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Nanosecond plasmas in liquids are being used for water treatment,electrolysis or biomedical applications. The exact nature of these very dynamicplasmas and most important their ignition physics are strongly debated. Theignition itself may be explained by two competing hypothesis: (i) ignition viafield effects or (ii) via electron multiplication in nanovoids. Both hypothesisare supported by theory, but experimental data are very sparse due to thedifficulty to monitor the very fast processes in space and time. In this paper,we are using fast camera measurements and fast emission spectroscopy ofnanosecond plasmas in water applying a positive and a negative polarity to asharp tungsten electrode. It is shown that plasma ignition is dominated byfield effects at the electrode-liquid interface either as field ionization forpositive polarity or as field emission for negative polarity. This leads to ahot tungsten surface at a temperature of 7000 K for positive polarity, whereasthe surface temperature is much lower for the negative polarity. At ignition,the electron density reaches 4 $\cdot$ 10$^{25}$ m$^{-3}$ for positive and only2 $\cdot$ 10$^{25}$ m$^{-3}$ for the negative polarity. At the same time, theemission of the \Ha~light for the positive polarity is 4 times higher than thatfor the negative polarity. During plasma propagation, the electron densitiesare almost identical of the order of a 1 to 2 $\cdot$ 10$^{25}$ m$^{-3}$ anddecay after the end of the pulse over 15 ns. It is concluded that plasmapropagation is governed by field effects in a low density region that iscreated either by nanovoids or by density fluctuation in super critical watersurrounding the electrode that is created by the pressure at the moment ofplasma ignition.

Ignition and propagation of nanosecond pulsed discharges in distilled water -- negative vs. positive polarity applied to a pin electrode