The time-resolved electric field in a fast ionization wave discharge in a diffuse nanosecond pulse discharge plasma in atmospheric pressure air is measured using the Electric Field Induced Second Harmonic (E-FISH) diagnostic. The electric field is placed on an absolute scale by calibration against a Laplacian field. At relatively low peak voltages, when the plasma is generated only near the pin high-voltage electrode, the electric field is measured ahead of the ionization wave during the entire voltage pulse, exhibiting a strong field enhancement compared to the Laplacian field, by about an order of magnitude. As the peak voltage is increased and the ionization wave traverses the laser beam, the electric field is measured both ahead of the wave and behind the ionization front, where the field drops rapidly due to the charge separation and plasma selfshielding. When the wave reaches the grounded electrode, the discharge transitions into a conduction phase in which the potential is redistributed within the gap. The electric field in the vicinity of the pin then increases again, following the applied voltage waveform for the rest of the pulse. The effective time resolution of the present measurements is 150 ps. Based on the single shot data, we find that the peak electric field in the wave front is moderately influenced by the applied voltage and varies between 160 to 210 kV/cm. This study demonstrates the viability of the E-FISH diagnostic for this class of atmospheric pressure discharges and paves the way for future in-depth studies of this particular problem.

The present work is devoted to the study of the spatio-temporal distribution of the reduced electric field (REF) in a 10 ns diffuse atmospheric air discharge at very high overvoltage, in a pin-to-plane electrode geometry. The REF is derived through the intensity ratio of two wellknown transitions of molecular nitrogen: N 2 (C-B, v′=2, v″=5) and N 2 + (B-X, v′=0, v ″=0). The achieved temporal resolution is 500 ps, while the spatial resolution is better than 300 μm and 400 μm in the axial and radial direction, respectively. Due to the fast rise time of the voltage pulse, the total electric field is dramatically disturbed by the contribution of the Laplacian field, contrary to low-voltage streamer discharges. Electric field values above the ionization threshold are sustained all along the plasma channel. The dynamics of the high-voltage diffuse discharge seem similar to those of classical streamers with a very high field zone propagating towards the plane electrode then followed by a backward neutralization wave. However, some noticeable discrepancies are reported between the experimentally-obtained distributions of the axial electric field at 65 and 85 kV and those computed by means of a fluid model. They stress in particular the origin and the role of the background electrons in the discharge dynamics. Several limitations of the applicability of the intensity-ratio method for the study of very transient phenomena are also discussed. At first, the effect of the temporal integration of the signals is addressed by comparing them with an artificial averaging of the modeling results. Then, the effect of the non-stationarity of the collected signals is put forward by applying the intensityratio method under steady-state assumption or not. Lastly, the overestimation of the electric field in the discharge front due the relatively long effective lifetime of N 2 (C, v′=2) compared to the discharge dynamics is discussed.