Transcript No Slide Title

PLASMA DYNAMICS AND THERMAL EFFECTS
DURING STARTUP OF METAL HALIDE
LAMPS*
Ananth N. Bhoja), Gang Luob) and Mark J. Kushnerc)
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
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
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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.
 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 metalhalide salts.
 Initiation consists of high pressure
breakdown of the cold gas, heating of the
cathode and housing, vaporizing the metal
(-salts).
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STARTUP OF HIGH PRESSURE HID LAMPS
 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
GEC03_02
85Kr.
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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.
[1] R. Moss, MS Thesis, UIUC
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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
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2-D MODEL: LAMPSIM
 LAMPSIM, a 2-dimensional model has been developed.
 2-d rectilinear or cylindrical unstructured mesh
 Poisson’s equation with volume and surface charge, and
material conduction:
 V

   q i i
 
       V   S
t
i


 S
     q i  i 1   i            jE
t
i






 Multi-fluid charged species transport equations are discretized
using the Scharfetter-Gummel technique.
Ni
t
 
   i  Si

 n  n i exp(   x
 i  1 / 2   D  i  1
 1  exp(   x )



 q    i 1   i 


 
  2 v BULK
 q  
x



 
D
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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.
N i (t   t )  N i (t )   N i t
 Ni  Ni (t   t)  Ni (t) 
Ni
t
(t   t)   t 

j
 N 
i 

N j
 N 
j 

 Circuit model
 Electron energy equation coupled with Boltzmann solution
for electron transport coefficients
 Surface chemistry.
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COUPLED PLASMA AND HYDRODYNAMICS
 To investigate effects of hydrodynamics in the startup phase, the
plasma dynamics model was coupled to a Navier-Stokes solver.
 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:


    v   0
t

 

 T 2  
  
  v 
    v v    p       v   v    v   I    g  S plasma
t
3

 
c pT
Energy
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:
t

    c p v T       T   S plasma
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MODEL GEOMETRY AND UNSTRUCTURED MESH
 Investigations into a cylindrically symmetric lamp based on the
experimental lamp geometry were conducted using an
unstructured mesh.
0.5cm
Dielectric
HEIGHT (cm)
Plasma
Air
Quartz
tube
ELECTRODE GAP = 1.6 cm
Powered
electrode
Dielectric
Grounded
electrode
CL
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Grounded
housing
Cylindrical center line
RADIUS (cm)
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PLASMA DYNAMICS: E/N
30 Torr,
Ar
70 Torr, 30 Torr,
Ar
Ar/Xe=90/10
 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.
ANIMATION SLIDE
1x10-16
E/N (V-cm2) 1x10-14
log scale
0-355 ns
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0-875 ns
0-275 ns
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Optical and Discharge Physics
PLASMA DYNAMICS: ELECTRON DENSITY
30 Torr,
Ar
70 Torr, 30 Torr,
Ar
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.
ANIMATION SLIDE
5x109
[e] (cm-3)
5x1012
log scale
0-355 ns
GEC03_10
0-875 ns
0-275 ns
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Optical and Discharge Physics
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.
GEC03_11
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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.
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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.
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BREAKDOWN AND GAS HEATING
[e]
Tgas
 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.
300
5x108
[e] (cm-3) 5x1012
log scale
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 Conditions:
Tgas (K)
450
Ar, 70 Torr, gap=0.8cm, 10 s
ANIMATION SLIDE
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HYDRODYNAMIC EFFECTS: NEUTRAL DENSITY
WITHOUT
WITH
 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:
9.2x1016
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Ar(cm-3)
2.3x1018
Ar, 70 Torr, gap=0.8cm, 10 s
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HYDRODYNAMIC EFFECTS: Te
WITHOUT
WITH
 Te is comparatively higher in
regions that show decreased
neutral densities.
 Conditions:
0.1
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Te (eV)
5
Ar, 70 Torr, gap=0.8cm, 10 s
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HYDRODYNAMIC EFFECTS: IONIZATION SOURCES
WITHOUT
WITH
 Higher Te helps maintain
sustained ionization sources
along the axis.
 Peak value of ionization sources
is higher.
 Conditions:
5x1017
S-E
(cm-3s-1)
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log scale
5x1019
Ar, 70 Torr, gap=0.8cm, 10 s
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Optical and Discharge Physics
SUMMARY
 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.
GEC03_18
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