  An influence of a high-voltage (3-17 kV) 20 ns pulse on a weakly-ionized
low-pressure (0.1-10 Pa) capacitively-coupled radiofrequency (RF) argon plasma
is studied experimentally. The plasma evolution after pulse exhibits two
characteristic regimes: a bright flash, occurring within 100 ns after the pulse
(when the discharge emission increases by 2-3 orders of magnitude over the
steady-state level), and a dark phase, lasting a few hundreds \mu s (when the
intensity of the discharge emission drops significantly below the steady-state
level). The electron density increases during the flash and remains very large
at the dark phase. 1D3V particle-in-cell simulations qualitatively reproduce
both regimes and allow for detailed analysis of the underlying mechanisms. It
is found that the high-voltage nanosecond pulse is capable of removing a
significant fraction of plasma electrons out of the discharge gap, and that the
flash is the result of the excitation of gas atoms, triggered by residual
electrons accelerated in the electric field of immobile bulk ions. The
secondary emission from the electrodes due to vacuum UV radiation plays an
important role at this stage. High-density plasma generated during the flash
provides efficient screening of the RF field (which sustains the steady-state
plasma). This leads to the electron cooling and, hence, onset of the dark
phase.
