Ionization in strong electric fields and dynamics of nanosecond-pulse plasmas

The paper describes experimental and computational studies of air plasmas sustained by high repetition rate high-voltage nanosecond pulses. Current and voltage measurements, together with earlier microwave diagnostics, allowed us to determine the efficiency of ionization. The energy cost per newly produced electron in these diffuse volumetric plasmas was found to be on the order of 100 eV, two orders of magnitude lower than in diffuse quasineutral DC and RF plasmas, and comparable with or even lower than in the cathode sheaths of glow discharges. A plasma kinetic model was developed and tested against the experimental Paschen breakdown curve in argon. The kinetic model was found to adequately describe the Paschen curve, and the important role of ionization by fast ions and atoms near the cathode, as well as the increase in secondary emission coefficient in strong fields described in the literature, was confirmed. Modeling of plasma dynamics in high-voltage nanosecond pulses yielded the energy cost of ionization, which was found to agree well with the experimental values. Both experiments and modeling revealed that the ionization cost per electron in these plasmas is relatively insensitive to the gas density. Detailed investigations of the plasma dynamics revealed a critical role of the cathode sheath that was found to take up most of the peak voltage applied to the electrodes. The extremely high E/N, much higher than the Stoletov's field at the Paschen minimum point, results in a very high ionization cost in the sheath. In contrast, the E/N in the quasineutral plasma is closer to that associated with the Stoletov's point, resulting in a near-optimal electron generation. This behavior (the reversal of ionization efficiencies in the sheath and in the plasma) is opposite to that in conventional glow discharges. The positive space charge in the sheath and its relatively slow relaxation due to the low ion mobility was also found to result in reversal of electric field direction in the plasma at the tail of the high-voltage pulse.