STARTING MECHANISMS FOR HIGHPRESSURE METAL HALIDE LAMPS*
Brian Lay**, Sang-Hoon Cho and Mark J. KushnerUniversity of Illinois
Department of Electrical and Computer EngineeringUrbana, IL 61801
http://uigelz.ece.uiuc.edu
June 2001
ICOPS01_title
* Work supported by General Electric and NSF** Present Affiliation: Sun Microsystems, Inc.
University of Illinois
Optical and Discharge Physics
AGENDA
ICOPS01_agenda
Metal-halide, HID Lamps
Description of Model
HID Startup with Trigger Electrode
Role of Photoionization
Startup of Hot Lamps
Concluding Remarks
University of Illinois
Optical and Discharge Physics
METAL HALIDE HIGH PRESSURE LAMPS
High pressure, metal-halide, High-Intensity-Discharge (HID) lamps are common illumination sources for large area indoor and outdoor applications.
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In the steady state, HID lamps are thermal arcs, producing quasi-continuum radiation from a multi-atmosphere, metal-vapor plasma.
Cold-fills are 50-100 Torr Ar with doses of metal or metal-halide salts.
Initiation consists of high pressure breakdown of the cold gas, heating of the cathode and housing, vaporizing the metal (-salts).
Glass Housing
Quartz DischargeTube
GE R400
5 cm
University of Illinois
Optical and Discharge Physics
STARTUP OF HIGH PRESSURE HID LAMPS
Breakdown of cold, high pressure HID lamps is often assisted by small additions of 85Kr for preionization.
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An auxiliary trigger electrode is employed for further “preionization”.
Multi-kV pulses are next used to breakdown the gap.
Issues:
Power Electrode
Trigger Electrode
Condensed Hg, Metal-Halide
Condensed Hg, Metal-Halide
2 cm 2 cm
Quartz Tube
Ground Electrode
Power Electrode
Trigger Electrode
Lifetime (minimizing sputtering of electrodes)
High-pressure restart
Reduction/removal of 85Kr.
University of Illinois
Optical and Discharge Physics
MODELING OF STARTUP IN HIGH PRESSURE LAMPS
To better understand and develop more optimum startup sequences for high pressure, metal-halide lamps, LAMPSIM has been developed, a 2-dimensional model.
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2-d rectilinear or cylindrical unstructured mesh
Implicit drift-diffusion for charged and neutral species
Poisson’s equation with volume and surface charge, and material conduction.
Circuit model
Local field or electron energy equation coupled with Boltzmann solution for electron transport coefficients
Optically thick radiation transport with photoionization
Secondary electron emission by impact
Thermally enhanced electric field emission of electrons
Surface chemistry.
University of Illinois
Optical and Discharge Physics
DESCRIPTION OF MODEL
Continuity with sources due to electron impact, heavy particle reactions, surface chemistry, photo-ionization and secondary emission.
Photoionization:
Electric field and secondary emission:
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N
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Pirr4
r3drr
exp)r(N)r(N
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j
jijSS
1/20
3W
2ESi j,
kT
E/qexpATj,jS
University of Illinois
Optical and Discharge Physics
DESCRIPTION OF MODEL (cont.)
Poisson for Electric Potential:
Volumetric Charge:
Surface Charge:
Solution: Equations are descritized using finite volume techniques and Scharfetter-Gummel fluxes, and are implicitely solved using an iterative Newton’s method with numerically derived Jacobian elements.
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SV
iii
V qt
iEiii
S jqt
1
tN)t(N)tt(N iii
jj
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iiiii N
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t
N)t(N)tt(NN
University of Illinois
Optical and Discharge Physics
MODEL GEOMETRY AND UNSTRUCTURED MESH Investigations of a cylindrically symmetric lamp were conducted using an
unstructured mesh to resolve electrode structure.
Cylindrical symmetry is questionable with respect to the trigger electrode.
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Powered Electrode
"Windings"
Trigger Electrode
Grounded Electrode
Quartz TubePlasma
Fin
Air
Fin
Grounded Housing
1 cm
Cylinder Center-lineCL
University of Illinois
Optical and Discharge Physics
BIAS WAVEFORMS
Startup is initiated by a -600V, 100ns pulse on the trigger electrode with the power electrode grounded.
The sustain pulse (trigger and powered electrodes) is -3500V, 275 ns.
