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Microhollow Cathode Discharge & Cathode Boundary Layer Discharge
Wei-Dong Zhu
Saint Peter’s College Jersey City, New Jersey, USA
Summer Institute on Complex PlasmasAugust 5, 2008, Hoboken, New Jersey
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Outline
• Introduction: what leads us to small size?• Microhollow Cathode Discharge (MHCD)
-Two scaling laws- Excimer formation- Plasma Plume
• Cathode Boundary Layer Discharge:• Visible and Vacuum Ultra Violet Emissions• Self-organization• Expending the active area: parallel operation• Attempts to explain self-organization
• Summary
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Types of plasmas•
High-temperature plasmas (HTP)
Ti
≈
Te
≥107
Ke.g., fusion plasmas
•
Low-temperature plasmas (LTP)Thermal LTP
Ti
≈
Te
≈
Tg
≤
2 x 104
Ke.g. arc plasma at normal pressure
Non-thermal LTPTi
≈
Tg
≈
300 KTi
<< Te
≤
105
Ke.g. low-pressure glow discharge
high-pressure non-thermal plasma
Hippler R./ Kersten H./ Schmidt M./ Schoenbach K. H.“Low Temperature Plasmas, Fundamentals, Technologies and Techniques”
(2001, 2008)
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Low Pressure Non-Equilibrium Plasmas
-1mTorr to a few Torr
-
Semi-conductor industry has achieved great success using plasma processing (e.g. in the manufacture of microchips)
-
Essentially all plasmas used in semiconductor processing are low-pressure plasmas.
There are many applications where the vacuum enclosure required for a low-pressure plasma is an obstacle for its technological use
Air Pressure Non-Equilibrium Plasmas
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Atmospheric pressure Non- Equilibrium Plasmas
No vacuum is involved at normal pressure, but ran into some new challenges such as glow to arc transition (GAT).
Arc Discharge: thermal plasma
-
Gas temperature can reach as high as 2x104 K- Low voltage drop at cathode- High cathode current density
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The first Atmospheric Non- Equilibrium Plasmas: 1857
“Ozone Production in an Atmospheric-Pressure Dielectric Barrier Discharge” (1857)Werner V. Siemens
Atmospheric-Pressure
More to see in next talk by Dr. J. Lopez
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Achieve Non-Equilibrium at Atmospheric PressureA few solutions to achieve stable, glow discharge at atmospheric
pressure
Transient plasmas: In atmospheric plasmas, for efficient gas heating at least 100-1000 collisions are necessary. Thus, if the plasma duration is shorter than 10-6
– 10-5
s, gas heating is limited. Of course, for practical purposes such plasma has to be operated in a repetitive mode, e.g., in trains of microsecond pulses with millisecond intervals.
Micro-plasmas: Gas heating occurs in the plasma volume, and the energy is carried away by thermal diffusion/convection to the outside. If the plasma has a small volume and a relatively large surface, gas heating is limited.
Dielectric Barrier Discharges: These plasmas are typically created between flat parallel metal plates, which are covered by a thin layer of dielectric or highly resistive material. The dielectric layer plays an important role
in suppressing the current: the cathode/anode layer is charged by incoming positive
ions/electrons, which reduces the electric field and hinders charge transport towards the electrode. DBD also has a large surface-to-volume ratio, which promotes diffusion losses and maintains a low gas temperature.
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Reducing the Distance between Anode and Cathode
J.P. Boeuf, E. Marode, J. Phys. D 15, (1982), 2169
Reducing the electrode distance to the order of the cathode fall reduces the discharge mainly to cathode fall and negative glow where the electron energy distribution is highly non-thermal: beam electrons.
electron energy distributions that are strongly non-
Maxwellian
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Micro Hollow Cathode Discharge(MHCD)
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A HC discharge device consists of a cathode with hollow structure (hole, aperture, etc.) in it and an arbitrarily shaped anode.
Two scaling laws
1. Paschen Breakdown Law (p, d)The product of pressure (p) and the anode-cathode separation (d), “p·d”
and the break down voltage obeys the well-known Paschen breakdown law, which applies to all discharges and determines the required breakdown voltage for given values of p, d, and the operating gas.
Hollow Cathode (HC) Discharge
Normally running at low pressure
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A HC discharge device consists of a cathode with hollow structure (hole, aperture, etc.) in it and an arbitrarily shaped anode.
