Laboratory experiments on positive streamer properties

1. Laboratory experiments on positive streamer properties S. Nijdam1, E.M. van Veldhuizen1, U. Ebert1,2 1) Eindhoven University of Technology, Department of Applied Physics, EPG, E-mail: s.nijdam@tue.nl 2) Centrum Wiskunde & Informatica (CWI), Amsterdam, The Netherlands

2. Propagation of positive streamers • Propagate against electron drift direction • Free electrons required in front of streamer • Photo-ionization (air) • Background ionization • Natural • Leftover from previous discharges • Artificial radioactivity • ….. • Electrons mostly attached to oxygen (O2-) Photo- ionization

3. Experimental set-up • Positive voltage pulse (10-55 kV) applied on anode, 4 – 16 cm above grounded plate • 25 – 1000 mbar • High purity gasses • Up to 7.0 purity (0.1 ppm) • ICCD Camera • Various spectrometers

4. Photo- and background ionization

5. Variation of O2 content in N2 160 mm 1000 mbar ～23 kV 200 mbar ～10 kV 25 mbar ～15 kV • Pure N2 • (< 1 p.p.m.) • 10-4 O2 in N2 • 2·10-3 O2 in N2 • 2·10-1 O2 in N2

6. Variation of O2 content in N2 160 mm

7. p*dmin as function of pressure • p*dminroughly constant

8. Propagation velocity Streamer propagation velocities at 200 mbar • Velocity similar for all investigated gasses

9. Background ionization sources • Natural level at 1 bar: 103-104 cm-3 • Leftover from previous discharges: • We use 0.01-10 Hz • Artificial radioactivity: • We add 9 ppb of 85Kr to pure nitrogen which gives ~2·106 cm-3 at 1 bar

10. Effects of repetition rate (200 mbar) 160 mm Theoretical background ionization level: 9·106 cm-3 9·105 cm-3 9·104 cm-3 9·103 cm-3

11. Addition of 85Kr • Quite similar but longer feathers/side branches with 85Kr added 160 mm

12. Repetition rate with 85Kr (200 mbar) • Not much difference between 1 Hz and slower. • Estimated background ionization levels: • From repetition rate at 1 Hz: 9·105 cm-3 • From addition of krypton-85: 4·105 cm-3 160 mm

13. Feathers

14. Feathers investigated • 200 mbar • Pure argon/nitrogen • Roughly 102 feathers/cm3 in both gasses

15. Interpretation of feathers Feather structure Smooth structure Ek = critical field for breakthrough (~ 30 kV/cm in air STP) lphoto = photo ionization length (~2 mm in air STP)

16. Is it that simple? No. • Electrons can be attached to O2 • Not Ek, but Edetachdetermines avalanche radius • Overall picture similar • Photo-ionization role decreases when either O2 or N2 is not present • Without photo-ionization, background ionization is needed • Results are the same as with lphoto>>Ek

17. Spectra

18. Spark and streamer spectra

19. Streamer spectrum simulations with SpecAir • Results only indicative • Different normalization needed for different wavelength regions

20. Conclusions • Even in high purity gases, we still see positive streamer propagation with roughly the same velocity as in N2:O2 mixtures. So photo-ionization seems to play a smaller role than expected. • Background ionization density has significant influence on streamer morphology • Theoretical estimates of effects of repetition frequency and addition of 85Kr seem to fit • Feathers appear at low photo- and background ionization levels • The spectra of streamers (and sprites) are very different from sparks (and lightning)

21. Thank you for your attention. (proof that streamers do not follow thesame path as their predecessors)Photo-ionization work published in:Nijdam et al., J. Phys. D: Appl. Phys., vol. 43, p. 145204, 2010.Other work will be published in my thesis on Feb. 3rd 2011.