What Are Plasma Jets?

Plasma jets are directed streams of partially or fully ionized gas that emerge from a plasma source and propagate into an ambient environment. They consist of a mixture of ions, electrons, excited neutral atoms and molecules, reactive radicals, and photons, and can sustain plasma chemistry at distances of several centimeters beyond the electrode region. Plasma jets span a broad temperature range: thermal plasma jets operate at temperatures of thousands to tens of thousands of Kelvin and are used in high-energy material processing, while non-equilibrium atmospheric-pressure plasma jets maintain gas temperatures near ambient while sustaining high electron energies and rich reactive chemistry. Both classes draw on plasma physics, gas discharge engineering, and chemistry.

Plasma jets differ from enclosed plasma reactors in that the active plasma region is open to the surrounding atmosphere or directed at a target. This geometry enables treatment of thermally sensitive substrates, including living tissue, food surfaces, and polymer films, that cannot tolerate the conditions inside a sealed processing chamber.

Atmospheric-Pressure Plasma Jet Operation

Non-equilibrium atmospheric-pressure plasma jets (APPJs) are generated by applying radiofrequency or microwave power to a gas, typically helium or argon with trace admixtures of oxygen or nitrogen, flowing through a pair of electrodes or a dielectric tube. The resulting discharge creates a plasma plume that extends into ambient air, producing reactive oxygen and nitrogen species such as ozone, atomic oxygen, and hydroxyl radicals at room-temperature gas conditions. The IEEE Transactions review of the atmospheric-pressure plasma jet characterizes APPJs by their glow-discharge-like behavior, stability at atmospheric pressure, and the suite of active species they generate without requiring vacuum enclosures.

Plasma Chemistry and Reactive Species

The utility of plasma jets in many applications rests on the complex mix of reactive species they produce. Electron-impact dissociation and excitation of the carrier and feed gases generates short-lived atomic and molecular fragments that drive surface chemistry on the substrate. Ozone concentrations, nitric oxide yield, ultraviolet photon flux, and ion bombardment intensity can be adjusted by changing gas composition, flow rate, applied power, and electrode geometry. Reactions between plasma-generated species and biological or polymer surfaces produce oxidation, crosslinking, and functionalization without bulk heating. A review in Reviews of Modern Plasma Physics surveys how the reactive species inventory controls outcomes in surface treatment, sterilization, and biomedical applications.

Thermal Plasma Jets and Material Processing

Thermal plasma jets, also called plasma torches or arc-jet systems, operate at much higher energy densities. An arc discharge or induction-coupled discharge heats the carrier gas to temperatures of 6,000 to 20,000 K, producing a high-enthalpy jet capable of melting and spraying metallic or ceramic particles, cutting metal sheets, or synthesizing nanomaterials. The ScienceDirect overview of plasma jets places thermal jets in the context of plasma spray coating, waste destruction, and synthesis of refractory materials. The high thermal energy flux and the ability to process materials that cannot be handled in conventional furnaces make thermal plasma jets indispensable in heavy industry.

Applications

Plasma jets have applications across a broad range of disciplines, including:

• Plasma spray coating for thermal barrier and wear-resistant layers in aerospace and power generation
• Atmospheric-pressure plasma surface activation of polymers for adhesion improvement
• Sterilization and decontamination of medical instruments and packaging
• Wound healing and cancer therapy in plasma medicine
• Nanomaterial and nanoparticle synthesis via thermal plasma decomposition
• Food surface treatment for pathogen reduction and shelf-life extension