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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
* Work supported by General Electric and NSF
** Present Affiliation: Sun Microsystems, Inc.
ICOPS01_title
AGENDA
 Metal-halide, HID Lamps
 Description of Model
 HID Startup with Trigger Electrode
 Role of Photoionization
 Startup of Hot Lamps
 Concluding Remarks
ICOPS01_agenda
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.
 In the steady state, HID lamps are
thermal arcs, producing quasicontinuum radiation from a multiatmosphere, 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).
ICOPS01_01
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 85Kr for preionization.
 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 85Kr.
ICOPS01_02
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 2dimensional model.
 2-d rectilinear or cylindrical unstructured mesh
 Implicit drift-diffusion for charged and neutral species
 Poisson’s 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.
ICOPS01_03
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.


N i
   qN ii     DiN i   Si
t
 Photoionization:
 
  r  r


 N i ( r )ij N j ( r ) exp 

 

SPi ( r )  
 2
4

r  r


 
d3r 

 Electric field and secondary emission:
SSi    j,
ICOPS01_04


1/2  
 
    W  q 3E/ 0  
 ,
jE  AT 2 exp  


kTS




jS    ij j
j
University of Illinois
Optical and Discharge Physics
DESCRIPTION OF MODEL (cont.)
 Poisson for Electric Potential:
    V  S
 Volumetric Charge:
 V
     q ii 
t
i
 Surface Charge:
 S
     qii 1   i          jE 
t
i
 Solution: Equations are descritized using finite volume techniques and
Scharfetter-Gummel fluxes, and are implicitely solved using an iterative
Newton’s method with numerically derived Jacobian elements.
Ni ( t  t )  Ni ( t )  Nit
 N i 
N i
N j
N i  N i ( t  t )  N i ( t ) 
( t  t )  t   


t
j  N j 
ICOPS01_05
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.
1 cm
Fin
Grounded
Electrode
Air
Powered
Electrode
"W indings"
Trigger
Electrode
Plasma
Quartz Tube
Grounded
Housing
Fin
CL Cylinder Center-line
ICOPS01_06
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.
BIAS (V)
0
 Roughness on the trigger
electrode provides
sufficient electric field
enhancement for electron
emission.
 No other initial sources of
electrons are allowed.
-500
-1000
TRIGGER
-1500
-2000
-2500
TRIGGER AND
POWER ELECTRODE
-3000
-3500
-4000
0
ICOPS01_07
100
200
300
TIME (ns)
400
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 1011 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 photoionization of Hg.
 Pulsation occurs at the cathode.
4 x 107 - 2 x 1011 cm-3
 75 Torr, Ar/Hg = 75/0.001 (slightly
warm), 450 ns.
3 x 108 - 2 x 1012 cm-3
ICOPS01_08
University of Illinois
Optical and Discharge Physics
LEADING EDGE OF TRIGGER PULSE ([e] and Te)
 Te closely follows the electric field. The electron density is sufficiently
low that little shielding occurs.
Electron Temperature
Electron Density
 As the voltage ramps to 600 V (15 ns), electric field
emission seeds the minigap.
 Avalanche preferentially
occurs near the windings
where the gross electric
field and Te are largest.
 75 Torr, Ar/Hg = 75/0.001
(slightly warm), 0 - 30 ns.
0 - 6 eV
7 x 106 - 7 x 1010 cm-3
ICOPS01_09
University of Illinois
Optical and Discharge Physics
LEADING EDGE OF TRIGGER PULSE (e-SOURCES)
 Electron impact ionization occurs near the trigger electrode tip and near
the windings closely tracking the electron temperature.
Electron Impact Ionization
Photoionization
 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.
7 x 106 - 7 x 1010 cm--3s-1
9 x 1012 - 9 x 1016 cm--3s-1
ICOPS01_10
University of Illinois
Optical and Discharge Physics
PHOTIONIZATION LEADS ELECTRON IMPACT
 Photoionization of Hg provides seed electrons in advance of the
electron impact avalanche front, similar to stream propagation.
[Photoionization][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.
MIN
ICOPS01_11
MAX
University of Illinois
Optical and Discharge Physics
PHOTIONIZATION LEADS ELECTRON IMPACT AT ANODE
 The leading of electron impact of photoionization is best illustrated at
the anode.
Electron Density
 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.
5 x 108 - 5 x 1011 cm-3
ICOPS01_12
University of Illinois
Optical and Discharge Physics
[e] vs TEMPERATURE
 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.
100/ 0.001
Ambient
99.9/0.1
50 C
97/3
140 C
Ar (75 Torr cold fill) / Hg
ICOPS01_13
7/3
220C
5 x 108 - 5 x 1011 cm-3
0-450 ns
University of Illinois
Optical and Discharge Physics
CONCLUDING REMARKS
 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.
ICOPS01_14
University of Illinois
Optical and Discharge Physics