A Beam of Reactive Chemistry
An atmospheric plasma jet is deceptively simple: noble gas flows through a quartz tube with internal electrodes, driven by kilohertz-frequency high voltage. Yet this simple device produces a remarkable plume — a centimeters-long beam of cold, reactive plasma that can be directed precisely onto biological surfaces. The plume carries a rich cocktail of reactive oxygen and nitrogen species (RONS) at near-body temperature, making it a precision tool for medicine, surface treatment, and materials processing.
Non-Equilibrium Physics
The key to cold plasma jets is non-equilibrium: electrons are heated to tens of thousands of kelvin by the electric field, while heavy particles (ions, neutrals) remain near room temperature. The hot electrons drive dissociation and ionization reactions that produce RONS, but the bulk gas stays cool enough to touch. This two-temperature physics — enabled by the low collision frequency at atmospheric pressure for light electrons — is what distinguishes cold plasma from thermal arcs and torches.
Plume Dynamics
The visible plasma plume is actually a series of fast-propagating ionization wavefronts called 'plasma bullets' that travel at 10–100 km/s. High-speed imaging reveals these discrete structures within what appears to the naked eye as a continuous glow. The plume length depends on voltage (driving ionization), gas flow (transporting reactive species), and nozzle geometry (shaping the flow field). Helium jets produce longer plumes than argon due to higher ionization efficiency and lower gas density.
Biomedical Applications
Plasma jets are the workhorse of plasma medicine. The kINPen MED — a CE-certified helium plasma jet — is used clinically for wound sterilization and healing acceleration. Researchers use jets for cancer cell treatment (selective apoptosis), dental applications (cavity sterilization and bleaching), and dermatology (acne treatment and skin rejuvenation). The ability to tune RONS output through voltage, gas, and geometry makes plasma jets uniquely adaptable to diverse therapeutic needs.