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 discharges Industry: 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 R 1, R 2 = 25 M  R 3 = 1 k  R 4 = 2.75  C = 250 pF

Experimental setup Positive streamers Gap: mm Point-plane Voltage: 0-40 kV Rise time: ~ 40 ns Pressure: 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 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

Measurement of energy I capacitive = C g *dV/dt Energy V I capacitive air, 1 bar, 15 kV, 17 mm gap I corona = I measured – I capacitive I measured

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

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 / mm 2 - E per pulse ~ 5 mJ current duration: ~ 200ns TLT-supply: - V = 45 – 50 kV - I ~ 60 A; J ~ 1 A / mm 2 - E per 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 (N 2, Ar, O 2 ) Larger electrode gaps Time resolved photographs Optical fibers Laser triggering Homogeneous electric field argon – 6 kV no streamer? argon kV air kV Aim: clean characterisation of short time streamer dynamics