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.nl2) 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

- --

-----

-------+++

+++

++++

++++

++

+

++

++++

+

+

+++

++

+

+

+

-- -

-

----

-

--

-

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

Photo- and backgroundionization

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

Variation of O2 content in N2

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

Addition of 85Kr

160 mm

• Quite similar but longer feathers/side branches with 85Kr added

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

Feathers

Feathers investigated

• 200 mbar• Pure

argon/nitrogen

• Roughly

102 feathers/cm3 in both gasses

E=Ek

E=Ek

lphotol

photo

Streamer heads

e-

e-

e-

e-

e-e-

e-

e-

e-

e-

e- e-

e-

e-e-e-

e-

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-

e-

e-e-

e-

e-

e-

e-

E=Ek

e- e-

e-

e-e-e-

e-

e-

e-

e-

E=Ek

lphoto

Spectra

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)

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.