  Nanosecond plasmas in liquids are being used for water treatment,
electrolysis or biomedical applications. The exact nature of these very dynamic
plasmas and most important their ignition physics are strongly debated. The
ignition itself may be explained by two competing hypothesis: (i) ignition via
field effects or (ii) via electron multiplication in nanovoids. Both hypothesis
are supported by theory, but experimental data are very sparse due to the
difficulty to monitor the very fast processes in space and time. In this paper,
we are using fast camera measurements and fast emission spectroscopy of
nanosecond plasmas in water applying a positive and a negative polarity to a
sharp tungsten electrode. It is shown that plasma ignition is dominated by
field effects at the electrode-liquid interface either as field ionization for
positive polarity or as field emission for negative polarity. This leads to a
hot tungsten surface at a temperature of 7000 K for positive polarity, whereas
the surface temperature is much lower for the negative polarity. At ignition,
the electron density reaches 4 $\cdot$ 10$^{25}$ m$^{-3}$ for positive and only
2 $\cdot$ 10$^{25}$ m$^{-3}$ for the negative polarity. At the same time, the
emission of the \Ha~light for the positive polarity is 4 times higher than that
for the negative polarity. During plasma propagation, the electron densities
are almost identical of the order of a 1 to 2 $\cdot$ 10$^{25}$ m$^{-3}$ and
decay after the end of the pulse over 15 ns. It is concluded that plasma
propagation is governed by field effects in a low density region that is
created either by nanovoids or by density fluctuation in super critical water
surrounding the electrode that is created by the pressure at the moment of
plasma ignition.
