plasma dynamics and thermal effects during startup of metal halide lamps * ananth n. bhoj a), gang...
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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
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 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
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 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.
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
University of Illinois
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
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
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.
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 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.
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.
0vt
plasmaSgIvT
vvpvvtv
32
plasmapp STTvct
Tc
Continuity :
Momentum:
Energy :
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
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
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-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
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
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.
University of IllinoisOptical and Discharge PhysicsGEC03_12
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.
University of IllinoisOptical and Discharge PhysicsGEC03_13
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] Tgas
5x108 5x1012
300 450
[e] (cm-3)
Tgas (K)
log scale
Conditions: Ar, 70 Torr, gap=0.8cm, 10 s
ANIMATION SLIDE
University of Illinois
Optical and Discharge Physics
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.
Conditions: Ar, 70 Torr, gap=0.8cm, 10 s
University of Illinois
Optical and Discharge Physics
HYDRODYNAMIC EFFECTS: Te
GEC03_16
Te is comparatively higher in regions that show decreased neutral densities.
Te (eV)0.1
5
Conditions: Ar, 70 Torr, gap=0.8cm, 10 s
WITHWITHOUT
University of Illinois
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.
Conditions: Ar, 70 Torr, gap=0.8cm, 10 s
log scale
WITHWITHOUT
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.
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