Two scaling laws
2. White-Allis Law (p, D)
-It relates the discharge sustaining voltage V to the product of pressure (p) and the cathode opening dimension (D), “p·D”.
-
If the product p . D is between a lower and a higher limit (e.g. 0.1 to 10 Torr . cm), the discharge develops in stages, each with a distinctive I-V characteristics.
Hollow Cathode (HC) Discharge
Lower Limit:the mean free path for ionization cannot exceed the hole diameter.
Higher Limit:the distance between “opposite”
cathodes cannot exceed the combined lengths of the two cathode fall regions plus the glow region.
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Micro Hollow Cathode Discharge (MHCD)
MHCD: Pushing HC towards atmospheric pressure.Sometimes referred to as high pressure HC.
Critical dimensions at atmospheric pressure:d: <500 μmD: 10 –
300 μm (assuming at room temperature)
Human Hair: 60 –
100 μmMost of the experimental studies are in rare gases and rare gas halide mixtures, with an increasing interest on atmospheric pressure air .
Sandwich Structure:Electrode –
Dielectric -
Electrode
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Electrode Geometries, Materials, and Fabrication Techniques
Electrode Material: molybdenum, silver, stainless steel, nickel platinum, copper
Dielectric Material: mica, aluminum oxide wafer, silicon oxide film, even polymer in some cases
Fabrication Techniques: 1)Openings: drilled mechanically, milled ultrasonically, drilled with excimer laser, wet chemical etching2)Packaging: high temp glue, direct plasma spray, micro-fabrication
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MHCD as UV radiation source –
Xenon as an example
•
High electron energy (E>8.4 eV) is required for the generation of excimer precursor: Xe (2P3/2
)6s.
XeXeXeXeXe +→++ *2
*
•
High pressure is needed to provide a favorable environ-
ment for three body processes to form excimers.
*XeeXe →+
2nd
excimer continuum at 172 nm
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K.H. Schoenbach, A. El-Habachi, W. Shi and M. Ciocca, Plasma Sources Sci. Technol. 6 (1997) 468
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Calculated I-V characteristic (solid squre) and peak gas temperature (cross) vs current. The dashed lines sid and siid are characteristics calculated for discharges sustained by electron emission from inside the hollow cathode or from the outer surface of the cathode, respectively.
J. P. Boeuf, L. C. Pitchford and K.H. Schoenbach, Appl. Phys. Lett. 86, 071501 (2005)
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MHCD as UV radiation source –
Xenon as an example
Pressure dependence of the VUV emission spectrum of MHCD in xenon. V: 215 -
235 V and I: 3 -
3.3 mA
Measured and calculated efficiency of the xenon excimer radiation vs pressure. I: 3–3.5 mA
A. El-Habachi and K. H. Schoenbach, Appl. Phys. Lett., Vol. 73, No. 7, 17 August 1998
MgF2
window and vacuum is used to reduce the absorption of UV by air!!
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Plasma ParametersElectron Temperature:
-
Evaluated via emission spectroscopy (line intensity) shows electron temperature ~ 1 eV in argon MHCD.
-
The fact that MHCDs are efficient source of excimer radiation suggest that large concentrations of high-energy electrons (in excess of the excitation energy of rare gas atoms) must be present.
Electron Density:-
Evaluated via Stark broadening and shift of argon lines at 801.699 and 800.838 nm, and the hydrogen Balmer-β
line at 486.1 nm-
Evaluated via a heterodyne infrared interferometry-
Electron density is in the range of 1013
– 1015
cm-3
depending on the mode of operation (pulsed or DC) and on operating current
Gas Temperature:-
Evaluated via optical emission spectroscopy (by fitting experimental data of the rotational (0,0) band of the second positive N2
system to modeling data) or absorption spectroscopy (Doppler Broadening of argon lines )
-
ranging from 400 K to 2000 K (higher for molecular gas, lower for low atomic weight rare gas)
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Plasma-Reactors (e.g., Dr. R. Besser, Dr. K.H. Schoenbach)
Other Applications of MHCD
C. Jiang, A.-A. H. Mohamed, R. H. Stark, J. H. Yuan, and K.l H. Schoenbach, IEEE TRANSACTIONS ON PLASMA SCIENCE, 33, 2005
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Other Applications of MHCDPlasma display cell (e.g., Dr. G. Eden)
More to see in Dr. G. Eden’s talk
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When a potential is placed across the electrodes and a gas is directed through the openings, the MHCD assembly is capable of generating a non-thermal, atmospheric pressure plasma micro jet.
