[Paper Review] Experimental study of atmospheric pressure single-pulse nanosecond discharge in pin-to-pin configuration

Xingxing Wang, Adam Patel+2 more|arXiv (Cornell University)|Jun 16, 2021

Plasma Applications and DiagnosticsMedicine60 references21 citations

TL;DR

This experimental study investigates single-pulse nanosecond discharges in pin-to-pin configurations at atmospheric pressure, using multi-diagnostic techniques to characterize spark and corona regimes. Key findings show that spark discharges at gaps <6 mm and 0.6–1 mJ/mm energy yield peak electron density (7.5×10¹⁵ cm⁻³), current (22 A), and gas temperature (4,000 K), with 85–89% of energy deposited before secondary breakdown, while corona discharges at larger gaps use ~0.1 mJ/mm and exhibit significantly lower parameters.

ABSTRACT

In this work, we present an experimental study of nanosecond high-voltage discharges in a pin-to-pin electrode configuration at atmospheric conditions operating in single-pulse mode (no memory effects). Various discharge parameters, including voltage, current, gas density, rotational/vibrational/gas temperature, and electron number density, were measured. Several different measurement techniques were used, including microwave Rayleigh scattering, laser Rayleigh scattering, optical emission spectroscopy enhanced with a nanosecond probing pulse, fast photography, and electrical parameter measurements. Spark and corona discharge regimes were studied with discharge pulse duration of 90 ns and electrode gap sizes ranging from 2 to 10 mm. The spark regime was observed for gaps < 6 mm using discharge pulse energies of 0.6-1 mJ per mm of the gap length. Higher electron number densities, total electron number per gap length, discharge currents, and gas temperatures were observed for smaller electrode gaps and larger pulse energies, reaching maximal values of about 7.5x10^15 cm-3, 3.5x10^11 electrons per mm, 22 A, and 4,000 K (at 10 us after the discharge), respectively, for a 2 mm gap and 1 mJ/mm discharge pulse energy. Initial breakdown was followed by a secondary breakdown occurring about 30-70 ns later and was associated with ignition of a cathode spot and transition of the discharge to cathodic arc. A majority of the discharge pulse energy was deposited into the gas before the secondary breakdown (85-89%). The electron number density after the ns discharge pulse decayed with a characteristic time scale of 150 ns governed by dissociative recombination and electron attachment to oxygen mechanisms. For the corona regime, substantially lower pulse energies (~0.1 mJ/mm), peak conduction current (1-2 A), and electron numbers (3-5x10^10 electrons per mm), and gas temperatures (360 K) were observed.

Motivation & Objective

• To characterize single-pulse nanosecond discharges in pin-to-pin electrode geometry at atmospheric pressure, free from memory effects.
• To compare spark and corona discharge regimes across varying electrode gaps (2–10 mm) and pulse energies.
• To measure and analyze key plasma parameters including electron number density, gas temperature, current, and energy deposition dynamics.
• To determine the temporal evolution and decay mechanisms of electron density post-pulse, identifying dominant recombination processes.
• To establish baseline data for future studies on nanosecond repetitively pulsed (NRP) discharges by isolating single-pulse behavior.

Proposed method

• Used a 90 ns high-voltage nanosecond pulser (25 kV peak) with 1 Hz repetition rate to ensure no memory effects.
• Employed microwave Rayleigh scattering for electron number density and gas density measurements.
• Applied laser Rayleigh scattering and optical emission spectroscopy (OES) with nanosecond probing to determine rotational (Trot) and vibrational (Tvib) temperatures.
• Used fast photography and current/voltage probes (3 GHz bandwidth) to capture discharge dynamics and conduction current.
• Measured conduction current by subtracting displacement current (calculated from dV/dt and capacitance) from total current.
• Conducted experiments at 2–10 mm gap distances with adjustable pulse energy via series resistors (200–400 Ω).

Experimental results

Research questions

• RQ1 What are the key differences in plasma parameters between spark and corona discharge regimes in single-pulse nanosecond discharges at atmospheric pressure?
• RQ2 How does the energy deposition profile evolve during the discharge, particularly the fraction deposited before secondary breakdown?
• RQ3 What are the dominant electron loss mechanisms governing the decay of electron number density after the pulse?
• RQ4 How do discharge parameters (current, electron density, temperature) scale with electrode gap distance and pulse energy?
• RQ5 At what repetition frequency do memory effects begin to emerge, based on recovery times of gas density and temperature?

Key findings

• For spark discharges at 2 mm gap and 1 mJ/mm pulse energy, peak electron number density reached 7.5×10¹⁵ cm⁻³, current peaked at 22 A, and gas temperature reached 4,000 K at 10 μs after discharge.
• In the spark regime, 85–89% of the discharge pulse energy was deposited into the gas before the secondary breakdown, which occurred 30–70 ns after initial breakdown.
• Secondary breakdown was linked to cathode spot ignition and transition to a cathodic arc, indicating a need to limit pulse duration to 30–70 ns to avoid this regime.
• Electron density decay was governed by dissociative recombination and electron attachment to oxygen, with a characteristic decay time of 150 ns.
• For corona regime at 8–10 mm gaps, peak conduction current was only 1–2 A, electron number density was ~3–5×10¹⁰ electrons per mm, and gas temperature was 360 K.
• The onset of memory effects in nanosecond repetitively pulsed (NRP) operation is expected for pulse repetition frequencies >1 kHz, based on recovery times of gas density and temperature.