A nonthermal plasma, cold plasma or non-equilibrium plasma is a plasma which is not in thermodynamic equilibrium, because the electron temperature is much hotter than the temperature of heavy species (ions and neutrals). As only electrons are thermalized, their Maxwell-Boltzmann velocity distribution is very different than the ion velocity distribution. When one of the velocities of a species does not follow a Maxwell-Boltzmann distribution, the plasma is said to be non-Maxwellian.
A kind of common nonthermal plasma is the mercury-vapor gas within a fluorescent lamp, where the "electron gas" reaches a temperature of 20,000 kelvins while the rest of the gas, ions and neutral atoms, stays barely above room temperature, so the bulb can even be touched with hands while operating.
In the context of food processing, a nonthermal plasma (NTP) or cold plasma is specifically an antimicrobial treatment being investigated for application to fruits, vegetables and other foods with fragile surfaces. These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. While the applications of nonthermal plasma were initially focused on microbiological disinfection, newer applications such as enzyme inactivation, protein modification and pesticide dissipation are being actively researched. Nonthermal plasma also sees increasing use in the sterilization of teeth and hands, in hand dryers as well as in self-decontaminating filters.
The term cold plasma has been recently used as a convenient descriptor to distinguish the one-atmosphere, near room temperature plasma discharges from other plasmas, operating at hundreds or thousands of degrees above ambient (see Plasma (physics) § Temperatures). Within the context of food processing the term "cold" can potentially engender misleading images of refrigeration requirements as a part of the plasma treatment. However, in practice this confusion has not been an issue. "Cold plasmas" may also loosely refer to weakly ionized gases (degree of ionization < 0.01%).
The nomenclature for nonthermal plasma found in the scientific literature is varied. In some cases, the plasma is referred to by the specific technology used to generate it ("gliding arc", "plasma pencil", "plasma needle", "plasma jet", "dielectric barrier discharge", "Piezoelectric direct discharge plasma", etc.), while other names are more generally descriptive, based on the characteristics of the plasma generated ("one atmosphere uniform glow discharge plasma", "atmospheric plasma", "ambient pressure nonthermal discharges", "non-equilibrium atmospheric pressure plasmas", etc.). The two features which distinguish NTP from other mature, industrially applied plasma technologies, is that they are 1) nonthermal and 2) operate at or near atmospheric pressure.
|NTP technology class|
|I. Remote treatment||II. Direct treatment||III. Electrode contact|
|Nature of NTP applied||Decaying plasma (afterglow) - longer lived chemical species||Active plasma - short and long-lived species||Active plasma - all chemical species, including shortest lived and ion bombardment|
|NTP density and energy||Moderate density - target remote from electrodes. However, a larger volume of NTP can be generated using multiple electrodes||Higher density - target in the direct path of a flow of active NTP||Highest density - target within NTP generation field|
|Spacing of target from NTP-generating electrode||Approx. 5 – 20 cm; arcing (filamentous discharge) unlikely to contact target at any power setting||Approx. 1 – 5 cm; arcing can occur at higher power settings, can contact target||Approx. ≤ 1 cm; arcing can occur between electrodes and target at higher power settings|
|Electrical conduction through target||No||Not under normal operation, but possible during arcing||Yes, if target is used as an electrode OR if target between mounted electrodes is electrically conductive|
|Suitability for irregular surfaces||High - remote nature of NTP generation means maximum flexibility of application of NTP afterglow stream||Moderately high - NTP is conveyed to target in a directional manner, requiring either rotation of target or multiple NTP emitters||Moderately low - close spacing is required to maintain NTP uniformity. However, electrodes can be shaped to fit a defined, consistent surface.|
|Examples of technologies||Remote exposure reactor, plasma pencil||Gliding arc; plasma needle; microwave-induced plasma tube||Parallel plate reactor; needle-plate reactor; resistive barrier discharge; dielectric barrier discharge|
Magnetohydrodynamic power generation, a direct energy conversion method from a hot gas in motion within a magnetic field was developed in the 1960s and 1970s with pulsed MHD generators known as shock tubes, using non-equilibrium plasmas seeded with alkali metal vapors (like caesium, to increase the limited electrical conductivity of gases) heated at a limited temperature of 2000 to 4000 kelvins (to protect walls from thermal erosion) but where electrons were heated at more than 10,000 kelvins.
Aerodynamic active flow control solutions involving technological nonthermal weakly ionized plasmas for subsonic, supersonic and hypersonic flight are being studied, as plasma actuators in the field of electrohydrodynamics, and as magnetohydrodynamic converters when magnetic fields are also involved.
Studies conducted in wind tunnels involve most of the time low atmospheric pressure similar to an altitude of 20–50 km, typical of hypersonic flight, where the electrical conductivity of air is higher, hence non-thermal weakly ionized plasmas can be easily produced with a fewer energy expense.
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