Optical study of the breakdown phenomenon in High Intensity Discharge lamps Freddy Manders, Marco Haverlag Philips Lighting Eindhoven Paul Aben, Job Beckers, Winfred Stoffels Technical University of Eindhoven

Outline Introduction Experimental Set-up Experimental results HID lamps Background approach Experimental Set-up Power supply Lamps under investigation Experimental results DC ignition of low aspect lamps DC ignition of High aspect lamps AC ignition Conclusions

Philips One of the biggest lamp manufactures in the world In Eindhoven pre-development of all lighting sources Halogen Fluorescent Compact fluorescent LED HID etc Introduction Experimental set-up results Conclusions

PRACTICAL APPLICATIONS USED FOR: High efficiency High colour rendering PRACTICAL APPLICATIONS Stadiums Tennis courts Parking lots Automotive Shopping malls Beamers Introduction Experimental set-up results Conclusions Burner is filled with: Starting gas (Noble gases, e.g. Xe & Ar) Buffer gas (e.g. mercury) Radiation emitting substance (e.g. Na, Ce, Dy, Ca, …)

We need to lower the ignition voltage of the gas discharge Background HID lamps Ignition voltage ~ 4 kV (300 mBar, pulse ignition, standard HID lamps) ~ 18 kV (10 Bar Xe, pulse ignition, Automotive lamps) Issues 1 st electron, small lamps ( Automotive, UHP) insulation materials (cables, lamp cap, etc) Safety issues, bigger lamps More expensive electronics The lamp should always ignited We need to lower the ignition voltage of the gas discharge Introduction Experimental set-up results Conclusions

Approach Try to understand how breakdown in HID lamps happens Study breakdown process in model lamps with an ICCD camera and change a few parameters Gas type (Ar / Xe) Gas pressure (300 mBar, 700 mBar ) Length / diameter of the lamp Positive / negative voltage Voltage source (Pulse, AC) Introduction Experimental set-up results Conclusions

Experimental set-up Introduction Experimental set-up results Conclusions

Experimental set-up Lamps Voltage source Low aspect ratio burners Argon, p = 300 mbar, d = 7 mm High aspect ratio burners Diameter = 4 mm Argon and Xenon electrode distance: 1.5 cm and 2.7 cm 300 mbar and 700 mbar Introduction Experimental set-up results Conclusions Pulse source of 5 kV with a rise time of 10 nsec. AC voltage source from 25 kHz up to 3 MHz Voltage (kV)

Experimental results DC pulse ignition of low aspect lamps DC pulse ignition of high aspect lamps AC ignition Introduction Experimental set-up results Conclusions

Low aspect ratio: Argon 300 mBar, -4.0 kV High voltage side Grounded side 17 ns 0 ns 6 ns 12 ns Introduction Experimental set-up results Conclusions 18 ns 3 ns 7 ns 13 ns 19 ns 4 ns 9 ns 14 ns 21 ns 5 ns 10 ns 15 ns max

Low aspect ratio: Argon 300 mBar, -4.0 kV Introduction Experimental set-up results Conclusions 9 nsec High voltage ionization expands spherical High voltage ionization is not uniform, channels are visible, streamers. Grounded emission becomes, because of the interaction with the wall

Low aspect ratio: Argon 300 mBar -4.0 kV -3.0 kV -2.5 kV 20 nsec 41 nsec 59 nsec 73 nsec 99 nsec 7 nsec 11 nsec 16 nsec 17 nsec 21 nsec 16 nsec 29 nsec 40 nsec 51 nsec 59 nsec Introduction Experimental set-up results Conclusions

Low Aspect ratio: Argon 300 mBar, -2.5 kV Introduction Experimental set-up results Conclusions 59 nsec High voltage emission region expands spherical High voltage emission region is diffuse, burner is almost completely filled with ionization No grounded electrode emission is visible

Low aspect ratio: Argon 300 mBar -4.0 kV +4.0 kV +2.5 kV 2 nsec 7 nsec 39 nsec Introduction Experimental set-up results Conclusions 6 nsec 11 nsec 60 nsec 8 nsec 16 nsec 99 nsec 17 nsec 11 ns 128 nsec

Low aspect ratio: Argon 300 mBar -4 kV +4 kV +2.5 kV Introduction Experimental set-up results Conclusions 16 nsec 8 nsec 99 nsec In all 3 pictures mechanism is streamer Negative streamers are in general more diffuse. +4 kV: clearly streamer mechanism +2.5 kV: slower and more diffuse

Conclusions low aspect ratio lamps At high negative over voltage (-4 kV) discharge is a streamer For lower voltage there is a transition to a Fast Ionisation wave (-2 kV) Negative streamers are more diffuse then positive Discharge always starts at powered electrode because of interaction with wall Introduction Experimental set-up results Conclusions

High aspect ratio burners Argon, p = 300 mbar, d = 1.5 cm, V = +4kV Introduction Experimental set-up results Conclusions 0 ns 52 ns 94 ns 8 ns 58 ns 99 ns 15 ns 69 ns 107 ns 23 ns 75 ns 109 ns 25 ns 84 ns 121 ns 39 ns 85 ns 48 ns   93 ns        

High aspect ratio burners: Ar300 mBar, +4kV Introduction Experimental set-up results Conclusions 85 nsec streamer along burner wall very little branching grounded emission expands spherical and is diffuse

