Nonthermal plasma

A nonthermal plasma is in general any plasma which is not in thermodynamic equilibrium, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell-Boltzmann distribution.

Applications

In the context of food processing, a nonthermal plasma (NTP) is specifically an antimicrobial treatment being investigated for application to fruits, vegetables and other foods with fragile surfaces^[1]. These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. 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 here for more on plasma 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.

Nonthermal plasma also sees increasing use in the sterilization of teeth ^[2][3] and hands^[4].

Nomenclature

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", 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. An upcoming technology will add the capabilities of nonthermal plasma to dentistry.

Technologies

	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
References	Gadri et al., 2000. Surface Coatings Technol 131:528-542 Laroussi and Lu, 2005. Appl. Phys. Lett. 87:113902 Montie et al., 2000. IEEE Trans Plasma Sci 28:41-50	Lee et al., 2005. Surface Coatings Technol 193:35-38 Niemira et al., 2005. P2. IFT NPD Mtg., Wyndmoor, PA NIemira et al., 2005. P2-40. IAFP Mtg., Baltimore, MD Sladek and Stoffels, 2005. J Phys D: Appl Phys 38:1716-1721 Stoffels et al., 2002. Plasma Sources Sci. Technol. 11:383-388	Deng et al., 2005. Paper #056149, ASAE Ann. Mtg., Tampa, FL Kelly-Wintenberg et al., 1999. J. Vac. Sci. Technol. A 17(4):1539-44 Laroussi et al., 2003. New J Phys 5:41.1-41.10 Montenegro et al., 2002. J Food Sci 67:646-648 Niemira et al., 2005. P2. IFT NPD Mtg., Wyndmoor, PA NIemira et al., 2005. P2-40. IAFP Mtg., Baltimore, MD

References

1. ^ "Decontamination of Fresh Produce with Cold Plasma". U.S. Department of Agriculture. http://www.ars.usda.gov/research/projects/projects.htm?ACCN_NO=409114. Retrieved 2006-07-28. 
2. ^ "Plasma rips away tenacious tooth bacteria". 6/11/2009. http://www.rdmag.com/ShowPR.aspx?PUBCODE=014&ACCT=1400000101&ISSUE=0906&RELTYPE=PR&PRODCODE=00000000&PRODLETT=GD&CommonCount=0. Retrieved 2009-06-20. 
3. ^ Beth Dunham (June 5, 2009). "Cool plasma packs heat against biofilm". http://uscnews.usc.edu/science_technology/cool_plasma_pack_heat_against_biofilms.html. Retrieved 2009-06-20. 
4. ^ Eisenberg, Anne (2010-02-13). "Hospital-Clean Hands, Without All the Scrubbing". The New York Times. http://www.nytimes.com/2010/02/14/business/14novel.html?_r=1&ref=technology. Retrieved 2011-02-28.