1. CHARACTERISTICS OF ARRAYS OF INDEPENDENTLY CONTROLLED RF MICRO-DIELECTRIC BARRIER DISCHARGES
Jun-Chieh Wanga), Napoleon Leonib), Henryk Bireckib),
Omer Gilab), and Mark J. Kushnera)
a)University of Michigan, Ann Arbor, MI USA
b)Hewlett Packard Research Labs, Palo Alto, CA USA
64th Gaseous Electronics Conference (GEC)
14-18 Nov. 2011, Salt Lake City, Utah, USA
* Work supported by Hewlett Packard Research Labs
Good afternoon, I am Jun-Chieh Wang.
Welcome to my presentation today.
During my talk, I am going to present interesting findings of our study
titled “INTERACTION OF MULTIPLE RF MICRO-DIELECTRIC BARRIER DISCHARGES”
This work is supported by Hewlett Packard Research Labs

2. University of Michigan Institute for Plasma Science & Engr.
AGENDA
Introduction to micro-Dielectric Barrier Discharges (mDBD)
Current extraction
Description of model
Scaling of mDBD Arrays
mDBD sustained in N2 (1 atm)
Aperture Spacing, Dielectric Constants, Frequencies and
Moving Surface
Concluding Remarks
I will first begin with an introduction to micro-dielectric barrier discharges
and current extraction
followed by a description of the model we use,
and the simulation result of mDBD arrays sustained in Nitrogen at atmospheric pressure.
We compare the current extraction of different excitation phase, aperture spacing, dielectric and frequencies
Gas Mixture.
University of Michigan
Institute for Plasma Science & Engr.
GEC2011_1

3. DIELECTRIC BARRIER DISCHARGES
The plasma in DBDs is sustained between electrodes of which one (or both) is covered by a dielectric.
When the plasma is initiated, the underlying dielectric is electrically charged, removing voltage from the gap.
The plasma is terminated when the gap voltage falls below the self-sustaining value and so preventing arcing.
On the following half cycle, a more intense electron avalanche occurs due to the higher voltage across the gap from previously charged dielectric.
The plasma in DBD is sustained between parallel electrodes where
one or both of the electrodes are covered by dielectrics
which separate the electrodes from the gas.
When the plasma is initiated and impinges on the dielectrics,
the dielectrics charge and reduce (remove) the voltage across the gap.
When the voltage falls below the self-sustaining value,
the plasma is quenched, thereby preventing the formation of an arc.
When the polarity changes, a more intense electron avalanche occurs
due to the higher voltage across the gap from the previously charged dielectric,
University of Michigan
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GEC2011_2

4. University of Michigan Institute for Plasma Science & Engr.
MICRO-DBD ARRAYS
mDBDs arrays can be optimized for producing photons, excited states or charge species by independently controlling apertures, repetition rate, pulse shape, geometry and materials.
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\HP_microdischarge_patents\structure_from_ tif
These tens to hundreds “of” microns of micro-plasma are interesting for planar current sources
due to its ability to fabricate large arrays.
So the non-arcing micro-DBDs using rf voltages are attractive for these arrays due to inexpensive mass production and area-selective modification
Here is an application based on the principle of mDBDs.
This electro-graphic appara’tus is applied to surface patterning at atmosphe’ric pressure.
The electron beams extracted from these arrays form charge dots at selective locations on the imaging surface.
These dots make up the latent image.
mDBDs are being developed for electrographic surface patterning at atmospheric pressure.
With aperture sizes of 10s m, spacings of 10s to 100s m, electron beams are extracted from individually addressable arrays of mDBDs.
Electron beams extracted from arrays form charges (dots) at selected locations on the imaging surface to create the latent image.
 Ref: N. Leoni, et al. US Patent , 2010
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GEC2011_3

5. SCALING OF MICRO-DBDs ARRAYS
mDBDs hold great potential for inexpensive and robust latent image production.
We report on the application of first-principles modeling to investigate the properties and scaling of mDBD arrays.
Fundamentals of operation
Geometry
Frequency
Materials properties
Gas composition
Also, the properties of mDBD arrays can be optimized for producing excited states, photons, or charge species by independently controlling the repetition rate, pulse shape and materials
In our study, we have computationally investigated the extraction of electron currents
from the mDBDs arrays by varying geometry, frequency and dielectric materials
University of Michigan
Institute for Plasma Science & Engr.
GEC2011_4

