STARTING MECHANISMS FOR HIGH PRESSURE METAL HALIDE LAMPS *

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

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* Work supported by General Electric and NSF** Present Affiliation: Sun Microsystems, Inc.

University of Illinois

Optical and Discharge Physics

AGENDA

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Metal-halide, HID Lamps

Description of Model

HID Startup with Trigger Electrode

Role of Photoionization

Startup of Hot Lamps

Concluding Remarks



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



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.



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.



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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iiiiii SNDqN

t

N

2

jiji

Pirr4

r3drr

exp)r(N)r(N

)r(S

j

jijSS

1/20

3W

2ESi j,

kT

E/qexpATj,jS



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

j

iiiii N

N

Nt)tt(

t

N)t(N)tt(NN



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



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.



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 photoionization 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



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.



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”.


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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



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.


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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



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



[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



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.

STARTING MECHANISMS FOR HIGH PRESSURE METAL HALIDE LAMPS *

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