All atmospheric pressure plasmas either result from electric arc discharges that are intentionally created, producing thermal plasmas, or have a means to suppress or inhibit arcing in order to form a non-thermal plasma. The most common forms of atmospheric pressure plasmas are illustrated below.

Dielectric Barrier Discharge (also called "silent discharge")
Invented in the late 1800s, this broad class of plasma source has an insulating (dielectric) cover over one or both of the electrodes and operates with high voltage power ranging from low frequency AC to 100 KHz. A multitude of random, numerous arcs forms between the two electrodes. The purpose of the dielectric is to extinguish the arcs after they form by charging the surface. These discharges may use helium for a more uniform appearance, but they can also produce ozone. Ozone is useful in some applications, such as water treatment, but it would need to be properly contained and exhausted to avoid hazardous exposures for employees. The principal disadvantage is that they produce a relatively low average density of atomic species because most of the active atoms are produced inside the narrow confines of an arc and are rapidly lost to recombination. Regardless of what common or trade name is applied, if a dielectric film is needed for operation, the plasma source is considered a dielectric barrier discharge. This form of discharge is often confused with corona discharges.

Corona Discharge
Used extensively over the past 100 years, corona discharges result from the high electric field that surrounds a sharply pointed electrode powered with high continuous or pulsed DC voltages. The high electric field causes ionization of air or other gases, and a weak plasma is created. These discharges are about 10X lower in density of active species than the dielectric barrier discharge but have the advantage of an arc-free process. They are inherently non-uniform and are often used for treating polymeric films to avoid the tendency of the dielectric barrier discharge to punch holes in the films.

Microwave Plasmas
Microwave devices provide a means to ionize at lower power than with radio frequency (RF) or DC/AC power, but the much smaller wavelength also means that the plasma will be less uniform than an RF plasma and these plasmas are hampered with scale-up problems. Microwaves are also prone to substrate heating problems and are prone to non-uniform processing. Microwave plasmas are either smaller, highly localized discharges, or are thermal plasmas which are unsuitable for material processing applications. Microwave plasmas are frequently used in vacuum-based technology as a downstream method because they provide higher dissociation rates than RF-based vacuum plasmas. Users of microwave plasmas should be vigiliant about health issues caused by exposure to microwave radiation.

Atmospheric Pressure Plasma Jet (APPJ)
First patented in 1999 by the founders of APJeT, this unique source has no dielectric cover and is capable of producing a chemical flux 100x greater than a dielectric barrier discharge and 1000x greater than a corona discharge. The use of helium, to inhibit arcing, the absence of the dielectric cover, the gap spacing between the electrodes and the RF frequencies used for powering the discharge are all patented aspects of this technology. Many different source designs have been demonstrated, and power densities of up to 500 W/cm³ have been demonstrated with gas temperatures below 150°C. Substrates may be treated using a downstream approach or they may be placed directly inside the discharge.