6. University of Michigan Institute for Plasma Science & Engr.
MODEL:
nonPDPSIM
Plasma Hydrodynamics
Poisson’s Equation
Gas Phase Plasma
Liquid Phase Plasma
2-unstructured mesh with spatial dynamic range of 104.
Fully implicit plasma transport.
Time slicing algorithms between plasma and fluid timescales.
Bulk Electron Energy
Transport
Kinetic “Beam”
Electron Transport
Neutral Transport
Navier-Stokes
Neutral and Plasma
Chemistry
Radiation Transport
Surface Chemistry
and Charging
Ion Monte Carlo
Simulation
Circuit Model
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GEC2011_5 6 6

7. SINGLE APERTURE: GEOMETRY
The mDBD is a “sandwich” of alternating electrodes and dielectrics.
rf metal electrode embedded in a printed-circuit-board.
Negatively DC biased discharge electrode separated from rf by dielectric sheet - 35 m opening
Less-negatively DC biased screen electrode separated from the discharge electrode acts as an anode switch
Extracts charge out of cavity
Narrow the current beam
Apertures are circular – modeled as 2D slots to enable modeling of multiple apertures.
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_181
AVS_2010_mark_geometry_whole.lay; 1500*411
AVS_2010_mark_geometry_single.tif; 600*501
The model geometry of a single aperture is shown here.
The mdbd is a sandwich geometry of alternating electrodes and dielectrics.
An rf metal electrode is embeded in a printed-circuit-board
A negatively dc biased discharge electrode is seperated from the rf electrode by a dielectric sheet with a 35 micron opening
A less negatively dc biased screen electrode separated from the discharge electrode acts as an anode switch,
This screen electrode has dual functions:
Charge extraction from the cavity, and current beam narrowing.
Although the actual apertures are circular, we modeled them as 2d slots to enable modeling of multiple apertures.
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GEC2011_6

8. MULTIPLE APERTURES: GEOMETRY
Grounded electrode covered with 25 m dielectric sheet separated from screen electrode by 410 m gap.
rf: -2 kV DC, 1.4 kV AC at MHz.
Discharge: -2 kV, Screen: V, top electrode grounded
0 V
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_181
AVS_2010_mark_geometry_whole.lay; 1500*411
AVS_2010_mark_geometry_single.tif; 600*501
This slide shows the whole model geometry of multiple apertures
The grounded electrode is covered with a 25 micron dielectric sheet,
and separated from the screen electrode by a 410 micron gap
Negative 2 kilovolt DC and 1.4 kilovolt AC voltage at 2.5~25 megaHz is applied to the rf electrodes and
-2 kilovolt and v voltage is applied to the discharge and screen electrode
The top extraction electrode is grounded.
The spacing between the apertures is 300 microns.
300 m
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, MHz
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GEC2011_7

