EXPERIMENTAL STUDY OF HIGH-ENERGY SINGLE-PULSE NANOSECOND DISCHARGES IN PIN-TO-PIN CONFIGURATION

Nanosecond pulsed discharges are versatile tools in plasma science, with applications ranging from plasma-assisted combustion to flow control. However, most prior studies have examined relatively low pulse energies (typically <10 mJ per pulse, <1 MW peak power), and significantly less data is available on high-energy nanosecond discharges (>10 mJ per pulse, >1 MW peak power). Such high-energy pulses create extreme plasma conditions – including elevated temperature, high degrees of ionization, and ultrafast gas heating – that necessitate detailed time-resolved diagnostics. The present work addresses this gap by experimentally investigating a high-energy nanosecond spark discharge in a pin-to-pin electrode configuration in open air. A comprehensive suite of measurements was employed to capture the discharge’s rapid evolution, including electrical waveform characterization, fast optical imaging via intensified charge-coupled device (ICCD), vibrational N₂ temperature using coherent anti-Stokes Raman scattering (CARS), and electron density distribution via optical interferometry. The discharge was generated between two tungsten electrodes (3–7 mm separation) by a 20 kV, 11ns FWHM pulse (~39.5 mJ pulse energy) from a nanosecond generator. The discharge current was recorded with a calibrated back current shunt (BCS) installed in the transmission line, enabling calculation of the discharge voltage, and the deposited power and cumulative energy. A gated ICCD camera captured the plasma emission, providing time-resolved images of streamer initiation and spark channel formation. The nitrogen vibrational temperature in the plasma was measured using picosecond CARS (probing a ~1 mm region of the channel), and a Michelson interferometer with a 532 nm continuous-wave laser was utilized to determine the electron number density from phase shifts in the interferograms caused by the plasma. The high-speed electrical diagnostics showed that breakdown occurred about 5–8 ns after the pulse, resulting in large current surges on the order of several hundred amperes. Peak current values reached ~400 A for 3 mm and 5 mm gaps and about 200 A for a 7 mm gap, reflecting the greater energy dissipation in the larger gap. Time-gated ICCD images captured a two-stage discharge development: an initial sub-nanosecond streamer phase followed by the main spark channel. Streamers were observed emanating from both electrodes and converging near the gap center to establish a continuous plasma path. The cathode-directed streamer expanded with an approximately spherical front, while the anode-directed streamer was highly branched; their meeting point formed a constricted “hourglass” junction in the early plasma channel. After this merger, the resulting spark channel rapidly intensified, with the brightest initial emission along the cathode-originating segment, whereas the anode-side streamer introduced branching and shot-to-shot path variability. Notably, larger gaps (e.g. 7 mm) produced more pronounced streamer branching and less axial symmetry prior to breakdown, consistent with the lower average electric field across a longer gap. CARS measurements revealed very high initial N₂ vibrational temperatures in the spark channel immediately after the discharge, followed by a rapid decay within the first ~500 ns as vibrational energy relaxed into translational modes (ultrafast gas heating). Thereafter, the vibrational temperature approached a plateau, indicating partial equilibration with the gas thermal state. Spatially, the peak vibrational temperature was higher near the anode than at mid-gap or near the cathode, implying asymmetric energy deposition along the channel (more intense vibrational excitation toward the positive electrode). Optical interferometry provided further insight into the plasma density in the channel. Electron number densities on the order of - were measured in the early stages of discharge, depending on the gap length and time after breakdown. In all cases, the electron density was maximum adjacent to the cathode and decreased toward the anode, consistent with a cathode-initiated ionization wave in which the cathode supplies seed electrons that propagate downstream. Shorter gaps yielded higher peak electron densities than longer gaps, reflecting the greater energy density deposited in a smaller volume. These trends in plasma density and vibrational temperature underscore the strong influence of pulse energy and electrode geometry on the discharge’s thermodynamic and kinetic properties. In conclusion, this experimental study provides a cohesive, time-resolved picture of how a high-energy nanosecond pin-to-pin discharge evolves, from its fast streamer-induced breakdown to the formation of a hot, dense plasma channel. The results highlight the unique features of high-energy pulses – notably, the swift breakdown, intense localized emission, rapid energy transfer to heat, and strong spatial gradients in excitation and ionization – which distinguish them from lower-energy nanosecond discharges. These findings serve to bridge the knowledge gap in the high-energy regime and offer valuable benchmarks for the development of plasma-based technologies. In particular, the insights into streamer dynamics, energy deposition, and plasma parameter profiles can inform the design of high-power plasma systems for applications such as flow control and high-speed combustion ignition, where understanding the behavior of energetic nanosecond discharges is critical.