Plasma micro jet
[A. Mohamed, et al., US patent application 20060028145]
The non-thermal plasma micro jet is usually in the dimensions of sub-
millimeter in diameter and less than 1 centimeter in length.
MHCD Plasma Plume
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Metal
Insulator
5 k
100 - HV
Gas
Flo
w
MHCD Plasma Plume
•
J. K. Kolb, A.-A.H. Mohamed, R.O. Price, R. J. Swanson, A. Bowman, R.L. Chiavarini, M. Stacey and K.H. Schoenbach, Appl. Phys. Lett.
2008, in press•
H.Q. Feng, P. Sun, Y.F. Chai, G.H. Tong, J. Zhang, W. Zhu and J. Fang, IEEE Trans. Plasma Sci., 2008, transcript in revision
Direct CurrentGas: Air, Oxygen, Nitrogen, Noble GasFlow Rate: 0.3 –
3 slmPower: ~ 10 –
15 W
Opening diameter: ~0.8 mmLength of plume: ~ 1cm
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A B C D E F-5
0
5
10
15
20
25
30
35
40
Surv
ival
Rat
e (%
)
Type of Bacteria
Treated Area Untreated Area
100 - HV
1.0 cm
Ø 9.0 cm
Petri Dish
2.0 cm
2.0 cm
E. Coli
Bacteria
A Escherichia coli
B Staphylococcus aureus
C Micrococcus luteus
D Bacillus megaterium
E Bacillus subtilis
F Bacillus natto
Plasma Plume Treatment of Bacteria
More to see in Dr. K. Becker’s Talk
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Plasma-Liquid Interaction
Gas
Liquid
The plasma activated species (in the case of air: O, O3
, NOx
(very small amount), OH…) is directly injected into the liquid.
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Comparing to other Plasma- Liquid Interaction Techniques
Spark Discharge Corona discharge Gliding arc discharge
H2
O2
production is used as an indication of the oxidative ability
Pictures courtesy of Nachiket Vaze (Drexel University)
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Discharge Voltage (kV) Power (W) H2
O2 (g kW−1h−1) References
Gliding arc 12 250,500 0.0652, torch mode
Burlica R0.544, normal mode
Pulsed 40 30 < 100 Hz 0.843
Anpilov,
A.
M.25 200 < 100 Hz 0.905
Pulsed 46 22.8, 60 Hz 3.64 Grymonpr´e D R
Pulsed streamer corona 40 140, 60 Hz 0.137 Joshi A A
Glow 1.55 90 4 Ivannikov A AGlow 0.5–0.8 10–160 — Hickling AContact glow 0.4–0.5 1.6–3.6 — Sengupta S KGlow 3 30 5.44 Titova U VDC diaphragm 2.8 456 0.74 Stara
Z
Pulsed streamer corona 25–40 50 Hz — Sharma A K
Capillary 1 mm 3.01 76.7 4.1 A Yu Nikiforov
Capillary 5 mm 5.4 18.3 7.8 Direct Injection of PAS 0.4 -
0.5 8 ~ 40 This work
[Table partially from “A Yu Nikiforov and Ch Leys 2006 Plasma Sources Sci. Technol.
16 (2007) 273–280”]
Hydrogen Peroxide Production
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Degradation of Rhodamine WT
200 ml of de-ionized water contaminated with Rhodamine WT (pink dye) (200 ppb). Rhodamine WT was reduced from 200 ppb to ~80 ppb in 1
hour.
