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Laboratory experiments on positive streamer properties

This study delves into the properties of positive streamers through laboratory experiments, focusing on factors like propagation against electron drift direction, free electrons required, photo-ionization, and background ionization. The experiments involve varying oxygen content in nitrogen gas, different ionization levels, and the effects of repetition rate and artificial radioactivity on streamer morphology. Results show that even in high-purity gases, positive streamers propagate similarly to N2:O2 mixtures, with less emphasis on photo-ionization than anticipated. The background ionization density significantly influences streamer morphology. Spectra analysis reveals distinctive differences between streamers and sparks/lightning. This research provides valuable insights into streamer behavior.

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Laboratory experiments on positive streamer properties

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  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.

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