High aspect ratio burners: +4kV Ar 300 mBar Ar 700 mBar Xe 300 mBar 23 ns 52 ns 75 ns 85 ns 121 ns 109 ns 99 ns 23 ns 51 ns 71 ns 85 ns 115 ns 98 ns 89 ns 22 ns 39 ns 57 ns 85 ns 140 ns 135 ns 106 ns Introduction Experimental set-up results Conclusions

High aspect ratio burners: +4kV 85 nsec, Ar 300 mBar Introduction Experimental set-up results Conclusions 85 nsec, Ar 700 mBar 85 nsec, Xe 300 mBar Ar300: little branching, intense emission at grounded electrode Ar700: much branching Xe300: much branching, very little emission at grounded electrode

High aspect ratio burners: Ar 700 mBar - 4kV Introduction Experimental set-up results Conclusions 0 ns 97 ns 269 ns 9 ns 108 ns 272 ns 20 ns 143 ns 301 ns 21 ns 188 ns 303 ns 29 ns 212 ns 313 ns 39 ns 242 ns 320 ns 257 ns 54 ns   340 ns    

High aspect ratio burners: -4kV Introduction Experimental set-up results Conclusions 257 nsec High voltage ionization (negative voltage) very diffuse and at a very low intensity At grounded electrode streamers are formed Two different mechanisms during one discharge

Conclusion high aspect lamps More interaction with the wall because the distance to the wall is smaller Positive discharge branch more then negative Negative discharge is more diffuse and slower The higher the pressure the more branching Xe branches more then Ar Introduction Experimental set-up results Conclusions

Comparison of velocities at + 4 kV velocity / (106 m/s) d = 1.5 cm d = 2.7 cm Ar300 1.9 ± 0.1 1.6 ± 0.1 Ar700 1.3 ± 0.1 1.1 ± 0.1 Xe300 1.7 ± 0.1 1.4 ±0.1 Xe700 1.2 ± 0.1 1.0 ± 0.1 Introduction Experimental set-up results Conclusions

Comparison of velocities at + 4 kV velocity / (106 m/s) d = 1.5 cm d = 2.7 cm Ar300 1.9 ± 0.1 1.6 ± 0.1 Ar700 1.3 ± 0.1 1.1 ± 0.1 Xe300 1.7 ± 0.1 1.4 ±0.1 Xe700 1.2 ± 0.1 1.0 ± 0.1 Introduction Experimental set-up results Conclusions 1.2 1.2 1.2 1.2

Comparison of velocities at + 4 kV velocity / (106 m/s) d = 1.5 cm d = 2.7 cm Ar300 1.9 ± 0.1 1.6 ± 0.1 Ar700 1.3 ± 0.1 1.1 ± 0.1 Xe300 1.7 ± 0.1 1.4 ±0.1 Xe700 1.2 ± 0.1 1.0 ± 0.1 Introduction Experimental set-up results Conclusions 1.4 1.4 1.4 1.4

Comparison of velocities at + 4 kV velocity / (106 m/s) d = 1.5 cm d = 2.7 cm Ar300 1.9 ± 0.1 1.6 ± 0.1 Ar700 1.3 ± 0.1 1.1 ± 0.1 Xe300 1.7 ± 0.1 1.4 ±0.1 Xe700 1.2 ± 0.1 1.0 ± 0.1 Introduction Experimental set-up results Conclusions 1.2 1.1 1.1 1.1

Conclusions velocity measurements Out of the camera pictures it is possible to calculate the velocity of the discharge. The speed are all in the order of 10^6 m/s which is normal for a streamer discharge The velocity results for the high aspect ratio lamps with 4 kV show that there are some relations: Introduction Experimental set-up results Conclusions The factor the velocity changed Distance between electrodes changed from 1.5 cm to 2.7 cm 1.2 Gas pressure changed from 300 to 700 mBar 1.4 Gas type changed from Ar to Xe 1.1

Breakdown voltage for AC ignition for high aspect lamps Introduction Experimental set-up results Conclusions -29% -26% For DC pulse ignition ~ 4 kV is needed to ignited the 700 mBar Xe lamps so AC lowers the breakdown voltage with ~ 50 %

Experimental results (AC ignition - Xenon) Two regimes in which the burners can ignite Introduction Experimental set-up results Conclusions

Experimental results of 300 mBar Xe (AC ignition)

Discharge travels via the wall 700 mBar of Xe 30kHz Low frequencies: Discharge travels via the wall Looks like pictures of pulse ignition (streamer-like channels, branching) 80kHz Introduction Experimental set-up results Conclusions 140kHz High frequencies: Discharge travels through the gas Ionisation channel has the shape of a streamer … 200kHz 300kHz 400kHz

Maximum velocity of ionisation front: 300 mBar Xenon at 200 kHz Introduction Experimental set-up results Conclusions Maximum velocity of ionisation front: Contradiction: Shape looks like a streamer-like discharge Maximum velocity of the ionisation channel is in the order of those in Townsend discharges.

Conclusions on AC ignition AC ignition voltage about 50-60% lower than pulse ignition voltage Ignition voltage is a decreasing function of frequency At relatively low frequencies the ionisation channel travels along the wall At relatively high frequencies the ionisation channel travels through the gas Ionisation channel builds up step-wise over many periods Ionisation channel only grows during voltage maximum Introduction Experimental set-up results Conclusions Possible explanation: Due to alternating E-field more charged particles stay in the volume, and are able to ionise for a longer time. The higher the frequency, the more charged particles stay in the volume.

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