field case. The normal modes and their damping of
the plasma subjected to an external hf field are enumerated.
In the third and fourth chapters, the study of a
bounded plasma subjected to an external hf field is
taken up. The third chapter is devoted to the presentation
of mathematical techniques used in the derivation of
the dispersion relation and its connection with those
under various limiting situations, such as a pure electron
plasma, plasma with no external hf field, etc., is
established. The normal modes of the slab and semi?
infinite plasmas show that the usual modes are modified
by the hf field in that, the hf modes get an additional
dispersion due to the hf field and the phase velocity
of the low?frequency modes could be entirely determined
by the hf field. Under certain conditions, the antisymmetric
modes are affected to a larger extent than
the symmetric modes in the case of a slab plasma.
Another important observation is that the low?
frequency sound?like modes which exist only in the presence
of the external hf field have a different behaviour,
as compared to those in the absence of the
external hf field. While the sound modes in the
absence of external hf field in the bulk and the surface
could not be distinguished, those in the presence of an
external hf field have distinguishable character due to
their different frequencies.
There are no hf Langmuir waves or low?frequency
sound waves in the case where the external static magnetic
field is normal to the boundary. But the
Bernstein modes exist on the surface because of the
cyclotron harmonic structure of the particle perturbation
density across the magnetic lines of force.
The importance of the study of the spatial
structure of the surface waves lies in the fact that
the wave electric field could be measured outside the
plasma. In the fourth chapter, it is found that
those surface waves whose existence demands the
presence of the external hf field are true surface waves —
their field is localised within one wavelength — as
against the quasi?surface waves, like the ion sound
waves in the absence of hf field.
The case of a semi?infinite plasma in which
there is a drift of the electrons relative to the ions
along the surface is considered in Chapter V. The dispersion
relation yields two unstable modes, one having a
peak growth rate near the resonance point of the surface
plasma oscillations and the other lying close to volume
plasma oscillations. The comparison is made with the
bulk two?stream instability and the general feature is
that both bulk and surface instabilities are stabilised
by the external hf field of high intensity, because the
oscillation of electrons in the external hf field dominates
over the drift motion. In certain regions of the
electron stream velocity, the surface is stable, but is
flanked by two unstable regions.
Finally, in the last chapter, the Langmuir solitons
in the presence of an external hf field are studied.
The frequency–wave?number dependence of the mode is
important to the study of the turbulence. Since the
external hf field introduces an additional dispersion,
it is interesting to study the case of soliton turbulence.
When the thermal dispersion exceeds the dispersion
introduced by the hf field, we always get
Langmuir solitons. Once this situation reverses, then
Langmuir solitons, with vanishing asymptotic amplitude,
do not exist. Instead, what are known as "soliton
holes" exist. In the situation where the soliton holes
exist, the turbulence parameter is determined by the
ratio of the Langmuir frequency to the hf field frequency,
rather than the electron?to?ion mass ratio. If the
depth of modulation of the soliton holes is maximum,
then envelope shocks are formed. We conclude therefore
that the propagation of non?linear Langmuir waves
with and without the presence of the external hf field
could be entirely different. | |
