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Modelling of Argon Atmospheric Pressure High Frequency Discharges
M. Atanasova, J. J. A. M. van der Mullen, G. Degrez, E. Benova
St. Kliment Ohridsi University of Sofia, Sofia, Bulgaria
Due to the ease of their use the variety of plasma sources at atmospheric pressure is
already immense. They alter in terms of construction and operating conditions – working gas,
power, gas flow as well as in terms of plasma characteristics – electron temperature and
density, gas temperature. This work is focused on modelling of atmospheric pressure high
frequency (HF) discharges operated at relatively low power densities. Two types of devices
are considered – the radio frequency capacitively coupled plasma jet (RF CCP) and the
microwave discharge sustained by surface electromagnetic waves (SWD). Both of the
considered devices operate in argon at atmospheric pressure. However, the difference in the
frequency of the power coupling mechanism induces a big difference in plasma properties.
This implies also that different modelling approaches have to be employed.
Both models follow the subdivision: configuration, transport and chemistry.
Configuration deals with the impact of the environment on the plasma and thus considers the
shape and sizes of the plasma, the boundary conditions, the energy coupling modus, and etc.
Transport describes the transport of species, momentum and energy in the plasma (and
afterglow). Chemistry deals with the processes of creation and destruction of plasma species
and accounts their impact on the sources and the coefficients that drive and facilitate
transport.
A theoretical description of an RF CCP with the form of a shower head has been built.
This device is quite complex. Apart from the sheath structure, the performance of this plasma
is also determined by the field augmentation and flow acceleration in the holes together with
the flow recirculation in between the plates. The understanding of the concerted action of
these phenomena was facilitated by two-dimensional time-dependant modelling. The model
allowed us to investigate the role of the sheaths and space charge, the influence of the shower
holes on the field distribution and the effect of the flow on the species profiles.
An intrinsic characteristic of SWDs is the interplay between exciting wave and
resulting plasma. An adequate description requires a model accounting for these two aspects
in a self-consistent manner. By means of such model a study on the propagation
characteristics of the surface wave maintaining the plasma as well as on the plasma
characteristics is performed. The effect of the geometry and the high pressure conditions is
examined.