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 ICOPS01_title * Work supported by General Electric and NSF ** Present Affiliation: Sun Microsystems, Inc.

2. 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

3. 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. ICOPS01_01 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).

4. 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 85 Kr for preionization. ICOPS01_02 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 85 Kr.

5. 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. ICOPS01_03 2-d rectilinear or cylindrical unstructured mesh Implicit drift-diffusion for charged and neutral species Poissons 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.

6. 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: ICOPS01_04

7. 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 Newtons method with numerically derived Jacobian elements. ICOPS01_05

8. 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. ICOPS01_06

9. 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. ICOPS01_07 Roughness on the trigger electrode provides sufficient electric field enhancement for electron emission. No other initial sources of electrons are allowed.

10. 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 10 11 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. ICOPS01_08 4 x 10 7 - 2 x 10 11 cm -3 3 x 10 8 - 2 x 10 12 cm -3

11. University of Illinois Optical and Discharge Physics LEADING EDGE OF TRIGGER PULSE ([e] and T e ) 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 T e are largest. 75 Torr, Ar/Hg = 75/0.001 (slightly warm), 0 - 30 ns. ICOPS01_09 0 - 6 eV 7 x 10 6 - 7 x 10 10 cm -3 Electron Temperature Electron Density T e closely follows the electric field. The electron density is sufficiently low that little shielding occurs.

12. 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. ICOPS01_10 9 x 10 12 - 9 x 10 16 cm --3 s -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 10 6 - 7 x 10 10 cm --3 s -1

13. 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. ICOPS01_11 MIN Photoionization of Hg provides seed electrons in advance of the electron impact avalanche front, similar to stream propagation. [Photoionization]- [Electron impact] MAX

14. 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. ICOPS01_12 The leading of electron impact of photoionization is best illustrated at the anode. Electron Density 5 x 10 8 - 5 x 10 11 cm -3

15. University of Illinois Optical and Discharge Physics [e] vs TEMPERATURE ICOPS01_13 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 10 8 - 5 x 10 11 cm -3 0-450 ns Ar (75 Torr cold fill) / Hg 100/ 0.001 Ambient 99.9/0.1 50 C 97/3 140 C 7/3 220C

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