**3.1. Some fundamental aspects of plasma science**

A plasma is an ionized gas [123–131]. The term *plasma* was coined by Langmuir in the 1920's and it is derived from the ancient Greek word *πλάσμα* (plasma), freely translated to something "*moldable*". A plasma consists of ions (with ion number density n<sup>i</sup> ) and electrons (with an electron number density n<sup>e</sup> ), and on the average it is quasi-neutral, and for singly ionized gases ne ≈ni . Thus, a prerequisite for plasma formation is ionization. Singly-charge ionization (in the form of ion-electron pair formation) is done by detaching an electron from a neutral gaseous atom or molecule. Although there are other ways of detaching an electron (e.g., thermally), one way doing it is by placing a gas between two electrodes and by applying an electric field with a sufficiently field-strength to ionize the gas (**Figure 7**), thus forming an *electrical gas discharge*. Because neutral gaseous atoms or molecules (ordinarily insulators) become ion-electron pairs, they also become (partial) conductors. Partial because to an approximation, conductivity depends on the degree of ionization (this is important for weakly ionized plasmas).

To obtain electrical gas breakdown, the dielectric strength of the gas must be exceeded. The dielectric strength is the maximum electric field-strength (in V/m) an insulating gas can endure without breaking down into ions and electrons. If there is a sufficiently large field-strength, breakdown of the dielectric strength will cause formation of (typically) a low-current **spark** (i.e., a momentary electrical discharge, an example is electrostatic discharge from static electricity), or formation of a continuous electric-**arc** requiring continuous application of an electric field from an external power supply (**Figure 7**) capable of providing high-current (often in the Amp range). Arcs find applicability in welding of metals.

**Conditions for sustaining continuous plasma operation:** Following gas breakdown, there must be continuous application of external power to sustain a plasma. Other criteria include an electrode distance d that must be > > λD and that ne λ3 D must be > > 1 (this is easy to satisfy

**Figure 7.** Ideal plasma formed in a gas-tight and pressure-controlled enclosure. The plasma is formed between two conducting plates or electrodes positioned at a distance (or gap) d from each other. For dc operation, pertinent literature should be consulted [124].

for the plasmas of interest to this work), where λD is the Debye length [133–137]. These will be briefly discussed later in this section.

For microplasmas formed inside fluidic microchannels, in addition to gas breakdown and to continuous application of power, a microplasma must be formed in a constrained microchannel.
