Development of Fast Ionization Wave Discharges at High Pulse Repetition Rates

Abstract

Positive and negative polarity fast ionization wave discharges in nitrogen and dry air are studied in a rectangular geometry channel over a wide range of time delays between the discharge pulses, τ = 10 ms to 25 μs (pulse repetition rates of ν = 100 Hz–40 kHz). The discharge is generated by alternating polarity high-voltage pulses (peak voltage 18–23 kV, pulse duration 50–60 ns). Ionization wave speed and electric field distributions in the discharge are measured by a calibrated capacitive probe. Plasma images show that for long time delays between the pulses, above τ ~ 1 ms (pulse repetition rate up to ν ~ 1 kHz), the positive polarity wave propagates along the channel walls and the negative polarity wave tends to propagate along the centerline. For time delays below τ ~ 0.5–1.0 ms (above ν ~ 1–2 kHz), both positive and negative polarity waves become diffuse, while wave speed and peak electric field in the wave front become nearly independent of the polarity. Wave speed remains nearly constant for τ ~ 1–10 ms (up to ν ~ 1 kHz), 0.8–0.9 cm/ns, and decreases for shorter time delays, by 30–40 % at τ = 25 μs (ν = 40 kHz). Peak electric field in the positive polarity wave decreases significantly as the pulse repetition rate is increased, from 2.7–3.0 kV/cm to 1.2–1.5 kV/cm. Comparison of experimental results in nitrogen with kinetic modeling calculations at high pulse repetition rates, when the discharge is diffuse, shows good agreement. Calculation results show that peak electric field reduction at high pulse repetition rates is mainly due to higher electron density in the decaying plasma generated by the previous discharge pulse. Reduction of wave speed and coupled energy at high repetition rates is primarily due to slower voltage rise and lower pulse peak voltage, limited by the pulse generator. The model shows that at high pulse repetition rates most of the discharge power is coupled to the plasma behind the wave, at E/N ~ 200–300 Td. Discharge energy loading at ν = 1–40 kHz is 1–2 meV/molecule/pulse.