plasma dynamics and thermal effects during startup of metal halide lamps * ananth n. bhoj a), gang...

PLASMA DYNAMICS AND THERMAL EFFECTS DURING STARTUP OF METAL HALIDE

LAMPS*

Ananth N. Bhoja), Gang Luob) and Mark J. Kushnerc)

University of IllinoisUrbana, 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

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



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 10’s-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).

Glass Housing

Quartz DischargeTube

GE R400

5 cm



STARTUP OF HIGH PRESSURE HID LAMPS

GEC03_02

Multi-kV pulses are commonly used to breakdown the gap.

Auxiliary electrodes and 85Kr, 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 85Kr.



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


Optical and Discharge PhysicsGEC03_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



2-D MODEL: LAMPSIM

LAMPSIM, a 2-dimensional model has been developed.

GEC03_05

2-d rectilinear or cylindrical unstructured mesh

Poisson’s equation with volume and surface charge, and material conduction:

SV

iii

V qt

iEiii

S j1qt

iii S

t

N

)xexp(1

xexp(nnD i1i

2/1i

D

v2xq

qBULK

i1i

Multi-fluid charged species transport equations are discretized using the Scharfetter-Gummel technique.



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 Newton’s method with numerically derived Jacobian elements.

GEC03_06

tN)t(N)tt(N iii

jj

j

iiiii N

N

Nt)tt(

t

N)t(N)tt(NN

Circuit model Electron energy equation coupled with Boltzmann solution

for electron transport coefficients

Surface chemistry.



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.

0vt

plasmaSgIvT

vvpvvtv

32

plasmapp STTvct

Tc

Continuity :

Momentum:

Energy :



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

Groundedhousing

Air

Groundedelectrode

Poweredelectrode

Quartz tube

Plasma

Cylindrical center line

Dielectric

CL

0.5cm

0.5 cmRADIUS (cm)

HE

IGH

T

(cm

)

EL

EC

TR

OD

E G

AP

= 1

.6 c

m



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-cm2) 1x10-14

0-355 ns 0-875 ns 0-275 ns

ANIMATION SLIDE

log scale

30 Torr, Ar

70 Torr, Ar

30 Torr, Ar/Xe=90/10



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

5x109 [e] (cm-3) 5x1012

0-355 ns 0-875 ns 0-275 ns

ANIMATION SLIDE

log scale

30 Torr, Ar

70 Torr, Ar

30 Torr, Ar/Xe=90/10

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 IllinoisOptical and Discharge PhysicsGEC03_11

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.


EFFECT OF VARYING APPLIED BIAS

B decreases at higher applied voltage for a constant gas pressure and composition.

After VB=1800 V, B saturates as ionization reaction rates begin to saturate as a function of E/N.




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] Tgas

5x108 5x1012

300 450

[e] (cm-3)

Tgas (K)

log scale

Conditions: Ar, 70 Torr, gap=0.8cm, 10 s

ANIMATION SLIDE



HYDRODYNAMIC EFFECTS: NEUTRAL DENSITY

GEC03_15

WITHWITHOUT

Ar(cm-3)9.2x1016 2.3x1018

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.




HYDRODYNAMIC EFFECTS: Te

GEC03_16

Te is comparatively higher in regions that show decreased neutral densities.

Te (eV)0.1

5


WITHWITHOUT


Optical and Discharge PhysicsGEC03_17

S-E(cm-3s-1)

5x1017 5x1019

HYDRODYNAMIC EFFECTS: IONIZATION SOURCES

Higher Te helps maintain sustained ionization sources along the axis.

Peak value of ionization sources is higher.


log scale

WITHWITHOUT



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.

plasma dynamics and thermal effects during startup of metal halide lamps * ananth n. bhoj a), gang...

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