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Cathode Boundary Layer Discharge (CBLD)
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Cathode Boundary Layer (CBL) Discharges
Materials:Electrodes: MolybdenumDielectric: Alumina
Dimensions:Electrode Thickness: 100 μm to 250 μmDielectric Thickness: 100 μm to 250 μm Opening Diameter: 300 μm to 4.5 mm
CBL discharges are DC driven and have:
High excimer
intensity (~ 1 W/cm2)
Efficiency of 3-7%
Low operating voltage
(<400 V)
Simple structure
and 2-D flat panel feasibility
K.H. Schoenbach, M. Moselhy, and W. Shi, Plasma Sources Scie. Technol. 13, 177 (2004)
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Experimental Setup
•
Gas: Xenon (Scientific grade)
•
Spectral filter: 170 nm with FWHM of 26.8 nm
•
Sample: Either mechanically assembled at Old Dominion University or plasma sprayed at University of Minnesota
HELP
ALPHA
SHIFT
ENTERRUN
DG ER FI
AJ BK CL
7M 8N 9O
DG DG DG
DG T 3U
0V .WX Y Z
TAB
% UTILIZATION
HUB/MAU NIC
2BNC4Mb/s
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Self-organization in the visible
2.32 0.42 0.12 0.1
3.9 1.1 0.67 0.49
6.7 2.7 1.75 0.64
7.6 5.1 3.4 1.9
75
200
400
760
Pres
sure
(Tor
r)
Current (mA)
2.32 0.42 0.12 0.1
3.9 1.1 0.67 0.49
6.7 2.7 1.75 0.64
7.6 5.1 3.4 1.9
75
200
400
760
Pres
sure
(Tor
r)
Current (mA)
visible
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Self-organization in VUV
1 0.4 0.13 0.09
4 1 0.62 0.53
7 2.5 1.6 0.42
7.6 5 3.4 1.7
75
200
400
760
Pres
sure
(Tor
r)
Current (mA)
1 0.4 0.13 0.09
4 1 0.62 0.53
7 2.5 1.6 0.42
7.6 5 3.4 1.7
75
200
400
760
Pres
sure
(Tor
r)
Current (mA)
VUV (172nm)
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current: 0.37 mACurrent density: 85 mA/cm2
75Torr xenond = 290μm
Cathode Boundary Layer Discharge between Molybdenum Cathode and Indium Tin Oxide
(transparent) Anode
cathode
anode (with Indium Tin Oxide film on glass slide)
750μm
290μm
direction of observation
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75 Torr xenon; cathode diameter: 750μm
Reducing the Current Leads to Self- organization
0.165 mA 0.090 mA
0.036 0.052 mA0.064 mA
0.136 mA
0.108 mA0.146 mA0.373 mA
mA
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Single Filament
0.60 mA 0.09 mA
0.07 mA 0.05 mA
50 Torr
0.60 mA 0.09 mA
0.07 mA 0.05 mA
50 Torr
with reduced pressure or cathode diameter
0.041mA
0.20mA 0.085mA
0.062mA
, D = 750μm 62Torr , D = 300μm
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0.0 0.2 0.4 0.6 0.8 1.0 1.2
270
300
330
360
390
420
450
Vol
tage
[V]
Current [mA]
75 Torr
I-V Characteristics (close-up)
Instability at transition from abnormal to (sub)normal discharge leads to formation of filaments.
1
2
3
4
5
6
75 Torr XenonD = 750μmd = 250μm
1
2
34
56
7
7
unstable
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Information on Cathode Fall Voltage and Secondary Emission Coefficient from
Measurements of Cathode Fall Length
camera
Schematic diagram of setup
camera
quartzTungsten
camera
Schematic diagram of setup
camera
quartzTungsten
camera
Schematic diagram of setup
camera
quartzTungsten
I = 0.092 mAI = 0.179 mAI = 0.224 mA
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Cathode Fall Thickness
Measured normal cathode fall thickness depending on pressure
The observed cathode fall thicknesses, as a function of current I
. The self-organization is exhibited when I
< 0.2 mA
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γ
(secondary emission coefficient) is calculated using information on the thickness of the cathode for normal glow operation:
Apdn
)/11ln(76.3 γ+=
Secondary Emission Coefficient
40 60 80 100 120 1400
50
100
150
200
γ=0.01
γ=0.03
γ=0.07
γ=0.2
Calculated Dataγ=0.03
Experimental Data
Thic
knes
s of
cat
hode
fall
[ μm
]
Pressure [Torr]
0.1 1
270
300
330
360
390
420
450
Vol
tage
[V]
I [mA]0.1 1
270
300
330
360
390
420
450
Vol
tage
[V]
I [mA]
Normal glow
↓
γ
= 0.03
A is the inverse of the product of pressure and mean free path for ionization. For xenon, A = 26 (cm Torr)−1
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UV-Optical Power and Efficiency decreases when the discharge transits into the abnormal glow mode
Current (mA)0.1 1 10
Pow
er (m
W)
0
5
10
15
20
25
75 Torr200 Torr400 Torr760 Torr
Xenon
Current (mA)0.1 1 10
Effi
cein
cy (%
)
0
1
2
3
4
5
6
75 Torr200 Torr400 Torr760 Torr
Xenon
Moselhy M and Schoenbach K H, 2004 J. Appl. Phys. 95 1642-49
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Rate coefficients are taken from D. Eckstrom et.al., J. Appl. Phys. 64, 15 (1988).
