Investigation into the role of ions and excited molecules in the corona process in air
Date
2016
Authors
Swanson, Andrew Graham
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Abstract
Corona is the partial breakdown of a gas in an electric field and involves
many complex phenomena including the production of space charge and excited
molecular states of its constituents. Air is predominantly made up
of nitrogen and oxygen, both of which have a wide range of excited states.
Oxygen itself is additionally an electronegative gas and has the ability to attach
electrons. Together these phenomena combine to influence the electric
field and the production of secondary electrons to give corona its distinctive
modes.
A particular state of excited molecular oxygen, known as singlet oxygen,
has the characteristic that it remains excited for a relatively long period of
time. It is known that singlet oxygen plays a role in the detachment of electrons
from the negative oxygen ions and its role in corona discharges has
been accounted for through theory and models, but due to the difficulties
of measurement of the singlet oxygen, the influence it does have on corona
discharges is not entirely clear.
On this basis the gas processes associated with the corona discharge in air have been explored, where a Boltzmann equation solver, the electron energy
distribution function, transport coefficient and Townsend coefficients
are used to understand the phenomena and provide input to a flux correct
transport algorithm model. The model clearly indicates the presence of singlet
oxygen, however its the space charge that plays the critical role in the
repetitive nature of corona due to the collapse and recovery of the electric
field.
Experimentally it was convenient to define and compare the modes of corona
under diāµerent conditions by its peak amplitude and repetition rate. The
experiments included manipulated the environment with air-flow and exciting
oxygen through a 1064 nm Nd:YAG laser as well as detecting 1270 nm
emission from singlet oxygen. There was no evidence to suggest that the
repetition rate of the corona discharge could be related to singlet oxygen.
There were no emissions detected from various configurations, whilst visible
corona was clearly seen.
Together with the evidence from the model, the results showed that the presence
of both negative ions and singlet oxygen is relatively low in the region
that should provide seed electrons. It could be inferred that a relationship
between repetition rate and the presence of singlet oxygen is not a dominant
process in the corona discharge.