1. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. (a) Experimental setup to measure the nitrogen fluorescence at 337 nm. (b) The photomultiplier signal shows a clear delay between the filament and the discharge event. In this case, the electrode separation is 13 cm. The discharge event is characterized by the high-frequency ringing in the signal. This noise is due to the high voltage (HV) power supply and its internal circuits. Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

2. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. (a) The delay time varies for consecutive experiments at 10 cm separation. (b) The delay time seems to be independent of the electrode separation. Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

3. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. (a) Experimental setup for the electric field measurement. (b) The oscilloscope traces from the photodiode (black) and the electric field probe (red) are shown. The characteristic delay can be seen between the filament and the breakdown event. Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

4. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. (a) For different electrode separations, the traces of the electric field are shown. The dynamics show similar features. (b) The position of the electric field probe between the electrodes is varied. Electrode separation is 25 cm. The position of the electric field probe is measured from the second electrode. Four traces are shown at 19 (closest to first electrode), 14, 7, and 3 cm (closest to second electrode). Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

5. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. The comparison of the electric field dynamics in the case of (a) self-induced discharge (electrode separation <10 cm) and (b) filament guided discharge. The insets show pictures of the discharge event taken with a high-speed camera. Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

6. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. Slightly different dynamics of the electric field can be observed for (a) positive voltage and (b) negative voltage applied to the electrode. The discharge occurs at t=0. Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

7. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. Shown here are the electric field measurement traces for the three different geometries. (a) The laser pulse is passing through a 1 cm in the first electrode but hitting the second electrode. (b) The laser pulse is passing through 1-cm holes in the two electrodes. (c) The laser pulse is grazing the top of both solid electrodes. The height of the electric field probe is kept constant, resulting in a smaller signal pertubation from the discharge to the electric field for case (c) in comparison to (a) and (b). Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504

8. Date of download: 9/19/2016 Copyright © 2016 SPIE. All rights reserved. The electric field data are fitted with an exponential growth function for two traces. The electrode separation was 28 cm with the electric field probe in the center. The characteristic growth time τ is (a) 230±1 ns and (b) 534±2 ns. Figure Legend: From: Electric field measurements during filament-guided discharge Opt. Eng. 2014;53(5):051504. doi:10.1117/1.OE.53.5.051504