laboratory experiments on positive streamer properties s. nijdam 1, e.m. van veldhuizen 1, u. ebert...
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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: [email protected]) 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 (O2
-)
Photo-ionization
- --
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-------+++
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++++
++++
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++++
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++
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+
-- -
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----
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--
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E
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
Variation of O2 content in N2
2·10-1 O2 in N2
Pure N2
(< 1 p.p.m.)
10-4 O2 in N2
2·10-3 O2 in N2
1000 mbar~ 23 kV
200 mbar~ 10 kV
25 mbar~ 15 kV
160 mm
p*dmin as function of pressure
• p*dmin roughly constant
10 100 10000.00
0.05
0.10
0.15
20% O2 in N
2
0.2% O2 in N
2
0.01% O2 in N
2
N2 6.0
p·d m
in (
bar·
mm
)
Pressure (mbar)
Propagation velocity
• Velocity similar for all investigated gasses
0 10 20 30 400
2
4
6
8 20% O
2 in N
2
0.2% O2 in N
2
0.01% O2 in N
2
N2 6.0
N2 7.0
Vel
ocity
(10
5 m/s
)
Voltage (kV)
Streamer propagation velocities at 200 mbar
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
1 7 6 3 1 with 10 10( ) · cm s at 1 barrec reckn t t k
rec
Sn
k
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
Repetition rate with 85Kr (200 mbar)
160 mm
• 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
E=Ek
E=Ek
lphotol
photo
Streamer heads
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e-
e- e-
e-
e-e-e-
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e-
e
Interpretation of feathers
Avalanches
Ek = critical field for breakthrough (~ 30 kV/cm in air STP)lphoto = photo ionization length (~2 mm in air STP)
Feather structure Smooth structure
Is it that simple? No.
• Electrons can be attached to O2
• Not Ek, but Edetach determines 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
e-
e-
e-
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e-e-
e-
e-
e-
e-
E=Ek
e- e-
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e-e-e-
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e-
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e-
E=Ek
lphoto
Spark and streamer spectra
0
200
400
600
300 400 500 600 700 800 9000
20
40
60
80
Strong spark
0
200
400
600
800
1000
N: N+: O: O+: Al+:
Streamer
Inte
nsit
y (a
.u.)
Wavelength (nm)
0.0
0.2
0.4
0.6
0.8N
2 SPS: N
2 FPS:
Streamer spectrum simulations with SpecAir
300 400 500 600 700 800 900
0
20
40
60
80
Inte
nsit
y (a
.u.)
Wavelength (nm)
Measurements
Specair results
0.0
0.2
0.4
0.6
0.8
Int
ensi
ty (
a.u.
)
N2 SPS: N
2 FPS: O:
Temperatures (K)
Electronic 40000
Rotational 800
Translational 300
Vibrational 5000
• 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 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)