Atmospheric-pressure plasma (or AP plasma or normal pressure plasma) is a plasma in which the pressure approximately matches that of the surrounding atmosphere – the so-called normal pressure.

Technical significance

Atmospheric-pressure plasmas have prominent technical significance because in contrast with low-pressure plasma or high-pressure plasma no

reaction vessel A chemical reactor is an enclosed volume in which a chemical reaction takes place. In chemical engineering, it is generally understood to be a process vessel used to carry out a chemical reaction, which is one of the classic unit operations in chem ...

is needed to ensure the maintenance of a pressure level differing from atmospheric pressure. Accordingly, depending on the principle of generation, these plasmas can be employed directly in the production line. The need for cost-intensive chambers for producing a partial vacuum as used in low-pressure plasma technology is eliminated.

Plasma generation

Various forms of excitation are distinguished: * AC (

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

) excitation * DC (

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...

) and low-frequency excitation * Excitation by means of

radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...

Microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

excitation Atmospheric-pressure plasmas that have attained any noteworthy industrial significance are those generated by DC excitation (

electric arc An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...

), AC excitation (

corona discharge A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone e ...

, dielectric barrier discharge, piezoelectric direct discharge and plasma jets as well as 2.45 GHz microwave microplasma).

Operating principle of a DC plasma jet

By means of a high-voltage discharge (5–15 kV, 10–100 kHz) a pulsed electric arc is generated. A process gas, usually oil-free compressed air flowing past this discharge section, is excited and converted to the plasma state. This plasma passes through a jet head to the surface of the material to be treated. The jet head determines the geometry of the beam, and is at earth potential to hold back potential-carrying parts of the plasma stream.

Operating principle of a microwave plasma jet

A microwave system uses amplifiers that output up to 200 watts of power radio frequency (RF) power to produce the arc that generates plasma. Most solutions work at 2.45 GHz. A new technology provides ignition and highly efficient operation with the same electronic and couple network. This kind of atmospheric-pressure plasmas is different. The plasma is only top of the electrode. That is the reason the construction of a cannula jet was possible.

Applications

Manufacturers use plasma jets for, among other things, activating and

cleaning Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning is often performed for aesthetic, hygienic, functional, environmental, or safety purposes ...

plastic and metal surfaces to prepare them for adhesive bonding and painting. Sheet materials up to several meters wide can be treated today by aligning a number of jets in a row. Surface modification achieved by plasma jets is comparable to the effects obtained with low-pressure plasma. Depending on the power of the jet, the plasma beam can be up to 40 mm long and attain a treatment width of 15 mm. Special rotary systems allow a treatment width per jet tool of up to 13 cm.Buske C., Förnsel P.: Vorrichtung zur Plasmabehandlung von Oberflächen (Device for the plasma treatment of surfaces); EP 0986939 Depending on the required treatment performance, the plasma source is moved at a spacing of 10–40 mm and at a speed of 5–400 m/min relative to the surface of the material being treated. A key advantage of this system is it can be integrated in-line in existing production systems. In addition the activation achievable is distinctly higher than in potential-based pretreatment methods (corona discharge). It is possible to coat varied surfaces with this technique. Anticorrosive layers and

adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can b ...

promoter layers can be applied to many metals without solvents, providing a much more environmentally friendly solution.

References

;Citations {{refimprove, date=June 2019 ;Bibliography * Tendero C., Tixier C., Tristant P., Desmaison J., Leprince P.: Atmospheric pressure plasmas: A review; Spectrochimica Acta Part B: Atomic Spectroscopy; Volume 61, Issue 1, January 2006, pp 2–30. * Förnsel P.: Vorrichtung zur Oberflächen-Vorbehandlung von Werkstücken (Device for surface pretreatment of workpieces); DE 195 32 412
EU-IP4Plasma e-learning portal: Basic facts on the fourth state of matter and its technical use

Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung (IFAM): Plasmatechnik und Oberflächen (Plasma technology and surfaces) – PLATO

Leibniz Institute for Plasma Science and Technology (INP Greifswald e.V.)

* ttps://www.relyon-plasma.com/technology/paa/?lang=en Pulsed Atmospheric Arc Technology
Atmospheric Plasma Treatment Explained in Simple Terms from the UK manufacturer Henniker Plasma

Plasmatreat US LP: Atmospheric Plasma Treatment

Basics of microwave driven atmospheric plasma
Plasma physics