9. SINGLE APERTURE - 25 MHz N2 (1 atm): [e]
0 V
[e]
(1016 cm-3, 5 dec)
Case_181:IWM6_2011_ne.lay
time# 700~780=>350~390 ns(sine); pot:-3400~-600 linear, ne:1e11~1e16, log; 1353x675;log,linearlog;25MHz(period=50)
Here is the simulation results of atmospheric nitrogen plasma. The animation on the left is the electron density in the cavity and the animation on the right is the gap. The rf votage of 1.4 kilo-volt at 25 MHz plus -2kV DC voltage is shown in the bottom of the animation.-2kilo-volt and volt is applied to the discharge and screen electrodes.
On the top is “a” grounded electrode covered with a sheet with a dielectric constant = 20.
1.At the beginning of this sinusoidal voltage, the previous positive surface charges reduce the voltage drop,
The electron plume extinguishes and the electron flux neutralizes the positively charged dielectric.
2.Before the “positive” peak of the rf voltage, the discharge electrode acts as a cathode
to enable the avalanche to occur in the mDBD cavity.The electrons charge the dielectric negatively.
As the rf voltage decreases to zero, no net potential “is applied”other than that produced by the negative charge on the dielectric.electrons are extracted from the cavity,
Before” the negative peak of the rf voltage, the electron plume “has already reached” its maximum,
Sufficient ions have been collected on the dielectric “which” reduces the net voltage drop ,“The” electron plume begins to diminish.
As the rf voltage returns to zero, net voltage across the gap is sufficiently small
due to the positive charges on the dielectric, and the electron plume is attracted to the dielectric and extinguished. This electron flux neutralizes the positively charged dielectric.
Vrf= -2kV DC, +1.4kV AC, 25 MHz, -2kV discharge, V screen
Vrf= Previous positive surface charges reduce voltage drop E E-plume extinguishes, e-flux neutralizes positively charged dielectric.
Vrf= 1.4 kV Avalanche in mDBD cavity. Electrons charge dielectric negatively.
Vrf= Grounded electrode extracts electrons from the cavity.
Vrf=- 1.4kV Positively charged dielectric reduces voltage drop E e-plume begins to diminish.
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MIN MAX
Animation Slide-GIF
GEC2011_8

10. SINGLE APERTURE - 25 MHz N2 (1 atm): E/N, Te, Se
E/N (40~4000 Td)
 Te (0.1~15 eV)
[Se+ Ssec](5x1024 cm-3 s-1,3dec)
Case_181:IWM6_2011_ne.lay
time# 700~780=>350~390 ns(sin()); pot:-3400~-600 linear, E/N-40~4000 Td linear, Te:0.1~15 ev linear, Se+Seimpact:5e21~5e24, log; 1353x673;25MHz(period=50)
We can see the plasma “is” nearly extinguished every cycle “from the previous slide”,
hence it requires re-initiation “each time”.The animations here are E/N on the left, electron temperature “in” the middle and electron sources due to electron impact ionizations in the bulk plasma and from secondary electrons on the right.When the electrons are extracted or recombined, larger E/N occurs at “the”zero crossing of rf voltage due to the previously charged dielectric,higher E/N increases the electron temperature and electron impact ionization.(at low f, higher E/N happens not only at zero-crossing=>so we have multi-pulses)The ionization due to secondary beam electrons is larger than bulk plasma. These secondary beam electrons are from the surface which has been seeded by positive ions and uv photons from long lived nitrogen excited states.When the electrons are created or attracted to the cavity,the conductive plasma shields out the electric field and lowers the E/N which reduces the ionization in the cavity.
Ionization reaction, threshold energy for ionizaiton:
E + N2 > N2^ + E + E; threshold energy=15.5 ev;E + N2V > N2^ + E + E; ev
E + N2* > N2^ + E + E ; ev;
Larger E/N at  Vrf zero-crossing due to previously charging of dielectric. Te, Se and Ssec follow E/N.
Ionization due to secondary electrons from surface seeded by positive ions and UV photons are slightly larger than bulk ionization.
Plasma shields electric field, lowers E/N, reduces ionization.
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MIN MAX
GEC2011_9
Animation Slide-GIF

11. 5 MHz: CHARGED PARTICLE INTRA-PERIOD DYNAMICS
The plasma chemistry of (even) N2 can be complex, with varying dynamics during the pulse.
Electron and ion densities are not in equilibrium throughout pulse.
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_184
IWM6_2011_3hread_ne_gap.sty; Time #:80~160, = 40~80 ns, 600*533 (2nd pulse)
IWM6_2011_pdpsim_flux1dint_FE_400ns.sty; 600*533
If the phase of the center rf electrode was delayed by a quarter cycle, the integrated electron flux between apertures increases
It is because there is a period of time when the electron plume propagates without interference by its neighbors.
During this time, the electron plume propagates not only upward to the top surface, but also diffuses laterally without being limited by its neighbor plumes.
Electrons
N2+, N4+, N+
Electric potential Contours
Positive space charge remains in gap to aid in electron extraction.
DC:-2 kV
AC:1.4 kV, 5 MHz
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GEC2011_10
Animation Slide-GIF

