In atmospheric-pressure dielectric barrier discharge plasma, mostly are generated by high voltage AC source. May I know what is the technical reason of AC source preferred over DC? Thank you :)
AC is used due to reactor's impedance (capacitive character). In DC sources one would have to limit the discharge current using high voltage resistors, which limits the power and efficiency.
The problem is in the presence of the dielectric barrier. With a DC source you can achieve the gas breakdown, but then, the charge carriers (electrons and ions) get trapped on the dielectric (it is non-conducting, they cannot get through) and their electric field shield the electric field from the external source. As a consequence, the gas in the discharge gap does not feel any electric field anymore and the discharge dies. If you want another round of discharges, you need to change the external voltage, either increase to get some field in the same direction, or decrease - in that case the field by the charge carriers trapped on the dielectric barrier is often sufficient to ignite a backwards discharge (that is how pulsed-DC DBD's work). The simplified idea of DBD's is having a gap covered by dielectrics and making the electrons jump from one dielectric surface to the other by switching the direction of the applied electric field.
Jan answer is correct, more detailed explanations you can find in open access article Article The Equivalent Circuit Approach for the Electrical Diagnosti...
I am working with point to plane electrode type of discharges. We have realised that AC has a lower energy consumption as compared to DC (for the same application) by a factor of 10 to 100 depending on other variables (electrode gap, water conductivity etc). I agree with Marcin Holub
Also an interesting article on the use of discharges for waste disinfection notes a 68% power reduction when using AC (DOI: 10.1021/acscentsci.9b00769)
When you mention AC, did you mean to imply a sine wave? There are many forms of alternating waveforms (sine, triangle, sawtooth, square) When you mentioned DC, did you mean continuous DC or pulsed DC? The wave shape and the power supply source impedance are of significance when discussing DBD reactors and discharges.
It also depends on the air or water environment in which you do research.
A simple answer: overcharge. On AC, one gets an overcharge of at least one of the electrodes in virtually any DBD setup (I assume you mean fixed electrodes, and not a floating electrode one).
You can use something like DC welding with adjustable AC current to achieve non-thermal plasma.