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

Abstract

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.

Ionization Mechanism in a Thermal Spark Discharge

“…The electron number density, measured by Stark broadening of N + , reached ne = 9×10 18 cm -3 , representing an ionization of 36%. Such high electron number densities and temperatures were also measured by other teams in pin-to-pin configurations [10][11][12][13][14] and nanosecond Dielectric Barrier Discharges [15,16].…”

“…However, no filter was used and the measurement of Orriere et al is an upper bound of their thermal spark diameter. Finally, in a 25-kV discharge of 40 ns at ambient conditions, Parkevich et al measured the ne distribution of a thermal spark by interferometry [14]. Their discharge looked homogeneous with conventional lenses but was indeed composed of several fully ionized filaments, 10 -50 μm in diameter.…”

“…Numerical parameters N2 (cm -3 ) O2 (cm -3 ) N2 + (cm -3 ) e-(cm -3 ) Tgas (K) Te (K) Time step (ps) 1.90×10 19 5.06×10 18 1.00×10 14 The applied voltage on the 1.2-mm gap is represented in Figure 10 and was reconstructed from the anode and cathode pulse shapes. The reduced field, E/N, is calculated assuming a Laplacian field between the electrodes.…”

Role of the excited electronic states in the ionization of ambient air by a nanosecond discharge

“…This mechanism explains in our opinion the fast transition (< 1 ns) from a non-equilibrium to a thermal plasma as experimentally observed with nanosecond discharges in Refs. [17,21,43]. Ionization rates of N2(X, A, B, C, a').…”

Динамика Пространственной Структуры Микросекундного Импульсного Барьерного Разряда В Воздухе Атмосферного Давления В Геометрии Острие-Плоскость При Различных Полярностях Питающего Напряжения

“…С другой стороны, на микроуровне импульсные разряды могут обладать структурой даже при кажущейся внешней однородности свечения канала. Так, в ряде работ с использованием метода теневого фотографирования высокой разрешающей способности, а также метода автографов было установлено, что внешне однородный канал искрового и диффузного разрядов в воздухе представляет собой пучок большого количества микроканалов [16][17][18][19][20][21]. При этом автографы (отпечатки) таких разрядов на поверхности электрода или его покрытия представляют собой скопления множества микрократеров.…”