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STARTING MECHANISMS FOR HIGH PRESSURE METAL HALIDE LAMPS *

STARTING MECHANISMS FOR HIGH PRESSURE METAL HALIDE LAMPS * Brian Lay**, Sang-Hoon Cho and Mark J. Kushner University of Illinois Department of Electrical and Computer Engineering Urbana, IL 61801 http://uigelz.ece.uiuc.edu June 2001. * Work supported by General Electric and NSF

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STARTING MECHANISMS FOR HIGH PRESSURE METAL HALIDE LAMPS *

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  1. STARTING MECHANISMS FOR HIGH PRESSURE METAL HALIDE LAMPS* Brian Lay**, Sang-Hoon Cho and Mark J. Kushner University of Illinois Department of Electrical and Computer Engineering Urbana, IL 61801 http://uigelz.ece.uiuc.edu June 2001 * Work supported by General Electric and NSF ** Present Affiliation: Sun Microsystems, Inc. ICOPS01_title

  2. 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 ICOPS01_agenda

  3. 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. • 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). University of Illinois Optical and Discharge Physics ICOPS01_01

  4. STARTUP OF HIGH PRESSURE HID LAMPS • Breakdown of cold, high pressure HID lamps is often assisted by small additions of 85Kr for preionization. • An auxiliary trigger electrode is employed for further “preionization”. • Multi-kV pulses are next used to breakdown the gap. • Issues: • Lifetime (minimizing sputtering of electrodes) • High-pressure restart • Reduction/removal of 85Kr. University of Illinois Optical and Discharge Physics ICOPS01_02

  5. 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. • 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 ICOPS01_03

  6. 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: University of Illinois Optical and Discharge Physics ICOPS01_04

  7. 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. University of Illinois Optical and Discharge Physics ICOPS01_05

  8. 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. University of Illinois Optical and Discharge Physics ICOPS01_06

  9. 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. • 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 ICOPS01_07

  10. 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. 4 x 107 - 2 x 1011 cm-3 3 x 108 - 2 x 1012 cm-3 University of Illinois Optical and Discharge Physics ICOPS01_08

  11. LEADING EDGE OF TRIGGER PULSE ([e] and Te) • Te closely follows the electric field. The electron density is sufficiently low that little shielding occurs. Electron Temperature Electron Density • 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. 0 - 6 eV University of Illinois Optical and Discharge Physics 7 x 106 - 7 x 1010 cm-3 ICOPS01_09

  12. LEADING EDGE OF TRIGGER PULSE (e-SOURCES) • Electron impact ionization occurs near the trigger electrode tip and near the windings closely tracking the electron temperature. Electron Impact Ionization Photoionization • 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. 7 x 106 - 7 x 1010 cm--3s-1 9 x 1012 - 9 x 1016 cm--3s-1 University of Illinois Optical and Discharge Physics ICOPS01_10

  13. PHOTIONIZATION LEADS ELECTRON IMPACT • Photoionization of Hg provides seed electrons in advance of the electron impact avalanche front, similar to stream propagation. [Photoionization]- [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. MIN MAX University of Illinois Optical and Discharge Physics ICOPS01_11

  14. PHOTIONIZATION LEADS ELECTRON IMPACT AT ANODE • The leading of electron impact of photoionization is best illustrated at the anode. Electron Density • 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. University of Illinois Optical and Discharge Physics 5 x 108 - 5 x 1011 cm-3 ICOPS01_12

  15. [e] vs TEMPERATURE • 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 100/ 0.001 Ambient 99.9/0.1 50 C 97/3 140 C 7/3 220C University of Illinois Optical and Discharge Physics Ar (75 Torr cold fill) / Hg ICOPS01_13

  16. CONCLUDING REMARKS • 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. University of Illinois Optical and Discharge Physics ICOPS01_14

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