  The characteristics of atmospheric-pressure microdischarges excited by
nanosecond high-voltage pulses are investigated in helium-nitrogen mixtures, as
a function of the parameters of the excitation voltage pulses. In particular,
cases of single-pulse excitation, unipolar and bipolar double-pulse excitation
are studied, at different pulse durations, voltage amplitudes, and delay times
(for the case of double-pulse excitation). Our investigations are carried out
with a particle-simulation code that also comprises the treatment of the VUV
resonance radiation in the plasma. The simulations allow gaining insight into
the plasma dynamics during and after the excitation pulse, the development and
the decay of charged particle density profiles and fluxes. We find a strong
dependence of the electron density of the plasma (measured at the end of the
excitation pulse) on the electrical input energy into the plasma and a weak
influence of the shape of the excitation pulse at the same input energy.
