Anode and Cathode Spots in High-Voltage Nanosecond-Pulse Discharge Initiated by Runaway Electrons in Air

Nanosecond pulsed streamer discharge has unique characteristics that differentiate it from longer discharges. The very fast voltage rise time, peak voltage plateau, short pulse duration, and fast fall time enable a large volume of uniform nonthermal plasma generation at atmospheric pressure. This review explains the physics of nanosecond discharge plasma through experimental and simulated studies for plasma processing techniques. The following are discussed and compared between sub-microsecond and nanosecond discharge plasma: discharge phase transition, discharge propagation, production of chemically active species, temperature change of gas during plasma propagation, electrode geometry, effect of voltage rise rate, voltage polarities, and N 2 /O 2 gas composition ratio in air seeding gas. Nanosecond pulse discharge plasma is characterized by a considerably faster streamer head propagation velocity and reduced gas heating, resulting in a higher energy efficiency for plasma processing. Ozone generation, nitric oxide treatment and volatile organic compound treatment results are given as examples of plasma processing.

“…Most studies on runaway electrons are related to electron beam generation and x-ray generation. Some studies report on runaway electrons at atmospheric pressure [77][78][79][80].…”

Section: Comparison Between Sub-microsecond and Nanosecond Discharge ...mentioning

confidence: 99%

Nanosecond pulsed streamer discharges: II. Physics, discharge characterization and plasma processing

Wang

Namihira

2020

“…Here the material and surface quality of the anode are essential to the spot generation as are different dielectric films [50,51] at the anode surface. In the case of the gaps with high strengths of the applied electric field, the anode spots may be generated by high-energy electron beams [14,15] incident on the anode surface. However, in our case, the factors mentioned above are insufficient to completely explain the extremely fast (=1 ns) formation of the anode spots.…”

Section: Enigma Of Anode Spotsmentioning

confidence: 99%

“…A key role in the development of the spark channels belongs to a highly ionized plasma, which is initially generated in micron-sized (∼1-10 μm) regions at the electrodes on time scales of 1 ns [7][8][9][10][11]. Generally, such plasma is observed in the discharge in the form of rapidly evolving cathode and anode spots [11][12][13][14][15] which then give rise to the spark channels. There are different theories describing the spot generation.…”

Section: Introductionmentioning

confidence: 99%

Extremely fast formation of anode spots in an atmospheric discharge points to a fundamental ultrafast breakdown mechanism

Parkevich¹,

Medvedev

Khirianova

et al. 2019

By employing multi-frame laser interferometry, shadow, and schlieren imaging, we trace the formation of a nanosecond spark discharge in millimeter-sized air gaps formed by a point cathode and flat anode or vice-versa. We discover that the electrical breakdown of the discharge gap is associated with extremely fast (=1 ns) explosive formation of micron-sized cathode and anode spots. We find that the characteristic delay between the instants of the anode and cathode spot initiation can be much shorter than 1ns. The spots appear as highly ionized near-electrode plasmas with an electron density n e ∼10 19 -10 20 cm −3 . The spots then give rise to highly ionized spark channels with pronounced filamentary structures. Our findings indicate that the extremely fast formation of anode spots is associated with an ultrafast gap breakdown promoted by an ultrafast ionization wave (UFIW). The role of the UFIW governed by the rapidly evolving cathode spot is discussed as a fundamental mechanism of the breakdown.

“…This is also the case for the processes [15][16][17][18][19] responsible for discharge structurization, due to which the discharge may develop nonuniformly, acquiring a filamentary structure [20,21] that greatly influences the discharge parameters. Notably, there is a number of proofs for the fact that the filamentary structure is inherent not only to the entire spark discharge but even to a single spark channel [22][23][24][25][26]. The spark can appear as multiple (∼10-100) micron-sized (∼10 μm) filaments, which, in turn, govern the parameters of the channel.…”

Section: Introductionmentioning

confidence: 99%

Fast fine-scale spark filamentation and its effect on the spark resistance

Parkevich

Medvedev

Ivanenkov

et al. 2019

Formation of a millimeter-sized spark discharge in ambient air is traced on a nanosecond time scale using multi-frame laser probing with an exposure time of 70 ps and spatial resolution of 3-4 μm. The discharge is initiated by a 25 kV voltage pulse with a rise time of 4 ns, with the pulse applied to the gap formed by a point cathode and flat anode. It is demonstrated that the gap breakdown is accompanied by the fast (∼1 ns) formation of a highly ionized homogeneous spark channel originating from the point cathode. We discover that the fast fine-scale filamentation of the homogeneous spark channel arises several nanoseconds after the breakdown and at some distance from the cathode, which results in a complex filamentary structure of the channel. We find that the growing spark channel, in fact, develops in the form of multiple (N  10) rapidlyevolving filaments that constitute micron-sized (∼10-50 μm) plasma channels with an electron density of -ñ 10 10 e 19 20 cm −3 and subnanosecond characteristic evolution time. First filaments appear at the top of the developing homogenous spark channel. Further, the growing filaments are split themselves, and their number is increased over time up to several tens. Our findings indicate that the fast fine-scale filamentation is one of the important mechanisms governing the spark channel resistance after the breakdown.