Plasma Science of Thermal, Atmospheric Pressure discharges
In general, plasmas generated for water purification (in contact with or within liquid water) are classified as cold, non equilibrium discharges. Here we refer to plasmas produced at atmospheric pressure where the electron temperature is much greater than the heavy particle temperature, which is typically near room temperature. In such plasmas, electrical energy is channeled primarily to the electrons. Electrons via collisions excite the air, input gas or water vapour, generating copious amounts of UV and reactive oxygen and nitrogen species.
Such plasmas have a host of applications ranging from environmental hazard mitigation to plasma medicine to the treatment of soft media. In general, the application of the sparking potential across an air gap (~ 30 kV/cm in 1 atm air) will result in a breakdown that transitions from avalanche to arc or spark if the applied voltage duration is sufficiently long. This transition from non-thermal ionisation front to thermal discharge is a consequence of the thermal instability.
Here as the avalanche builds, the electron neutral collision frequency increases, leading to gas heating and rarefaction of the gas, which in turn leads to an increased electric field to neutral density ratio (E/N). The increased E/N leads to greater electron energy gains between collisions that thus an associated increase in electron temperature.
This increase in electron temperature increases the ionisation rate, resulting in a runaway condition where through increases in the electron neutral collision frequency, the electron and gas temperatures merge. This so called thermal instability leads ultimately to the formation of the arc discharge.
To achieve a cold, non-thermal plasma this transition must be controlled. The transition can be mitigated a number of ways:
Ballasting
Use of dielectric barriers, use of fast rise time, limited duration (ns) voltage pulses
Control of duty cycle
Use of electron beams
Modification of gas composition
Increased flow rate to enhance cooling.
Our method is the use of fast rise time, high voltage pulses that terminate (typically nanosecond to microsecond widths) before the avalanche develops into a spark.
