Cold Plasma Discharge Types

Plasma, often referred to as the fourth state of matter, is a partially ionized gas composed of free electrons, ions, and neutral species. The ability to generate and control artificial plasmas under various conditions, using different gas types and discharge methods, has opened up a wide range of industrial and technological applications. These advancements rely heavily on manipulating plasma discharge parameters, such as pressure, gas composition, reactor design, and operating conditions, enabling innovations in thermal and non-thermal plasma technologies.

Plasma discharges can be categorized in several ways, reflecting the diverse nature of this state of matter. One of the most commonly used and practical approaches is classifying discharges based on thermal properties into two main types: thermal (hot) and non-thermal (cold and warm) plasmas. For many artificial plasma applications—especially those in non-thermal categories—the type of discharge determines operational performance, efficiency, and suitability for a given use case. While other categorization parameters exist, discharge type remains one of the most universally relevant methods due to its direct influence on application design and optimization.  

Classification of Non-Thermal Discharge Types 

While thermal plasma remains important for certain applications, recent innovations and advances in non-thermal plasmas have garnered significant attention across multiple fields. These systems have become increasingly popular due to their versatility and effectiveness in various applications. The classification of non-thermal discharges often depends on reactor design (e.g., dielectric barrier discharge, atmospheric pressure plasma jets, corona, gliding arc, microwave, and nanosecond pulsed discharge) and discharge characteristics (e.g., glow, micro, townend, filamentray, streamer, spark, and arc discharge), as they influence the operational stability, efficiency, and suitability for specific applications. Other factors, such as energy input (e.g., direct current, alternating current, pulsed) and gas composition (e.g., noble gas, air, reactive gas), are also fundamental for understanding plasma performance.   

Reactor Design

• Dielectric Barrier Discharge (DBD): DBD systems generate non-thermal plasma at atmospheric pressure using electrodes separated by a dielectric material. This design prevents arcing, ensuring stable plasma production. DBDs are widely used in surface modification, sterilization, and ozone generation.
• Atmospheric Pressure Plasma Jets (APPJs): APPJs produce jet-like streams of reactive plasma at atmospheric pressure, typically using noble gases or ambient air. Their localized and precise treatments make them effective for applications such as sterilization, surface modification, and enhancing chemical reactions.  
• Corona Discharge: Corona discharges are generated by applying a high electric field to sharp electrodes, resulting in the ionization of surrounding gas molecules. These discharges generate reactive species, making them suitable for applications such as air purification, surface treatments, and industrial ozone production.
• Gliding Arc Discharge: This type of reactor generates plasma as it "glides" along the surface of electrodes in an arc shape. Unlike cold plasmas, gliding arc discharge produces a warm plasma with higher energy, making it effective for applications such as waste treatment, enhanced combustion, and material processing. 
• Microwave Discharge: This reactor design generates plasma within waveguides or resonant cavities using high-frequency microwave radiation. Microwave discharges are particularly useful for applications such as thin-film deposition, nanoparticle synthesis, and low-temperature gas conversion.
• Nanosecond Pulsed Discharge: Nanosecond pulsed discharges deliver ultra-short, high-voltage pulses to generate non-thermal plasma under atmospheric pressure. These systems are suitable for high-speed processes, including ignition enhancement, rapid surface treatments, and plasma-assisted combustion.

Discharge Characteristics

• Glow Discharge: Glow discharges occur in low-pressure gas-filled tubes and are characterized by stable, uniform plasmas. Their controllability and stability make them key to applications like lighting (e.g., neon lights), thin-film deposition, and spectroscopic analysis.
• Micro-Discharge: Micro-discharges are small-scale, non-thermal plasmas confined to limited spaces. These discharges are valuable in precision applications, including biomedical treatments, localized surface processing, and environmental remediation.
• Townsend Discharge: Townsend discharges occur under low-pressure conditions and rely on the avalanche ionization effect of free electrons. These discharges are stable and non-thermal, making them suitable for gas ionization processes, such as specific-gas phase chemical reactions. 
• Filamentary Discharge: Filamentary discharges are transient, localized plasma channels that form under atmospheric-pressure conditions. They are highly reactive and are often used in applications such as wastewater treatment, sterilization, and plasma catalysis. 
• Streamer Discharge: Streamer discharges are high-electric-field-driven plasmas that propagate as narrow, ionized channels, commonly observed in gases with low breakdown voltages. Their ability to generate reactive species at near-room temperatures makes them effective for applications in pollution control, water treatment, and plasma-assisted chemical processes.
• Spark or Arc Discharge: Although spark and arc discharges are often associated with thermal operations, they can produce transient or localized non-thermal plasma in certain configurations. These discharges are typically used in ignition systems, plasma-assisted combustion, and high-temperature surface treatments.
   

Non-thermal plasma presents exciting opportunities for innovation across diverse fields. Understanding the various discharges is crucial for optimizing the performance and applicability of non-thermal plasma technologies, as they are essential for maintaining the stability, efficiency, and uniformity of plasma generation processes.

The characterization and classification of plasma discharges are essential for optimizing artificial plasma systems to meet specific functional requirements. By understanding the interplay between reactor design, discharge behavior, and process conditions, researchers and engineers can customize non-thermal plasma technologies for emerging and established applications. The discharge type often dictates key factors such as plasma stability, energy efficiency, and uniformity, which are critical for achieving desired outcomes in fields ranging from environmental remediation to biomedical innovation.  

Due to their versatility, non-thermal plasmas are becoming increasingly important. Their categorization by discharge type offers a practical framework for effectively studying, developing, and applying these systems.