  In this work, evolution of parameters of nanosecond repetitively pulsed (NRP)
discharges in pin-to-pin configuration in air was studied during transient
stage of initial twenty discharge pulses. Gas and plasma parameters in the
discharge gap were measured using coherent microwave scattering (CMS), optical
emission spectroscopy (OES) and laser Rayleigh scattering (LRS) for NRP
discharges at repetition frequencies of 1, 10 and 100 kHz. Memory effects (when
perturbations induced by the previous discharge pulse would not decay fully
till the subsequent pulse) were detected for the repetition frequencies of 10
and 100 kHz. For 10 kHz NRP discharge, the discharge parameters experienced
significant change after the first pulse and continued to substantially
fluctuate between the subsequent pulses due to rapid evolution of gas density
and temperature during the 100 us inter-pulse time caused by intense
redistribution of the flow field in the gap on that time scale. For 100 kHz NRP
discharge, the discharge pulse parameters reached a new steady state at about
five pulses after initiation. This new steady state was associated with
well-reproducible parameters between the discharge pulses and substantial
reduction of breakdown voltage, discharge pulse energy, and electron number
density in comparison with the first discharge pulse.