12. University of Michigan Institute for Plasma Science & Engr.
MULTIPLE APERTURES
With simultaneous extraction of electron current, the integrated flux from each aperture can be uniform provided plumes are below the “space charge” limit of affecting their neighbors.
300 m
0 V
ε/ε0=20
28 x 1011
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_181
AVS_2010_mark_ne.sty
Time #:80~160, = 40~80 ns, 600*533 (2nd pulse)
IWM6_2011_pdpsim_flux1dint_FE_400ns.lay; 600*533
The figures shown on the left is the multi-aperture current extraction, and the figure on the right is the time integrated electron flux.
3 rf electrodes are in phase, with 25 mhz, -2 kv dc and 1.4 kv rf voltage, v and -2kv on the screen and discharge electrode
The dielectric constant is 20
We found that with in phase excitation, the integrated peak current is pretty uniform, because the plumes are below the space charge limit of affecting their neighbors
7 x 1011
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, 25 MHz
Integrated Electron Flux
(10th pulse)
[e] (1x1015 cm-3, 4 dec)
N2, 1 atm,
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GEC2011_11
Animation Slide-GIF

13. MULTIPLE APERTURES: SPACING
Decreasing the aperture spacing (300m  110m ), electron plumes merge and are slightly focused in the gap due to the accumulating positive ions.
0 V
110 m
ε/ε0=20
35 x 1011
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_185
IWM6_2011_3hread_ne_gap
Time #:80~160, = 40~80 ns, 600*533 (2nd pulse) pot:-3400~0, (add in and -600 line, tot 42 lines)
IWM6_2011_pdpsim_flux1dint_FE_400ns.lay; 600*533
If the spacing of apertures is reduced from 300 to 110 microns, the electron plumes merge, and is slightly focused in the gap
Due to the accumulating positive ions.
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, 25 MHz
 Integrated Electron Flux (10th pulse)
[e] (1x1015 cm-3, 4 dec)
N2, 1 atm,
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GEC2011_12
Animation Slide-GIF

14. MULTIPLE APERTURES: SPACING
By decreasing the spacing of apertures, the three e-plumes merge to form a single current beam.
Due to this merging, the instantaneous electron flux is larger and broadened in width.
Case_181, IWM6_2011_pdpsim_flux1d_FE_anim.lay (1~400 ns; map skip 5)
Case_185, IWM6_2011_pdpsim_flux1d_FE_anim.lay (1~400 ns; map skip 5)600*533
The figures on the left and right are instantaneous electron flux of 300 micron and 110 micron spacing
Be decreasing the spacing of the apertures, the three electron plumes merge to form a single current beam.
And the instantaneous electron flux is larger and broader in width.
300 m Spacing
110 m Spacing
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GEC2011_13

15. MULTIPLE APERTURES: TOP DIELECTRIC CONSTANT
The surface charge on the top dielectric creates lateral local electric fields which broaden the electron plume.
By decreasing the dielectric constant (ε/ε0=20→1), dielectric charging effects start earlier. E-plume is not only broadened but directed towards less charged region.
300 m
0 V
[e] (1x1015 cm-3, 4 dec)
[V] (-3.4kV~0 V)
N2, 1 atm,
ε/ε0=1
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_183
IWM6_2011_3hread_ne_gap
Time #:3~400, = 1.5~200 ns, 900x598 (1~5th pulse), timestep skip=3( every 1.5 ns show 1 frame) , pot:0~-3400(40 level)
The surface charge on the top dielectric creates a lateral local electric field which broadens the electron plume, it is known as “blooming”
As demonstrated here, if the dielectric constant is decreased from 20 to 1,
The dielectric charging effect starts earlier, electron plume is not only broadened but also directed toward less charged regions, which renders the dot size larger than the diameter of the charge source beams
This “blooming effect” can reduce image quality.
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, 25 MHz
MIN MAX
University of Michigan
Institute for Plasma Science & Engr.
GEC2011_14
Animation Slide-GIF

