1. Laboratory experiments on positive streamer properties S. Nijdam 1, E.M. van Veldhuizen 1, U. Ebert 1,2 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 (O 2 - ) 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 O 2 content in N 2 2·10 -1 O 2 in N 2 Pure N 2 (< 1 p.p.m.) 10 -4 O 2 in N 2 2·10 -3 O 2 in N 2 1000 mbar ～ 23 kV 200 mbar ～ 10 kV 25 mbar ～ 15 kV 160 mm

6. Variation of O 2 content in N 2 160 mm

7. p*d min as function of pressure p*d min roughly constant

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

9. Background ionization sources Natural level at 1 bar: 10 3 -10 4 cm -3 Leftover from previous discharges: We use 0.01-10 Hz Artificial radioactivity: We add 9 ppb of 85 Kr to pure nitrogen which gives ~2·10 6 cm -3 at 1 bar

10. Effects of repetition rate (200 mbar) 160 mm Theoretical background ionization level: 9·10 6 cm -3 9·10 5 cm -3 9·10 4 cm -3 9·10 3 cm -3

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

12. Repetition rate with 85 Kr (200 mbar) 160 mm Not much difference between 1 Hz and slower. Estimated background ionization levels: From repetition rate at 1 Hz:9·10 5 cm -3 From addition of krypton-85:4·10 5 cm -3

13. Feathers

14. Feathers investigated 200 mbar Pure argon/nitrogen Roughly 10 2 feathers/cm 3 in both gasses

15. Interpretation of feathers E k = critical field for breakthrough (~ 30 kV/cm in air STP) l photo = photo ionization length (~2 mm in air STP) Feather structureSmooth structure

16. Is it that simple? No. Electrons can be attached to O 2 Not E k, but E detach determines avalanche radius Overall picture similar Photo-ionization role decreases when either O 2 or N 2 is not present Without photo-ionization, background ionization is needed Results are the same as with l photo >>E k

17. Spectra

18. Spark and streamer spectra

19. Streamer spectrum simulations with SpecAir Temperatures (K) Electronic40000 Rotational800 Translational300 Vibrational5000 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 N 2 :O 2 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 85 Kr 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 the same 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. 3 rd 2011.