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

The paper describes experimental and computational studies of weakly ionized air plasmas sustained by high repetition rate high-voltage nanosecond pulses. The experimentally determined energy cost per newly produced electron in these diffuse volumetric plasmas is on the order of 100 eV, two orders of magnitude lower than in diffuse quasineutral dc and rf plasmas, and close to that in the cathode sheaths of glow discharges. Modeling of plasma dynamics in high-voltage nanosecond pulses yielded the energy cost of ionization in good agreement 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 thin cathode sheath that was found to take up most of the peak voltage applied to the electrodes. The extremely high values of the ratio of electric field strength to the gas number density (EN), much higher than the Stoletov's field at the Paschen minimum point, result in a very high ionization cost in the sheath. In contrast, the value of EN 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.