16. MULTIPLE APERTURES: DIELECTRIC CONSTANT
At high dielectric constant (ε/ε0=20), e-plume initially increases due to positive charge accumulation in gap, then it decreases due to negative surface charge.
At low dielectric constant (ε/ε0=1), charging effects start earlier due to the lower capacitance. The e-flux is broadened and warped laterally towards less charged region.
ε/ε0=20
ε/ε0=1
Case_181, IWM6_2011_pdpsim_flux1d_FE_anim.lay (1~400 ns; map skip 5)
case_183, IWM6_2011_pdpsim_flux1d_FE_anim.lay (1~200 ns; map skip 5) 600*533
The figures on the left and right are instantaneous electron flux for dielectric constants equal to 20 and 1
At high dielectric constants, the electron plume initially increases due to positive charge accumulation in the gap, then it decreases as a consequence of surface charge
At low dielectric constants, the charging effect starts earlier due to the low capacitance. The electron flux is broadened and warped laterally toward less charged regions.
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GEC2011_15

17. ELECTRON DENSITY vs FREQUENCY
At high frequency, e-flux is limited by rf voltage.
[e] initially increases due to accumulation of positive ions, then decreases as a result of negatively dielectric charging.
At low frequency, e-flux limited by charging, [e] decreases.
[e] tends to be larger at higher frequency.
In Case_188: geometry_3.tif,
IWM6_2011_ne_188_189_190_191_192_193.tif; 600 dpi
A probe adjacent to the dielectric is used to measure the extracted electron density.
The dielectric constant is 12.5, -2 kv dc and 1.4 kv rf voltage of 2.5 to 25 MHz on the rf electrodes.
The figure shown here is the peak density at different rf cycles of frequency from 2.5 to 25 MHz
At high frequency, electron flux is limited by rf voltage, so the electrons initially increase due to accumulation of positive
ions, then decrease as a result of dielectric charging.
However, at low frequencies, electron flux is limited by charging on the dielectric, and peak electron density decreases
The electron density tend to be larger at higher frequency
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GEC2011_16

18. ELECTRIC POTENTIAL vs FREQUENCY
Electric potential on dielectric at 10th cycle and at the same voltage.
At high frequency, electric potential is more negative due to larger electron current extraction. At low frequency, e-flux is limited by dielectric charging, electric potential decreases.
Distribution at same peak value is independent of frequency.
In Case_188: geometry_3.tif,
IWM6_2011_case_188_189_190_191_192_193_top_diel_400ns.lay; 600*533
The figure shows the electric potential at the surface of the dielectric at the 10th rf cycle for 2.5 to 25 MHz
At high frequencies, electric potential is greater due to larger electron current extraction
At lower frequencies, electric potential decreases because the electron flux is limited by dielectric charging.
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GEC2011_17

19. E-FLUX, POTENTIAL vs O2 FRACTION
Electric potential (10th rf cycle) and instantaneous electron flux at dielectric surface for 1 atm N2 with 0.1% and 10% of O2 at 25 MHz.
[e], Se, Ssec is not sensitive to O2 content in the cavity (higher E/N).
[e] is largely attached to O2 on its path to the top sheet at higher O2 concentration. Massive negative ions (O-, O2-) reduces extraction field by neutralizing equally massive positive ions.
The electron flux and surface potential decreases at higher O2 content.
Voltage on top dielectric 10th rf cycle(V)
Instantaneous E-FLUX (x1019 cm-2s-1 )
case_188ex2_3head_RF_-2kv_1.4kV_25MHZ_TOP0kV_epson12.5_SCR_-1950V_DIS-2kV_1atm_0.1O2
case_188ex4_3head_RF_-2kv_1.4kV_25MHZ_TOP0kV_epson12.5_SCR_-1950V_DIS-2kV_1atm_10O2
GEC_2011_pdpsim_flux1d_fe.lay (1~400 ns; map skip 5) 600x
---
Case_188: geometry_5.tiff
Case_188ex2: top_diel_400ns_25mhz_0.1O2.dat; Case_188ex3: top_diel_400ns_25mhz_1O2.dat
Case_188ex4: top_diel_400ns_25mhz_10O2.dat; GEC_2011_case_188_ex2_ex3_ex4_top_diel_400ns_25MHz.lay
; 600*533
The figure shows
The figures on the left and right are instantaneous electron flux for dielectric constants equal to 20 and 1
At high dielectric constants, the electron plume initially increases due to positive charge accumulation in the gap, then it decreases as a consequence of surface charge
At low dielectric constants, the charging effect starts earlier due to the low capacitance. The electron flux is broadened and warped laterally toward less charged regions.
0.1% O2
10% O2
Higher O2
content
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GEC2011_22
GEC2011_18

