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Research Project
Institute for Plasmas and Nuclear Fusion
Funder
Authors
Publications
Simulation of pre-breakdown discharges in high-pressure air. I: The model and its application to corona inception
Publication . Ferreira, N. G. C.; Santos, D. F. N.; Almeida, P. G. C.; Naidis, G. V.; Benilov, M. S.
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.
Kinetic Bohm criterion in the Tonks-Langmuir model: assumption or theorem?
Publication . Benilov, M. S.; Almeida, N. A.
New first integral is found in the collision-free Tonks-Langmuir model. The integral has a clear physical interpretation: the weighted mean
inverse kinetic energy of ions, evaluated in the quasineutral approximation, equals ðkTe=2Þ
1 at all points in space. This feature is also pre sent in the full (not relying on the assumption of quasineutrality) model: for small values of the Debye length, the weighted mean inverse
kinetic energy is with good accuracy equal to ðkTe=2Þ
1 in the entire region of quasineutral plasma, including in the vicinity of the space charge sheath. These results constitute a mathematical proof of the kinetic Bohm criterion and provide a new look at the problem, which has
been discussed for several decades. In particular, these results show that the much-debated problem of divergence for slow ions stems from a
misinterpretation. Moreover, these results explain why no unique form of kinetic Bohm criterion, modified with the account of ionization
and/or collisional and/or geometrical effects in the sheath, has emerged: it cannot be postulated in a nonarbitrary way since there is simply
no definite value of the inverse mean kinetic energy with which the ions enter the sheath, if these effects are non-negligible.
A simple model of distribution of current over cathodes of vacuum circuit breakers
Publication . Cunha, Mário D.; Wenzel, Norbert; Almeida, Pedro G. C.; Hartmann, Werner; Benilov, Mikhail S.
There are several hundreds of spots operating
simultaneously on cathodes of vacuum arcs in high-power vac uum circuit breakers. In this work, the spot distribution along
the contact surface is simulated by means of an approach
that is based on the concept of surface density of spots and
represents a natural alternative to tracing individual spots.
An equation governing the evolution of the surface density
of the spots or, equivalently, the distribution of macroscopic
(averaged over individual spots) current density over the cathode
is obtained by generalizing the concept of random walk of a
single cathode spot in low-current vacuum arcs. The model
relies on empirical parameters characterizing individual spots
(the diffusion coefficient of the random motion of cathode spots
and the velocity of drift superimposed over the random motion),
which may be taken from experiments with low-current arcs,
and does not involve adjustable parameters. The model is simple
and physically transparent and correctly reproduces the trends
observed in the experiments under conditions where the cathode
arc attachment is diffuse. The distribution of the macroscopic
current density on the cathode, given by the model, represents
the boundary condition that is required for existing numerical
models of vacuum arcs in high-power vacuum circuit breakers.
Simple computation of ignition voltage of self-sustaining gas discharges
Publication . Almeida, P. G. C.; Almeida, R. M. S.; Ferreira, N. G. C.; Naidis, G. V.; Benilov, M. S.
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.
Simulating changes in shape of thermionic cathodes during operation of high-pressure arc discharges
Publication . Cunha, M. D.; Kaufmann, H. T. C.; Santos, D. F. N.; Benilov, M. S.
A numerical model of current transfer to thermionic cathodes of high-pressure arc discharges
is developed with account of deviations from local thermodynamic equilibrium occurring
near the cathode surface, in particular, of the near-cathode space-charge sheath, melting of the
cathode, and motion of the molten metal under the effect of the plasma pressure, the Lorentz
force, gravity, and surface tension. Modelling results are reported for a tungsten cathode of
an atmospheric-pressure argon arc and the computed changes in the shape of the cathode
closely resemble those observed in the experiment. The modelling has shown that the time
scale of change of the cathode shape during arc operation is very sensitive to the temperature
attained by the cathode. The fact that the computed time scales conform to those observed
in the experiment indicate that the model of non-equilibrium near-cathode layers in high pressure arc discharges, employed in this work, predicts the cathode temperature for a given
arc current with adequate accuracy. In contrast, modelling based on the assumption of local
thermodynamic equilibrium in the whole arc plasma computation domain up to the cathode
surface could hardly produce a similar agreement.
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Funders
Funding agency
Fundação para a Ciência e a Tecnologia
Funding programme
6817 - DCRRNI ID
Funding Award Number
UID/FIS/50010/2019