1. PLASMA DYNAMICS AND THERMAL EFFECTS DURING STARTUP OF METAL HALIDE LAMPS * Ananth N. Bhoj a), Gang Luo b) and Mark J. Kushner c) University of Illinois Urbana, IL 61801 a)Department of Chemical and Biomolecular Engineering b)Department of Mechanical and Industrial Engineering c)Department of Electrical and Computer Engineering http://uigelz.ece.uiuc.edu October 2003 * Work supported by General Electric R&D Center, EPRI, and NSF

2. University of Illinois Optical and Discharge Physics AGENDA GEC03_agenda Introduction: Metal-halide, HID Lamps Description of 2-D Model Dynamics of Plasma Properties Trends in Breakdown Times Gas Heating and Plasma Dynamics Summary

3. University of Illinois Optical and Discharge Physics METAL HALIDE HIGH PRESSURE LAMPS High pressure, metal-halide, High-intensity- Discharge (HID) lamps are sources for indoor and outdoor applications. GEC03_01 In the steady state, HID lamps are thermal arcs, producing quasi-continuum radiation from a multi-atmosphere, metal-vapor plasma. Cold-fills are 10s-100 Torr of rare gases, typically 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 GEC03_02 Multi-kV pulses are commonly used to breakdown the gap. Auxiliary electrodes and 85 Kr, are examples of strategies used to reduce starting voltages. High voltages can cause considerable sputtering and hence darkening of the tubes resulting in lumen loss. After breakdown, a glow discharge phase eventually becomes a thermal arc operating at a few atmospheres. High-pressure can cause considerable delays in restart time until lamp cools down. Issues: Extend lifetime (minimizing sputtering of electrodes) Reduce high-pressure restart time Reduction/removal of 85 Kr.

5. University of Illinois Optical and Discharge Physics BREAKDOWN TIME Experimental results[1] are available for breakdown times in mixtures of Argon/Xenon in a lamp geometry similar to a commercial metal halide lamp. Breakdown time ( B ) is defined as the time at which voltage across the gap drops by 5% of its peak value. GEC03_03 [1] R. Moss, MS Thesis, UIUC

6. University of Illinois Optical and Discharge Physics GEC03_04 MODELING OF STARTUP PHASE: 0-D and 2-D MODELS To address startup issues, 0-D and 2-D models have been developed and validated with the experimental data. The 0-D model under predicts B over a range pressures and compositions. Plasma parameters like electron density and E/N are spatially inhomogeneous on breakdown time scales. Propagation delays associated with this are not captured in a 0-D description of the problem. Conditions: 70 Torr, 2000 V bias, mixtures of Ar/Xe

7. University of Illinois Optical and Discharge Physics 2-D MODEL: LAMPSIM LAMPSIM, a 2-dimensional model has been developed. GEC03_05 2-d rectilinear or cylindrical unstructured mesh Poissons equation with volume and surface charge, and material conduction: Multi-fluid charged species transport equations are discretized using the Scharfetter-Gummel technique.

8. University of Illinois Optical and Discharge Physics DESCRIPTION OF 2-D MODEL Sources due to electron impact, heavy particle reactions, surface chemistry, photo-ionization and secondary emission due to ion bombardment and photons are included. Solution: Equations discretized using finite volume techniques implicitly solved using an iterative Newtons method with numerically derived Jacobian elements. GEC03_06 Circuit model Electron energy equation coupled with Boltzmann solution for electron transport coefficients Surface chemistry.

9. University of Illinois Optical and Discharge Physics COUPLED PLASMA AND HYDRODYNAMICS To investigate effects of hydrodynamics in the startup phase, the plasma dynamics model was coupled to a Navier-Stokes solver. GEC03_07 A single neutral fluid treatment. 2-d boundary fitting unstructured mesh. 2nd order finite volume method using the Semi-Implicit method for Pressure Linked Equations (SIMPLE) scheme. Continuity : Momentum: Energy :

10. University of Illinois Optical and Discharge Physics MODEL GEOMETRY AND UNSTRUCTURED MESH Investigations into a cylindrically symmetric lamp based on the experimental lamp geometry were conducted using an unstructured mesh. GEC03_08 Dielectric Grounded housing Air Grounded electrode Powered electrode Quartz tube Plasma Cylindrical center line Dielectric C L 0.5cm RADIUS (cm) HEIGHT (cm) ELECTRODE GAP = 1.6 cm

