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Time-dependent fluid simulation of negative point-to-plane DC corona discharge: Influence of neutral gas temperature

This study employs a time-dependent axisymmetric fluid model in COMSOL Multiphysics to demonstrate that increasing neutral gas temperature reduces electron, positive ion, and negative ion densities in negative DC point-to-plane corona discharges, while leaving electric potential and electron temperature largely unaffected.

Original authors: Samira Tajiknezhad, Fouziyeh Shafei, Ramin Mehrabifard

Published 2026-10-20

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Samira Tajiknezhad, Fouziyeh Shafei, Ramin Mehrabifard

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of electricity, there are moments when air, usually a perfect insulator, suddenly becomes a conductor. This happens in high-voltage systems where the electric field is so intense near a sharp point that it rips electrons away from air molecules, creating a glowing, ionized gas known as plasma. This phenomenon, called a corona discharge, is the invisible engine behind everyday technologies like the electrostatic precipitators that clean smoke from factory chimneys and the ozone generators that purify water. While these devices often operate at room temperature, real-world conditions can be much hotter. As machines run, they generate heat, and the surrounding air warms up. For engineers designing these systems, understanding how this rising heat changes the behavior of the plasma is critical. If the air gets too hot, will the device stop working efficiently? Will the flow of electricity change in unexpected ways? The answer lies in the microscopic dance between charged particles and the neutral gas that surrounds them.

A team of researchers from Iran and Slovakia set out to answer these questions by creating a detailed digital simulation of a negative DC point-to-plane corona discharge. They modeled a setup where a sharp metal needle, charged to a high negative voltage, faces a flat grounded plate, with dry air filling the space between them. Using powerful computer software, they watched how the plasma evolved over time as they systematically increased the temperature of the neutral gas from a comfortable 300 Kelvin to a scorching 600 Kelvin. Their goal was to see if heating the air would fundamentally alter the structure of the discharge or simply change its intensity. The simulation acted as a virtual laboratory, allowing them to track the movement of electrons and ions with a precision that is nearly impossible to achieve with physical sensors in such a small, fast-moving environment.

The results revealed a clear and consistent pattern: as the gas temperature rose, the plasma became significantly weaker, yet its basic shape remained the same. In the simulation, the researchers observed that the density of electrons, positive ions, and negative ions all dropped dramatically as the temperature increased. When the gas was heated from 300 Kelvin to 600 Kelvin, the peak number of electrons in the active region fell by about 80 percent. The number of positive ions dropped by roughly 66 percent, and negative ions decreased by about 75 percent. This happened because, at a constant pressure, heating a gas causes it to expand, which means there are fewer air molecules packed into the same space. With fewer neutral molecules available to collide with, the electrons had fewer opportunities to knock other electrons loose or attach to molecules to form ions. The process of creating new charged particles simply slowed down.

Despite this massive drop in the number of particles, the researchers found that the overall structure of the electric field and the temperature of the electrons themselves remained surprisingly stable. The electric potential, which is the driving force pushing the charges, looked almost identical across all the different temperatures. Similarly, the energy of the electrons, measured as electron temperature, changed very little, rising by only about 10 percent even as the gas heated up significantly. This suggests that while the crowd of particles thinned out, the individual particles continued to move and behave in much the same way. The electric field, dictated largely by the shape of the needle and the voltage applied, did not need to reorganize itself to accommodate the heat; it simply pushed a smaller group of particles with the same force.

The flow of energy told a similar story of reduction without structural change. The amount of electrical power transferred from the electric field to the plasma, known as capacitive power deposition, fell by nearly 85 percent as the temperature rose. Because there were fewer electrons to absorb this energy, the total energy delivered to the gas dropped sharply. Consequently, the energy lost through collisions with air molecules also decreased. The researchers noted that while the ability of a single electron to cause ionization actually improved slightly due to the lower density of the gas, this gain was completely overwhelmed by the sheer lack of electrons and neutral molecules to interact with. The net result was a discharge that was much less intense but fundamentally unchanged in its layout.

These findings offer a clear guide for the thermal design of atmospheric-pressure plasma devices. The study suggests that if a corona discharge device operates in a hot environment, engineers should expect a significant reduction in the number of charged particles and the total current, but they should not expect the discharge to suddenly shift its location or change its fundamental mode of operation. The discharge does not vanish or become chaotic; it simply becomes quieter and less dense. This insight is vital for optimizing equipment like ozone generators and air purifiers that must function reliably even as they heat up during prolonged use. By understanding that heat primarily acts to thin out the plasma rather than distort its shape, designers can better predict performance limits and ensure their systems remain effective under varying environmental conditions.

