Experimental investigation and nanosecond imaging of streamers

T.M.P. Briels, E.M. van Veldhuizen, U. Ebert

Workshop Leiden, 9-13 May 2005

Introduction

• High electric field, non conducting medium narrow ionised channels: streamers

• Nature: e.g. sprite dischargesIndustry: e.g. gas and water cleaning

• Presentation: - positive streamers- point-plane gap- air

Contents• Experimental setup• Fast photographs:

– shape of streamers as function of- voltage - electrode gap length- pressure

– diameters of streamers • Evolution of current and voltage:

– energy of streamers as function of- voltage - electrode gap length- pressure

• Influence of the electric circuit• Conclusions and future plans

Experimental setup

• R1, R2 = 25 M

• R3 = 1 k

• R4 = 2.75

• C = 250 pF

Experimental setup

• Positive streamers• Gap: 10-40 mm• Point-plane• Voltage: 0-40 kV• Rise time: ~40 ns• Pressure: 0.1-1

bar• Air

Measurements: photographs

• 40 mm gap• 30 kV• air• long exposure

time: 300 ns

Anode

Cathode

Measurements: photographs

• 40 mm gap• 30 kV• air• short exposure

time: 2 ns

Measurements: photographs

Exposure: 300 ns 50 ns 10 ns 2 ns (0 < t < 300 ns) (50 < t < 100 ns) (50 < t < 60 ns) (46 < t < 48 ns)

Photographs: voltage

• Increase voltage increase number of streamers increase diameters more streamers bridge gap

25 mm

7.5 kV 12.5 kV 17.5 kV

air, 1 bar

Photographs: electrode gap length

• Decrease gap pattern close to anode similar• Streamer pattern determined by local electric field,

not by averaged electric field

25 mm

17 mm

10 mm

air, 1 bar, 7.5 kV

Photographs: pressure

• Decrease pressure increase diameter

number at anode tip similar

1000 mbar 400 mbar 200 mbar 100 mbar

air, 40 mm gap, 10 kV

Measurement of diameter

• Measurement at:- FWHM- single streamer- in focus- no return stroke

• Statistical scatter:factor 3 – 4

• Here evaluation with long exposure time

400 mbar, 25 kV, 40 mm gap

Diameter: electrode gap length

• Increase V increase diameter• Varying gap diameters similar (e.g. at 15 kV)

air, 1 bar

Diameter: pressure

• Voltage increase diameter increase• Pressure increase diameter decrease

air, 40 mm gap

Diameter: pressure

• Roughly: diameter ~ 1/pressure

air, 40 mm gap

0 200 400 600 800

0.00

0.05

0.10

0.15

0.20

0.25

0

2

4

6

8

10

12

14

16

Cu

rre

nt (

A)

Time (ns)

Vo

ltag

e (

kV)

Measurement of energy

• Icapacitive = Cg*dV/dt

• Energy

V

Icapacitive

air, 1 bar, 15 kV, 17 mm gap

dtIVE corona **

• Icorona = Imeasured – Icapacitive

Imeasured

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

En

erg

y (m

J)

Voltage (kV)

10 mm gap 17 mm gap 25 mm gap 40 mm gap

Energy: electrode gap length

• Smallest gap highest energy (backstroke?)

air, 1 bar

[student Bert Lodewijks]

Energy: pressure

• Pressure decrease energy increase

air, 17 mm gap

0 2 4 6 8 10 12 14 16 18 20 22 24

0.01

0.1

1

10

0 2 4 6 8 10 12 14 16 18 20 22 24

0.01

0.1

1

10

0 2 4 6 8 10 12 14 16 18 20 22 24

0.01

0.1

1

10

0 2 4 6 8 10 12 14 16 18 20 22 24

0.01

0.1

1

10

1000 mbar

200 mbar 400 mbar

Ene

rgy

(mJ)

Voltage (kV)

100 mbar

Electric circuit

d = 1 – 2 mm thick d ~ 1 mm thin d = 200 – 400

m

d = 200 – 400 m

• Capacitor supply:- V = 40 kV

- I ~ 1 A; J ~ 1 A / mm2

- Eper pulse~ 5 mJ

• current duration: ~ 200ns

• TLT-supply:- V = 45 – 50 kV- I ~ 60 A; J ~ 1 A / mm2

- Eper pulse ~ 60 mJ

• current duration ~ 50 ns[PhD-student Lukas Grabowski]

Conclusions• Increase voltage increase number of streamers,

diameters, energy more streamers bridge gap

• Increase gap similar streamer pattern decrease energy

• Increase pressure decrease diameter(diameter ~ 1/pressure?)

similar streamer pattern at tip decrease energy

• Influence power supply: thin or thick streamers

Future

• Negative streamers

• Different gases (N2, Ar, O2)

• Larger electrode gaps• Time resolved photographs• Optical fibers• Laser triggering• Homogeneous electric field

argon – 6 kV

no streamer?

argon+ 3.5 kV

air+ 7.5 kV

Aim: clean characterisation of short time streamer dynamics