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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Interest of a better understanding

Goals of the StudyGoals of the Study

2000s

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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

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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

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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

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OutlinesOutlines

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-400 -300 -200 -100 0 V

K Apotential

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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

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OutlinesOutlines

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Optical Recording

AC Resonant IgnitionAC Resonant Ignition

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Electrostatic Recordings

AC Resonant IgnitionAC Resonant Ignition

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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

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AC Resonant IgnitionAC Resonant Ignition

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Goals of the study

Set-up

DC breakdown

AC resonant ignition

Summary

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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

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SummarySummary

PHILIPS TU/eANY QUESTIONS?ANY QUESTIONS?

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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

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Calculated Data from RC-Probe Output

Measured: =f(t,x)

Calculated:

radial E field: )/ln()/()/ln(),(

bcabaxtE

agR

disp. current:t

CI LD

linear charge: LL CQ

|E| field: 22),( RX EExtE

<E field:

R

X

E

EE 1tan

axial E field:x

xtEX

),(

total disp. current

total charge