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, 2 ) Centrum Wiskunde & Informatica (CWI), Amsterdam, The Netherlands
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
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
Photo- and background ionization
Variation of O 2 content in N 2 2·10 -1 O 2 in N 2 Pure N 2 (< 1 p.p.m.) O 2 in N 2 2·10 -3 O 2 in N mbar ~ 23 kV 200 mbar ~ 10 kV 25 mbar ~ 15 kV 160 mm
Variation of O 2 content in N mm
p*d min as function of pressure p*d min roughly constant
Propagation velocity Velocity similar for all investigated gasses Streamer propagation velocities at 200 mbar
Background ionization sources Natural level at 1 bar: cm -3 Leftover from previous discharges: We use Hz Artificial radioactivity: We add 9 ppb of 85 Kr to pure nitrogen which gives ~2·10 6 cm -3 at 1 bar
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
Addition of 85 Kr 160 mm Quite similar but longer feathers/side branches with 85 Kr added
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
Feathers
Feathers investigated 200 mbar Pure argon/nitrogen Roughly 10 2 feathers/cm 3 in both gasses
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
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
Spectra
Spark and streamer spectra
Streamer spectrum simulations with SpecAir Temperatures (K) Electronic40000 Rotational800 Translational300 Vibrational5000 Results only indicative Different normalization needed for different wavelength regions
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)
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 , Other work will be published in my thesis on Feb. 3 rd 2011.