113 research outputs found
What is the mathematical meaning of Steenbeck's principle of minimum power in gas discharge physics?
It is shown that Steenbeck’s principle of minimum power, or voltage, for discharges with fixed
current is not a corollary of the principle of minimum entropy production, in contrast to what
is frequently assumed; besides, the latter principle itself does not provide a reasonable
approximation in gas discharge physics. Similarly, Steenbeck’s principle is not a corollary of
mathematical models of gas discharges. Hence, this principle contradicts the mathematical
models. A methodically correct evaluation of the error caused by the use of Steenbeck’s
principle requires a comparison of a solution obtained with the use of this principle with an
exact solution to the same problem, rather than with experimental results or results deemed
reasonable from the point of view of common sense. Such a comparison is performed for two
examples from the theory of a cylindrical arc column. The examples show that the error
incurred by the usage of Steenbeck’s principle is uncontrollable and may be unacceptably high.info:eu-repo/semantics/publishedVersio
Reply to the Comment on ‘What is the mathematical meaning of Steenbeck's principle of minimum power in gas discharge physics?’
info:eu-repo/semantics/publishedVersio
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Visualising gas heating from an RF plasma loudspeaker
In an electro-acoustic transduction mechanism, an ac modulation (here in the audio frequency range) of the electric field in an atmospheric pressure air plasma gives rise to a rapid increase in the gas temperature and dimensions of the gas volume. As in natural lightning, the rapid expansion in the ionised column though the air produces external pressure variations at the modulation frequency.
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Spatial and temporal measurement of the gas temperature can identify the nature of the thermal expansion and provide a direct approach to understanding its relationship to the sound pressure wave that is generated. However, the established method through spectroscopic measurement of rotational line emission from nitrogen molecules is limited to the main current channel where relaxation and subsequent optical emission of the excited nitrogen molecules occurs. The wider picture is revealed through the use of the Schlieren method where the refractive index gradients caused by gas heating in the plasma are imaged
Simulation of pre-breakdown discharges in high-pressure air: II. Effect of surface protrusions
Analysis of deviations from the similarity law, observed at high and very high pressures in
experiments on discharge ignition and breakdown in corona-like configurations, can serve as a
useful, albeit inevitably indirect, source of information about microprotrusions on the surface of
the electrodes. In this work, such analysis was performed by means of 2D numerical modelling.
Conical or cylindrical protrusions on the surface of the inner electrode were studied and the
kinetic scheme includes the electrons, one species of positive ions, and negative ions O−
2
, O−,
and O−
3
. It is shown that the deviations from the similarity law, observed in the experiment, may
indeed be attributed to enhanced ionization of air molecules in regions of amplified electric field
near the microprotrusions. A qualitative agreement with the experiment in all the cases is
achieved for protrusion heights of the order of 50 µm. Such values may appear rather high,
however there is no other explanation in sight at present. The enhancement of the field electron
emission from the surface of the negative electrode due to the amplification of the electric field
on the microprotrusion was estimated and found insignificant in the range of values of the
protrusion aspect ratio where the enhanced ionization in the gas phase is already appreciable.info:eu-repo/semantics/publishedVersio
Modelling interaction of multispecies plasmas with thermionic cathodes
The model of the near-cathode plasma, developed previously for the case of a
single-species plasma-producing gas, is generalized for the case of multiple
plasma-producing species. Results are presented of calculation of a diffuse
mode of current transfer to tungsten cathodes in a mercury plasma with an
addition of sodium. It is found that the presence of 1% of sodium results in a
considerable expansion of the range of stability of the diffuse mode.info:eu-repo/semantics/publishedVersio
Modelling current transfer to cathodes in metal halide plasmas
This work is concerned with investigation of the main features of current
transfer to cathodes under conditions characteristic of metal halide (MH)
lamps. It is found that the presence of MHs in the gas phase results in a
small decrease of the cathode surface temperature and of the near-cathode
voltage drop in the diffuse mode of current transfer; the range of stability of
the diffuse mode expands. Effects caused by a variation of the work function
of the cathode surface owing to formation of a monolayer of alkali metal
atoms on the surface are studied for particular cases where the monolayer
is composed of sodium or caesium. It is found that the formation of the
sodium monolayer affects the diffuse mode of current transfer only
moderately and in the same direction that the presence of metal
atoms in the gas phase affects it. Formation of the caesium monolayer
produces a dramatic effect: the cathode surface temperature decreases
very strongly, the diffuse-mode current–voltage characteristic becomes
N–S-shaped.info:eu-repo/semantics/publishedVersio
Comment on “Electric field measurements under DC corona discharges in ambient air by electric field induced second harmonic generation” [Appl. Phys. Lett. 115, 244101 (2019)]
info:eu-repo/semantics/publishedVersio
Simple computation of ignition voltage of self-sustaining gas discharges
A robust, fast, and accurate numerical method is proposed for finding the voltage of the
ignition of DC self-sustaining gas discharges in a wide range of conditions. The method is
based on physical grounds and builds up from the idea that the ignition of a self-sustaining gas
discharge should be associated with a resonance that would occur in a non-self-sustained
discharge in the same electrode configuration. Examples of the application of the method are
shown for various configurations: parallel-plate discharge, coaxial and wire-to-plane corona
discharges, and a discharge along a dielectric surface. The results conform to the conventional
Townsend breakdown condition for the parallel-plate configuration and are in good agreement
with existing experimental data for the other configurations. The method has the potential of
providing a reference point for optimization of the hold-off capability of high-power
switchgear operating in low-frequency fields.info:eu-repo/semantics/publishedVersio
A practical guide to modeling low-current quasi-stationary gas discharges: Eigenvalue, stationary, and time-dependent solvers
The work is concerned with the modeling of low-current quasi-stationary discharges, including the Townsend and corona discharges. The
aim is to develop an integrated approach suitable for the computation of the whole range of existence of a quasi-stationary discharge from
its inception to a non-stationary transition to another discharge form, such as a transition from the Townsend discharge to a normal glow
discharge or the corona-to-streamer transition. This task includes three steps: (i) modeling of the ignition of a self-sustaining discharge,
(ii) modeling of the quasi-stationary evolution of the discharge with increasing current, and (iii) the determination of the current range
where the quasi-stationary discharge becomes unstable and the non-stationary transition to another discharge form begins. Each of these
three steps is considered in some detail with a number of examples, referring mostly to discharges in high-pressure air.info:eu-repo/semantics/publishedVersio
Simulation of pre-breakdown discharges in high-pressure air. I: The model and its application to corona inception
A ‘minimal’ kinetic model of plasmachemical processes in low-current discharges in high pressure air is formulated, which takes into account electrons, an effective species of positive
ions, and three species of negative ions. The model is implemented as a part of numerical
model of low-current quasi-stationary discharges in high-pressure air based on the use of
stationary solvers, which offer important advantages in simulations of steady-state discharges
compared to standard approaches that rely on time-dependent solvers. The model is validated
by comparison of the computed inception voltage of corona discharges with several sets of
experimental data on glow coronas. A good agreement with the experiment has been obtained
for positive coronas between concentric cylinders in a wide range of pressures and diameters
of the cylinders. The sensitivity of the computation results with respect to different factors
is illustrated. Inception voltages of negative coronas, computed using the values of the
secondary electron emission coefficient of 10−4
–10−3
, agree well with the experimental data.
A simplified kinetic model for corona discharges in air, which does not include conservation
equations for negative ion species, has been proposed and validated. Modelling of positive
coronas in rod-to-plane electrode configuration has been performed and the computed
inception voltage was compared with experimental data.info:eu-repo/semantics/publishedVersio
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