Calculation of Excimer
Density
Xe2* Xe*
Xe+
Xe
electronic collisionheavy particle collisionradiation
**
* 2)(XeXe
Xe nNKnnKKnNKdt
dngasexcimeredestepegasex ××−××+−××=
exderadgasexcimer nnKnKnNKdt
dnXeXeXe
Xe ××−×−××= − *2
*2
*
2*
2
{ }2)()(
3
*2
gasexcimerstepdexdeerad
edegas
NKKKnKnKnKNKn
e
excimer
Xe ×++××+×××
=−
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1014 1015 10160
1x1014
2x1014
3x1014
4x1014
5x1014
6x1014
Exci
mer
den
sity
[1/c
m3 ]
Electron density [1/cm3]
75 Torr, 400K 150 Torr, 600K 250 Torr, 800K
Higher Pressure Is Preferable for Excimer
Formation
Excessive electrons (slow electrons) inhibit excimer formation.
Current (mA)0.1 1 10
Pow
er (m
W)
0
5
10
15
20
25
75 Torr200 Torr400 Torr760 Torr
Xenon
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Optimization of the operating point
Visible Pictures
VUV Pictures (172 nm)
•
The peak of the excimer
emission coincides with the turning of the I-V curve, which is at a current of about 0.8 mA
•
In order to operate multi-holes in parallel, the I-V characteristic has to be positive. Around 1 mA/hole seems to be an optimum region.
0.4 0.8 1.2 1.6 2.0220
240
260
280
300
320
0.0
0.5
1.0
1.5
2.0
2.5
Voltage
Volta
ge (V
)
Current (mA)
Excimer Emission
Exc
imer
Em
issi
on In
tens
ity (
W/c
m2 )
250 Torr
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Excimer
Emission and Discharge Voltage over an Extended Time
0 5 10 15 20 25
230
240
250
260
270
280
1500
2000
2500
3000
3500
4000
Volta
ge (V
)
Time (hrs)
VoltageRefill
Excimer Emission
Exc
imer
Em
issi
on In
tens
ity (A
.U.) •
The excimer
emission intensity decays to about 50% in 20 hours.
•
The decrease in emission is always correlated to an increase in discharge voltage.
•
Exchanging the gas in the chamber returns the excimer
emission to almost its initial value.
•
Impurity due to leaking or outgassing
from the components in the chamber
•
Sealing up the device
0 4 8 12 16 20800
1000
1200
1400
1600
1800
2000
2200
OH
Emis
sion
Inte
nsity
(A.U
.)
Time (hrs)
Refill
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Scaling
Parallel operation of multiple openings –
Multi-CBL structure
Maintain the sandwich structure and scale up in one direction –
Micro-slit structure
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Parallel operation without individual ballasting
• Cathode: Mo ~0.25 mm thick
• Dielectric: Al2
O3
~0.25 mm thick
• Anode: Mo ~0.25 mm thick
• Hole diameter: ~0.75 mm
• Center to center distance: ~1.5 mm
Visible Picture of parallel operation of 9 holes
(Operating gas: xenon (scientific grade)
Base pressure: ~1 mTorr; Working pressure: 200 Torr
Cathode voltage: -398 V; Discharge current: 6 mA)
Without individual ballasting or other initial ignition assistance, the multi-hole samples tend to be more easily ignited and sustained at lower pressure (<~200 Torr)
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Parallel operation without individual ballasting
0 1 2 3 4 5 6 7
380
400
420
440
460
480 Increasing
Volta
ge (-
V)
Current (mA)
Decreasing
Current-Voltage characteristics of an eight circular-opening CBL discharge (visible images of the discharge are embedded at corresponding currents indicated by arrows).