20. MULTIPLE APERTURES: MOVING SURFACE
In continuous processing, the polymer sheet is in motion during treatment. To investigate the consequences on plasma properties, the polymer sheet is moved between sets of pulses.
After the surface is moved, e-plume sees an asymmetric charge (potential) distribution - current extraction is warped in response to new charge distribution.
300 m
0 V
ε/ε0=1.1
Time Slicing:
Plasma
2 cycle
(80 ns)
Plasma
2 cycle
(80 ns)
case_197_3head_RF_-2kv_1.4kV_25MHZ_TOP0kV_epson1.1_SCR_-1950V_DIS-2kV_2cy_48um_800cms
GEC_2011_pdpsim_ne_totch_gap.lay
2.5 ns ~ us (1200)
case_197_3head_RF_-2kv_1.4kV_25MHZ_TOP0kV_epson1.1_SCR_-1950V_DIS-2kV_2cy_48um_800cms(uigel )
from case_188_3head_RF_-2kv_1.4kV_25MHZ_TOP0kV_epson12.5_SCR_-1950V_DIS-2kV
use HP_ nlist, pdpsim_surface.dat from HP\corona\corona case\033ex2_1pin_-3kV_1atmN2_diel10_gap0.25mm_MC_pulsing_suf 425us_V200cm
dtsurf_move = dR/V = 4.8um / 800(cm/s) =4.8*10^-4 cm/800(cm/s) = 0.6 e-6 sec, (0.6 us move a grid)
Run 80 ns plasma, shift 6 us (10 grids ) move 48um
ns (2 cycle) us80ns us160 ns(2 cycle)
Plasma surface moving plasma
A time slicing technique is used to resolve this problem.
After 2 rf cycle of plasma treatment, the surface moves 48 um, and the consequential 2 rf cylce is applied.
Moving
Surface
48 m
[e] (1x1016 cm-3, 6 dec)
[V] (-3.4kV~0 V)
N2, 1 atm,
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, 25 MHz
MIN MAX
University of Michigan
Institute for Plasma Science & Engr.
GEC2011_19
Animation Slide-GIF

21. ELECTRON FLUX: MOVING SURFACE
After the surface moves, the e-plume sees an asymmetric potential distribution on top polymer sheet. A large electron flux is then directed to the less charged region.
By controlling the moving speed or voltage wave form, an relatively uniform potential (charge) distribution could be achieved.
2nd e-flux before surface motion.
1st e-flux after surface motion.
case_197_3head_RF_-2kv_1.4kV_25MHZ_TOP0kV_epson1.1_SCR_-1950V_DIS-2kV_2cy_48um_800cms
GEC_2011_pdpsim_flux1d_fe.lay
GEC_2011_pdpsim_flux1d_fe_static.lay
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GEC2011_20

22. University of Michigan Institute for Plasma Science & Engr.
CONCLUDING REMARKS
First principles computer modeling has been applied to investigation of independently controlled RF mDBD arrays and characteristics of electron current extraction.
Properties of charging fluxes can be controlled with frequency, voltage, geometry, material properties and gas composition.
Decreasing aperture spacing merges the e-plume and increases the flux. Electron collection on dielectric creates lateral local field that broaden the e-plume and eventually stops the current extraction.
Decreasing the dielectric constant reduces capacitance, decreasing charging time and warps the e-plume.
At high frequency, current extraction is limited by rf voltage; current increases initially then decreases, enabling some tuning of the collected fluxes.
Shape of voltage and charge distribution on dielectric are also controllable with O2 content and moving surface.
University of Michigan
Institute for Plasma Science & Engr.
GEC2011_21