Nanosecond pulsed atmospheric glow discharges without dielectric barriers

Summary form only given. Pulsed excitation is widely believed to offer an additional option in the quest to achieve superior performance of atmospheric pressure glow discharges (APGD), compared to the mainstream sinusoidally excited APGD. For example, very large applied voltage can be used over very short period of time to produce abundant and highly energetic electrons. Yet inherently pulsed excitation has a voltage-off period and this can reduce the averaged dissipated power of APGD below what is typical with sinusoidal excitation. Hence pulsed excitation allows APGD to deliver highly reactive plasma species with very high electrical energy efficiency. In the 1-300 kHz range, electrode insulation using dielectric barriers has long been considered as an essential condition for generating glow discharges at atmospheric pressure. For APGD to be deployed for material processing on an industrial scale, it is often desirable to remove the electrode-insulating dielectric layers that may become contaminated from continuous and heavy usage. From the standpoint of APGD physics on the other hand, it is also of significant interest to explore whether the removal of the dielectric barriers may bring about new and beneficial spatiotemporal behaviors for more superior surface treatment. In this contribution, we report the experimental observation of a versatile barrier-free APGD achieved with nanosecond excitation voltage pulses at 1 kHz instead of the usual sinusoidal excitation. The barrier-free mode of operation is shown to be viable over a very wide range of system parameters. Temporal behaviors of such barrier-free APGD are shown to be different from all other APGD reported so far, including pulsed atmospheric dielectric-barrier discharges. Pulsed barrier-free atmospheric plasma can achieve six current pulses every voltage pulse, suggesting high electrical energy efficiency. Also interesting is the observation that pulsed barrier-free APGD support both cathode and anode sheaths, even though the excitation is made of unipolar voltage pulses. In terms of plasma chemistry, they are also capable of producing a high flux of oxygen atoms. These desirable features are possible because of a unique combination of pulsed excitation and barrier-free operation