Technical Summary: Time-dependent fluid simulation of negative point-to-plane DC corona discharge: Influence of neutral gas temperature

Problem Statement
Negative DC corona discharges are critical for applications such as electrostatic precipitation, ozone generation, and surface treatment. While the effects of voltage, electrode geometry, and gas composition on discharge characteristics are well-documented, the specific impact of neutral gas temperature on the transient evolution of negative DC point-to-plane coronas remains incompletely understood. In practical operating conditions, gas temperature is rarely constant; it can rise due to discharge-induced heating or environmental changes. Such temperature variations alter neutral particle density and collision frequencies, thereby modifying ionization, attachment, and recombination kinetics. Although previous studies have addressed steady-state behaviors or focused on dielectric barrier discharges, a systematic time-dependent fluid study quantifying how neutral gas temperature influences the spatiotemporal evolution of charged species and space-charge accumulation in negative DC coronas is limited.

Methodology
The authors developed a two-dimensional axisymmetric, time-dependent fluid model using COMSOL Multiphysics to simulate a negative DC point-to-plane corona in dry atmospheric air.

Geometry and Conditions: The simulation features a sharp metallic point (50 μm tip radius) positioned 9.5 cm above a grounded plane, with a constant applied voltage of -50 kV. The initial neutral gas temperature ( $T_g$ ) was varied from 300 K to 600 K.
Governing Equations: The model solves coupled drift-diffusion equations for electrons, positive ions, and negative ions, alongside Poisson's equation for the electric potential. The system operates under the local field approximation, where transport coefficients and reaction rates depend on the reduced electric field ( $E/N$ ).
Physics and Chemistry: A reduced air-plasma chemistry scheme was employed, including electron-impact ionization, two- and three-body electron attachment, and recombination processes. Rate coefficients were derived from the LXCat database and evaluated as functions of $E/N$ . The neutral gas number density ( $N$ ) was calculated via the ideal gas law, linking temperature directly to density ( $N \propto 1/T_g$ at constant pressure).
Numerical Approach: A non-uniform finite-element mesh with local refinement near the needle tip resolved steep gradients. An adaptive time-stepping strategy was used to capture transient dynamics, with the cathode voltage ramped from 0 to -50 kV to ensure numerical stability.

Key Results
The simulations revealed that increasing the neutral gas temperature from 300 K to 600 K significantly alters discharge intensity while preserving the fundamental spatial structure of the ionization and drift regions.

Charged Species Densities: There is a pronounced reduction in the peak densities of all charged species as temperature increases. Specifically, peak electron density decreased by approximately 80% (from $7.36 \times 10^{11}$ to $1.44 \times 10^{11} \text{ cm}^{-3}$ ), positive ion density by ~66%, and negative ion density by ~75%. This reduction is attributed to the lower neutral gas density at higher temperatures, which decreases the frequency of electron-neutral collisions and suppresses both ionization and attachment rates.
Fluxes and Power Transfer: Consistent with density reductions, electron flux, positive-ion flux, and negative-ion flux all decreased significantly (by ~78%, ~16%, and ~58%, respectively). Similarly, capacitive power deposition and collisional power loss dropped by approximately 85% and 81%, respectively, indicating weaker energy coupling between the electric field and the plasma.
Electric Field and Potential: In contrast to the density changes, the electric potential distribution and electron temperature remained relatively insensitive to temperature variations. The peak electron temperature increased by only about 10%, and the spatial profile of the electric potential was nearly identical across all temperature cases. The peak electric field magnitude decreased moderately (~24%), but the overall spatial shape of the field remained governed by electrode geometry.
Ionization Kinetics: A nuanced finding concerns the ionization rate. While the ionization rate coefficient increases with temperature due to a higher reduced electric field ( $E/N$ ), the net ionization production rate decreases. This occurs because the reduction in the number of available neutral targets and electrons outweighs the increased efficiency per collision.

Significance and Claims
The paper claims to provide a systematic time-dependent analysis that fills a gap in understanding the transient thermal effects on negative corona discharges. The primary contribution is the demonstration that neutral gas temperature influences discharge kinetics primarily through the modulation of neutral particle density and collision frequencies, rather than through direct alterations of the electric field structure or electron temperature.

The authors assert that their findings offer useful guidance for the thermal design and operation of atmospheric-pressure corona devices (e.g., electrostatic precipitators, ozone generators) operating under varying environmental conditions. By highlighting that gas heating suppresses plasma density and discharge intensity while leaving the spatial structure largely intact, the study underscores the necessity of time-dependent modeling to accurately capture space-charge formation and discharge stabilization mechanisms. The results align with previous theoretical analyses (e.g., Popov) regarding the dominance of neutral density effects in temperature-dependent plasma kinetics.

Technical Summary: Time-dependent fluid simulation of negative point-to-plane DC corona discharge: Influence of neutral gas temperature

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