ignition studies of low-pressure discharge lamps m. gendre - m. haverlag - h. van den nieuwenhuizen...
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Ignition Studies of Low-Pressure Discharge Lamps
M. Gendre - M. Haverlag - H. van den Nieuwenhuizen - J. Gielen - G. Kroesen
Friday, March 31st 2006
Experiments on CFL Ignition
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Goals of the study
Set-up
DC breakdown
AC resonant ignition
Summary
OutlinesOutlines
Goals of the study
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Understanding Plasma Ignition
Physics: better comprehension of dielectric-plasmaphase transitions in general
Technology: understand how compact fluorescentlamps ignite under various conditions
Goals of the StudyGoals of the Study
Motivations
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Interest of a better understanding
Goals of the StudyGoals of the Study
Courtesy of R. Richter, private communication
1900s
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Understanding Lamp Ignition
Q: How does low-pressure breakdown work ?
Townsend model: electron avalanche between electrodes
Goals of the StudyGoals of the Study
cathode anode
E
atom
ionelectron
- homogeneous E field
- infinite electrode extension
Neglected by Townsend
- inhomogeneous field
- diffusion losses of charges toward the walls
- wall surface charges
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Goals and Approach
6/22
A: Thorough study of ignition in a ‘standard’ linear lamp
studies:
- different experiments on same lamp design
- different lamp configurations (gas, pressures…)
- control of experiments (repeatability, accuracy…)
- cross-comparisons between results
Global Overview of the Phenomenon
Goals of the StudyGoals of the Study
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Goals of the study
Set-up
DC breakdown
Back to AC resonant ignition
Summary
OutlinesOutlines
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Global Circuitry
Set-UpSet-Up
RLHV probe HV probe
cameracontroller
ICCDcamera
lens
TTL trigger
Faraday cage
lamp
capacitive probe driver
computer
digitaloscilloscope
ch
1
2
3ch 4
voltage regulator
IF
IG
poweramplifier
function generator
IA
pulse generation
low-voltage waveform amplified to 200-1200V
double pulse schemefor charge clean-up
load resistor for lampcurrent regulation
poweramplifier
function generator
IA
voltage regulator
electrode heating
active electrode at1000K for e- emission
electrode impedanceconstant over time
electrode voltage keptat constant value
no drift of electrodeperformances duringthe experiments
time reference
same time base for allinstruments
lamp voltage taken asthe main reference
simultaneous triggeringof scope and camera
cameracontroller
ICCDcamera
lens
computer
1- optical imaging
fast iCCD camera for50-500ns time resolution
one full lamp ignitionfor each image taken
20 to 60 images areadded for each time step
data processed to givespace-time diagrams capacitive
probe driver
2- potential probing
custom-built floating capacitive probe
lamp potential mappingin time and space
7mm and 1ms space and time resolutions
data processed to givespace-time diagrams
IF
IG
3- current recording
current measured atthree critical points
sub- s and A timeand current resolutions
current waveformsrecorded by oscilloscope
poweramplifier
IA
Faraday cage
lamp
Faraday cage
critical for the accuracyof the experiments
poweramplifier
IA
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probe rack
- house and locate the probe
lamp
- 145mm long, 10mm diameter
ITO window
- transparent and electrically conducting
Faraday’s cage
- stable electrostatic environment
9/22
Set-UpSet-Up
Experimental Frame of Reference
electrostatic probe
- senses the lamp surface potential
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Goals of the study
Set-up
DC Breakdown
AC resonant ignition
Summary
10/22
OutlinesOutlines
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-400 -300 -200 -100 0 V
K Apotential
11/22
Ignition Mechanism Overview
0
A
14 x (cm)
K
-500 +5000
light
global evolution identicalin both cases
apparent lag of light emission (max 1s)
smooth evolution of lamp potential
potential gradient in the wake of first wave
DC BreakdownDC Breakdown
Argon 3 torr –600V
Pre-breakdown wave:
- starts at the cathode
- propagates toward the anode
- speed and intensity decreases
Return strike:
- starts at end of first wave propagation
- propagates toward the cathode
- speed and intensity decreases
Argon 3 torr –600V
Pre-breakdown wave:
- starts at the cathode
- propagates toward the anode
- speed and intensity decreases
time
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3torr Ar-Hg lamp : -500V dt=100ns
Cathode-Initiated Breakdown
DC BreakdownDC Breakdown
K A (0)
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DC BreakdownDC Breakdown
Argon 3torr –200V : Failed Ignitionresolution potential: 10ns optical: 100ns
Argon 3torr –300V : Failed Ignitionresolution potential: 10ns optical: 500ns
Argon 3torr –400V : Successful Ignitionresolution potential: 10ns optical: 500ns
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Pre-Breakdown Wave Position vs. Voltage
- wave speed directly proportional to voltage
- ignition condition: first wave has to reach the anode
DC BreakdownDC Breakdown
10 - 3 km/s
51- 44 km/s
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Lamp Net Charges vs. Voltage
DC BreakdownDC Breakdown
- first wave charging effect increases with voltage
- decrease of net charge only for successful breakdown
- Ignition condition: charging threshold to be reached
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_ +
High radial/axial E-field component Local Townsend-like breakdown Wall charging and field displacement Further ionization in front of cathode
Global Overview of the Phenomenon
DC BreakdownDC Breakdown
Qualitative model
e
Wave propagation toward the anode
Decreasing electron current flux
Field rotation and enhancement Electrodes bridged, circuit closed
Steep current increase
Wave propagation toward the cathode
Global lamp charge decrease
Exponential current increase Current stabilized by ballast- ionization wave driven by front field, rate of wall charge
- wave speed dependent on E/p value
- gradual decrease of field and wave speed during propagation
- ignition condition : E/p high enough for 1st wave to reach anode
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Goals of the study
Set-up
DC breakdown
AC resonant ignition
Summary
17/22
OutlinesOutlines
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Correlation with DC Breakdown
- synchronous propagation of K and A waves
- importance of surface charge memory effect
- easier ignition in alternating potentials as a result
20/22
AC Resonant IgnitionAC Resonant Ignition
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Goals of the study
Set-up
DC breakdown
AC resonant ignition
Summary
21/22
OutlinesOutlines
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Global Overview
- multiple diagnostic tools running simultaneously
- cross comparisons between optical/electrical data
- various experimental conditions investigated
- correlation between wave propagation and lamp charging
- minimum lamp charging required for successful ignition
- new information inferred from data analysis
22/22
SummarySummary
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Global RC-Probe Circuit
Provides lamp surface potential vs. time/space
R
U
Faraday’s cage
Z
+
-
- limited field disturbance around the lamp
- Z chosen so total system transfer function = pure real
- little need for post-experiment data treatment
Set-UpSet-Up
x
r