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-4000
-3500
-3000
-2500
-2000
-1500
-1000
-500
0
0 100 200 300 400TIME (ns)
TRIGGER
TRIGGER ANDPOWER ELECTRODE
BIAS (V)
Roughness on the trigger electrode provides sufficient electric field enhancement for electron emission.
No other initial sources of electrons are allowed.
University of Illinois
Optical and Discharge Physics
ELECTRON DENSITY: BASE CASE (SLIGHTLY WARM)
Electric field emission from the trigger electrode initiates the discharge.
Densities of 1011 cm-3 are produced by the trigger pulse.
Avalanche in the main gap is anode directed due to cathode preionization. After gap closure, avalanche is cathode directed.
“Prearrival” of avalanche at anode occurs due to photo- ionization of Hg.
Pulsation occurs at the cathode.
75 Torr, Ar/Hg = 75/0.001 (slightly warm), 450 ns.
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4 x 107 - 2 x 1011 cm-3
3 x 108 - 2 x 1012 cm-3
University of Illinois
Optical and Discharge Physics
LEADING EDGE OF TRIGGER PULSE ([e] and Te)
As the voltage ramps to -600 V (15 ns), electric field emission seeds the mini-gap.
Avalanche preferentially occurs near the windings where the gross electric field and Te are largest.
75 Torr, Ar/Hg = 75/0.001 (slightly warm), 0 - 30 ns.
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0 - 6 eV
7 x 106 - 7 x 1010 cm-3
Electron Temperature Electron Density
Te closely follows the electric field. The electron density is sufficiently low that little shielding occurs.
University of Illinois
Optical and Discharge Physics
LEADING EDGE OF TRIGGER PULSE (e-SOURCES)
Photoionization of Hg, tracking excited states and not directly electric field, peaks dominantly near the trigger electrode.
As avalanche times are < 1 ns at electric fields of interest (100s Td), e-impact sources dominate.
Photoionization does penetrate “further, sooner”.
75 Torr, Ar/Hg = 75/0.001 (slightly warm), 0 - 30 ns.
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9 x 1012 - 9 x 1016 cm--3s-1
Electron Impact Ionization Photoionization
Electron impact ionization occurs near the trigger electrode tip and near the windings closely tracking the electron temperature.
7 x 106 - 7 x 1010 cm--3s-1
University of Illinois
Optical and Discharge Physics
PHOTIONIZATION LEADS ELECTRON IMPACT
As time progresses and the electric field increases, the delay between photo-ionization and impact decreases.
Photoionization by non-resonance radiation will have longer penetration distances and larger effects.
75 Torr, Ar/Hg = 75/0.001 (slightly warm), 0 - 15 ns.
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MIN
Photoionization of Hg provides seed electrons in advance of the electron impact avalanche front, similar to stream propagation.
[Photoionization]- [Electron impact]
MAX
University of Illinois
Optical and Discharge Physics
PHOTIONIZATION LEADS ELECTRON IMPACT AT ANODE
Electric field enhancement at the small radius anode produces “avalanche” class E/N, though lacking seed electrons.
Photoionization leading the avalanche front from the cathode seeds the high E/N region around the anode.
The resulting local avalanche begins a cathode directed breakdown wave.
75 Torr, Ar/Hg = 75/2.3 (warm), 185 - 450 ns.
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The leading of electron impact of photoionization is best illustrated at the anode.
Electron Density
5 x 108 - 5 x 1011 cm-3
University of Illinois
Optical and Discharge Physics
[e] vs TEMPERATURE
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The cw pressure of (hot) HIDs is many atm.
After turn off, the tube must cool (metal vapor condense), to reduce the density (increase E/N) so that the available starting voltage can reignite the lamp.
5 x 108 - 5 x 1011 cm-3
0-450 ns
Ar (75 Torr cold fill) / Hg
100/ 0.001Ambient
99.9/0.150 C
97/3140 C
7/3220C
University of Illinois
Optical and Discharge Physics
CONCLUDING REMARKS
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A model for startup of high pressure, metal halide, HID lamps has been developed.
Internally triggered lamps have been investigated, demonstrating role of photoionization and field emission in startup phase.
Restart of hot (cooling lamps) is ultimately limited by available voltage to “spark” high density (low E/N) of still condensing metal vapor .
Future developments will address heating of electrodes and onset of thermionic emission.