11. University of Illinois Optical and Discharge Physics PLASMA DYNAMICS: E/N Voltage is compressed ahead of the ionization front. At higher pressures, it takes longer for the ionization front to close the gap. The peak E/N transits the gap faster with small Xe fraction leading to faster breakdown time. GEC03_09 1x10 -16 E/N (V-cm 2 ) 1x10 -14 0-355 ns0-875 ns0-275 ns ANIMATION SLIDE log scale 30 Torr, Ar 70 Torr, Ar 30 Torr, Ar/Xe=90/10

12. University of Illinois Optical and Discharge Physics At higher pressure, lower available E/N leads to a slower electron avalanche. Electron density avalanches faster when Xe is present in small fractions. PLASMA DYNAMICS: ELECTRON DENSITY GEC03_10 5x10 9 [e] (cm -3 ) 5x10 12 0-355 ns0-875 ns0-275 ns ANIMATION SLIDE log scale 30 Torr, Ar 70 Torr, Ar 30 Torr, Ar/Xe=90/10

13. EFFECT OF VARYING GAS COMPOSITION Small Xe fractions reduce B by as much as 50%. The lower ionization potential (Xe: 12.13 eV, Ar: 15.76 eV) and the Penning effect increase available electron density. At higher Xe fractions, inelastic losses increase and B increases. University of Illinois Optical and Discharge Physics GEC03_11

14. EFFECT OF VARYING GAS PRESSURE At higher pressures, longer times are required for critical E/N needed to start the avalanche. Collision frequency increases at higher pressures and reduces electron mobility.Consequently, the movement of the ionization front is slower and B increases. University of Illinois Optical and Discharge Physics GEC03_12

15. EFFECT OF VARYING APPLIED BIAS B decreases at higher applied voltage for a constant gas pressure and composition. After V B =1800 V, B saturates as ionization reaction rates begin to saturate as a function of E/N. University of Illinois Optical and Discharge Physics GEC03_13

16. University of Illinois Optical and Discharge Physics BREAKDOWN AND GAS HEATING GEC03_14 During breakdown energy deposition is low and thermal effects are negligible. After breakdown, density and energy deposition increase. Thermal gradients develop first near the powered electrode. Higher energy deposition increases temperature along the arc tube axis. [e]T gas 5x10 8 5x10 12 300450 [e] (cm -3 ) T gas (K) log scale Conditions: Ar, 70 Torr, gap=0.8cm, 10 s ANIMATION SLIDE

17. University of Illinois Optical and Discharge Physics HYDRODYNAMIC EFFECTS: NEUTRAL DENSITY GEC03_15 WITHWITHOUT Ar( cm -3 ) 9.2x10 16 2.3x10 18 Higher temperatures along the axis of the arc tube give rise to transient velocity fields. Neutral density decreases at regions of higher temperature close to the axis and increases at larger radii closer to the walls. Conditions: Ar, 70 Torr, gap=0.8cm, 10 s

18. University of Illinois Optical and Discharge Physics HYDRODYNAMIC EFFECTS: T e GEC03_16 T e is comparatively higher in regions that show decreased neutral densities. T e (eV) 0.15 Conditions: Ar, 70 Torr, gap=0.8cm, 10 s WITHWITHOUT

19. University of Illinois Optical and Discharge Physics GEC03_17 S-E (cm -3 s -1 ) 5x10 17 5x10 19 HYDRODYNAMIC EFFECTS: IONIZATION SOURCES Higher T e helps maintain sustained ionization sources along the axis. Peak value of ionization sources is higher. Conditions: Ar, 70 Torr, gap=0.8cm, 10 s log scale WITHWITHOUT

20. University of Illinois Optical and Discharge Physics SUMMARY GEC03_18 A 2-D plasma dynamics model has been developed for startup of high pressure, metal halide, HID lamps. Breakdown times were investigated as a function of applied bias, composition, and pressure in Ar/Xe mixtures. The model was validated with experimental data. Breakdown times scaled inversely with E/N and non-monotonically with gas composition. In the post-breakdown phase, energy density rises with plasma density to set up thermal gradients and transient flow fields. Perturbations in density resulting from convection cause changes in E/N and these produce rapid changes in plasma properties.