Subsequent individual discharges occurred at voltages much lower than that which was required to ignite the discharge in the first opening:
-
pre-ionization of the gas in the surrounding openings by the VUV light emitted from previously ignited opening(s) and/or possibly the diffusion of long-living positive and negative ions towards neighboring opening(s)
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VUV emission- Only partial image (limited by the magnification of the system)- Low excimer emission (due to the lower pressure: 100 Torr)- At higher pressures (>150 Torr), stable operation with discharges in all openings is not easily achievable. - Also noticeable is that the excimer emissions from different openings differ in intensity, which may have been caused by the difference in shape and diameter between individual openings due to the mechanical drilling process.
Partial VUV image of an 8 circular-opening CBL discharge working in parallel
(100 Torr, 446 V and 5.07 mA)
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Self-organization
Xenon (100 Torr) Xenon (250 Torr)
(ignition assisted with mechanical switch)
Parallel operation of self-organization can be achieved by tuning down current steadily after all holes are ignited
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Operation of Micro-Slit CBL Structure
Visible picture of a Micro-slit discharge
(Working pressure: 250 Torr
(Xenon)
Cathode Voltage: 224 V
Discharge current: 9.3mA)
Slit width: ~ 0.2 mm
Slit length: ~ 1.3 cm
Dielectric thickness: ~ 0.1 mm
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More on Self Organization
Self-organization (Visible images) of a micro- slit CBL discharge: (a)50 Torr; (b)150 Torr; (c)245 Torr; (d)354 Torr and (e)homogeneous discharge at 100 Torr (249V and 4 mA)(The images are at different magnification for a better demonstration purpose)
(a)
(b)
(c)
(d)
(e)
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Current-Voltage characteristics of the Discharge from a Micro-Slit CBL Structure
Region I:
Plasma trying to fill the micro-slit
Region II:
Slit filled with plasma, self-organization patterns persist
Region III:
Evenly distributed plasma
The best operating condition to achieve high excimer
emission intensity is at the current indicated by the lower boundary (between region I and II).
230
280
330
380
430
0 2 4 6 8 10 12
Current (mA)
Vol
tage
(-V
)
50 Torr 75 Torr100 Torr 125 Torr150 Torr 175 Torr200 Torr 225 Torr250 Torr 300 TorrIII
II
I
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Explanation of self-organizationN Takano and K H Schoenbach: Coulomb Forces[Plasma Sources Sci. Technol. 15 (2006) S109–S117] Coulomb forces are assumed to be the reason for self-organization. The cathode fall is determined by a large positive space charge. Any filament developing in this region will therefore extend repulsive forces to neighbouring
cathode fall filaments. These repulsive forces are assumed to be balanced by electrostatic forces from positive surface charges at the surrounding dielectric spacer.
Hamiltonian (the sum of the electrostatic energies of the filaments) of a system which consists of n filaments
The potential ϕι
of a filament at a distance ri
from the centre is
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Explanation of self-organization
M. S. Benilov: Bifurcation analysis-
International Conference on Plasma Science (ICOPS), Travers City, Michigan, June, 2006; -
Physical Review E 77, 036408 2008
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Summary•
Stable (dc), high pressure glow discharges. Structure reduced to cathode fall, negative glow, and anode fall
•
CBL discharges in xenon show excimer
emission intensity up to 2 W/cm2
with internal efficiency up to 7%.•
Excimer
emission peaks at the transition from normal
glow to self-organization.•
Excimer
emission intensity decreases to about 50% in
20 hours without refilling and returns almost to the initial value after exchanging the gas.
•
Single opening CBL discharge can be expanded to multi-CBL structure without individual ballasting or to micro-slit CBL structure.
•
Self-organization patterns were observed in both micro- CBL discharge and micro-slit CBL discharge.
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Acknowledgement
•
Dr. K.H. Schoenbach•
Dr. K. Becker•
Dr. J. Lopez•
Dr. J. Kolb•
Dr. J. Heberlein•
Dr. J. Zhang•
Dr. R. Besser•
Dr. N. Takano•
Mr. R. Price•
Mr. P. Sun•
Ms. H. Feng