Ambient Pressure Cold Plasma Jets
The great American scientist Irving Langmuir was the first to use the term plasma in 1927 to describe the glowing, ionised state of matter, which hugged the contour of a glass vessel in which it was produced. Plasma is a fluid of electrons and ions and can take a variety of forms and be created using many methods. Most of the matter in the Universe is found in this form. The Sun, stars, nebulas, auroras and flashes of lightning are plasma manifestations. Neon signs which make our cities come alive at night and fluorescent lamps which light up our homes are artificial plasmas.
When plasma is produced by ionising a gas by an electrical discharge, like in a welding arc, the electrons absorb energy from the electric field and, being highly mobile, collide among themselves frequently and randomise their energy. Their temperature can reach many thousands of degrees Kelvin. When they collide with ions and neutral molecules, they transfer a small fraction (proportional to the electron-ion mass ratio) of their energy. Collisions being frequent at atmospheric pressure, the electrons and heavy particles reach a state of thermodynamic equilibrium, despite the low energy transfer fraction. As a result, all the species in the plasma remain at almost the same temperature. Such plasmas produced in arcs or lightning can reach temperatures over 10,000 degrees and are called the equilibrium of hot plasmas.
When the pressure is low, as in the Neon tube or fluorescent lamps, the collisions between electrons and heavier particles occur less frequently. The electrons remain hot while the heavier particles remain cold; there is no thermodynamic equilibrium. The electrons have a very high temperature (up to a few eV, 1 eV ≈ 11,600 K), whereas the temperature of heavy particles is relatively low. For this reason, they are called non-equilibrium or cold plasma. The presence of highly energetic electrons facilitates electron impact excitation, ionisation and dissociation of molecules at low gas temperatures. The presence of all these species makes cold plasmas chemically very active.
Can we make cold plasma in ambient atmospheric pressure? The trick is to realise that inelastic collisions with molecules drain the electron energy very efficiently. So if we switch off the electric field after the plasma is formed, the transfer of energy from electrons to neutrals can be switched off. This is called pulsed plasma. A repetitive train of pulses will create fresh bursts of short-lived plasma with energetic electrons and cold neutrals. This is one of the schemes invented by Plasma physicists for producing atmospheric pressure cold plasmas.
Cold plasma generated at atmospheric pressure contains reactive species like electrons, ions, free radicals, excited atoms, UV photons etc. These species drive gas-phase chemistry at a low gas temperature very efficiently. This has led to their finding use in fields that require low temperatures, such as biomedical applications and material processing. In recent years, many devices have been invented to produce cold plasma in ambient pressure and nearly room temperature (less than 40°C) at the contact zone.
One of the most versatile manifestations of the CAP plasma is the non-thermal ambient pressure plasma jets, not confined within electrodes or discharge tubes. As a result, objects, irrespective of their shape and size, can be directly exposed to the jet and receive active radicals and charged particles. Koinuma and colleagues first developed the CAP plasma jet in 1992 (1). They used an RF source for excitation.
Despite appearing homogenous to the naked eye, CAP plasma jet is discrete in nature when observed using fast imaging. This is because the plasma volume consists of discrete structures, which propagate at speeds of more than a few km/sec.
Another widely used technique for generating CAP plasma is the dielectric barrier discharge (DBD), excited by alternating or pulsed voltages with at least one of the two electrodes covered with a dielectric layer. The function of this layer is to limit the discharge current and thus suppress the transition of the glow discharge into a high current spark or arc discharge. The typical electrode gap in a DBD ranges from 0.1 mm to several centimetres. The dielectric materials used are glass, quartz, ceramics and polymers etc. They are powered by high voltage sources, which operate in frequencies in the kHz range. The geometries may be parallel plates or coaxial cylinders with a dielectric between them.
Fridman and co-workers (2) have developed a floating electrode DBD (FE-DBD). It consists of two electrodes: an insulated high voltage electrode and an active electrode. The difference between FE-DBD and DBD is that the second electrode is active, capacitively coupled to the ground. The second electrode can be the human body or any other target.
DBD plasma has a filamentary structure. However, DBDs can also produce homogeneous diffuse plasma. To make a glow DBD, a Townsend breakdown must be initiated instead of a streamer breakdown. The residual species from an earlier pulse of the applied voltage provide the seed electrons or enhance the initial field for the next discharge cycle. DBDs can be converted into plasma jets, powered by KHz ac, RF or pulsed dc sources.
A corona discharge is a crown-like glow around a sharp metallic object, like a pin. The sharp edge enhances the electric field near the electrode edge, with the field falling off with distance. The electric field strength is high enough to form a conductive region but insufficient to cause a full electrical breakdown of the gas. Instead, the gas breaks down near the sharp electrode and includes a non-equilibrium plasma.
A corona discharge can be powered by DC, AC, or pulsed voltage. It is ubiquitous in such applications as ozone synthesis, material processing, water purification, electro-photography, copier machine etc. This type of plasma provides substantial flexibility in treating various products and materials used in the medical industry; for example, syringe barrels, pill bottles, catheter tubing, IV tubes and surgical gowns etc. Cold plasma from corona discharge finds extensive use in a broad range of plasma processing applications.
Liberating cold plasmas from the confines of a vacuum chamber has dramatically impacted their industrial, environmental, agricultural and biological applications. They find extensive use in material processing, environmental remediation, nano-material synthesis, textile industry, food processing and biomedical applications etc. With the realisation of cold plasma at atmospheric pressure, high-speed processing of fibres, wovens and powders incompatible with vacuum processing becomes possible. Cold plasmas are permeating into newer and newer application areas, limited only by the innovator’s imagination.
References
“Cold Atmospheric Pressure Plasma Technology for Biomedical Application by Rakesh Ruchel Khanikar and Heremba Bailung” in https://www.intechopen.com/chapters/77718A is a detailed report on this topic.
1. Koinuma H, Ohkubo H, Hashimoto T, Inomata K, Shiraishi T, Miyanaga A and Ihayashi S (1992) Development and application of a microbeam plasma generator Appl. Phys. Lett. 60 816–817
2. Fridman G, Peddinghaus M, Ayan H, Fridman A, Balasubramanian M, Gutsol A, Brooks A and Friedman G (2006) Blood coagulation and living tissue sterilisation by floating-electrode dielectric barrier discharge in air Plasma Chem. Plasma Process. 26 425–442
