  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.
