Transcript PPT - The Michigan Institute for Plasma Science and Engineering
PLASMA DISCHARGE SIMULATIONS IN WATER WITH PRE-EXISTING BUBBLES AND ELECTRIC FIELD RAREFACTION Wei Tian and Mark J. Kushner
University of Michigan, Ann Arbor, MI 48109 USA [email protected], [email protected]
2 nd Michigan Institute for Plasma Science and Engineering (MIPSE) 21 September 2011, Ann Arbor, Michigan * Work supported by Department of Energy Office of Fusion Energy Science
AGENDA
Introduction to plasma discharges in liquids
Breakdown mechanism: Initiation and propagation
Description of model
Initiation: breakdown inside the bubble
Propagation: electric field rarefaction
Concluding Remarks
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PLASMAS IN LIQUIDS
Plasmas sustained in liquids and bubbles in liquids are efficient sources of chemically reactive radicals, such as O, H, OH and H 2 O 2 .
Applications include pollution removal, sterilization and medical treatment.
The mechanisms for initiation of plasmas in liquids are poorly known.
Plasma Sources Sci. Technol.
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(2008) 024010 MIPSE_SEP2011_2 Plasma Process. Polym.
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BREAKDOWN MECHANISM
Due to the high atomic/molecular density in liquids, for a given voltage, E/N (Electric Field/Number density) is small.
Plasma breakdown, consisting of initiation and propagation of a streamer, typically requires a critically large E/N.
To achieve this E/N, breakdown requires a mechanism to rarefy the liquid or to provide sources of seed electrons.
Initiation
Pre-existing bubbles
Localized internal vaporization
Molecular decomposition Electron-initiated Auger process
Propagation
Electric field rarefaction
Gas channel cavitation
Polarity effect
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MODELING PLATFORM: nonPDPSIM
Poisson’s equation:
(
j q j N j
s
)
Transport of charged and neutral species:
N j
t
j
S j
Electron Temperature (transport coefficient obtained from Boltzmann’s equation:
e
t
j
E
n e
i
i K i N i
5 2
e
e
T e
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MODELING PLATFORM: nonPDPSIM
Radiation transport and photoionization:
S m
(
r i
)
N m
k
(
r i mk
)
A k
N k
j k r
j
' ,
r i
d
3
r
j
'
G
r j
' ,
r i
exp
l
'
r i r
j
4
r
j lk
'
r i N l
2
j
'
d
r j
'
Electric field emission
J e
AT k
2 exp
work
q
3
E
0 0
k B T k
1 2
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INITIATION: PASCHEN’S CURVE FOR BUBBLES
The vapor phase in liquids will have pressures of at least 1 atm – usually the vapor of the liquid or the injected gas.
Even breakdown in these rarefied regions is challenging, needing to have large voltages.
“Paschen’s law”, Wikipedia, Septemeber 21, 2011 (http://en.wikipedia.org/wiki/Paschen%27s_law)
Bubble (20 ~ 75
m Pressure (1 ATM) Pd value (1 ~ 10 Torr cm) Voltage (20 ~ 50 kV)
Some E/N “amplification” may be required, as in electric field enhancement due to geometry, permittivities or ) charging.
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CONFIGURATION
Breakdown of liquids from pre-existing bubbles was numerically investigated.
Sharp-Tip Electrode
Bubble ~ 75 um
Parallel Electrode
Bubble ~ 50 um
Parallel Electrode
Bubble ~ 20 um
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MIN
INITIATION INSIDE BUBBLES
Initiation processes inside the bubble within 0.1 ns
Initiation processes are associated with electron impact ionization, photo ionization and field emission MAX University of Michigan Institute for Plasma Science & Engr.
SHARP-TIP ELECTRODE
[e] (10 18 cm -3, 3 dec)
E-field (5.0 ~ 7.0 MV/cm)
S e (10 27 cm -3 s -1 , 3 dec)
[S photo ] (10 22 cm -3 s -1 , 3 dec)
The sharp tip produces electric field enhancement to 5 MV/cm, E/N to 10,000 Td.
Electron density produces ionization of a few percent.
Electron impact ionization dominates over photo-ionization
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[e] (10 17
PARALLEL ELECTRODE: PHOTO-IONIZATION
cm -3, 3 dec)
[EF] (0.8 ~ 1.8 MV/cm)
[S e ] (10 27 cm -3 s -1 , 3 dec)
[S photo ] (10 22 cm -3 s -1 , 3 dec)
The electric field is enhanced due to the permittivity difference at the gas liquid interface
Electron density is uniform due to uniform electric field inside the bubble The electron impact ionization dominates over photo-ionization
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PARALLEL ELECTRODE: FIELD EMISSION
[e] (10 16 cm -3, 3 dec)
E-field (0.3 ~ 0.5 MV/cm)
S e (10 25
cm -3 s -1 , 3 dec) [S photo ] (10 22 cm -3 s -1 , 3 dec)
The electric field is concentrated at the top of the bubble
Electrons are emitted from the top of the bubble, where the electric field is strong enough
The field emission assists the ionization
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PROPAGATION: E-FIELD RAREFACTION
“Liquids can become phase unstable such that gas channels form along electric field lines.”
A streamer can propagate itself. The electric field is expelled and advanced at the streamer tip, because of free charges inside the streamer and ion accumulation at the tip.
E-field Enhancement
The enhanced electric field is so strong that a phase-like transition occurs there. The densities, compositions and other phase related properties are changed respectively. As a result, a low-density area is created.
Phase Transition
The streamer extends itself into the new low density area. The loop continues until the streamer reaches the grounded electrode.
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Streamer Extension
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PROPAGATION: PHOTO-IONIZATION
MIN MAX
Gap = 1 mm
V max =30 kV, with rising time of 0.1 ns
Average E-Field ~ 0.3 MV/cm
Speed ~ 400 km/s
Flood represents the electron density
Lines represent the potentials University of Michigan Institute for Plasma Science & Engr.
[e] (10 16 cm -3,
PROPAGATION: PHOTO-IONIZATION
3 dec)
E-field (1.0 ~ 2.5 MV/cm)
S e (10 22 cm -3 s -1 , 3 dec)
[S photo ] (10 25 cm -3 s -1 , 3 dec)
The streamer is a little wider than the bubble, because the photo ionization is isotropic
The photo-ionization is dominating in the bulk plasma; electron impact ionization only occurs at the head of the streamer
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PROPAGATION: FIELD EMISSION
Gap = 2 mm
V max =20 kV, with rising time of 0.1 ns
Average E-Field ~ 0.1 MV/cm
Speed ~ 100 km/s
Flood represents the electron density
Lines represent the potentials MIN MAX University of Michigan Institute for Plasma Science & Engr.
[e] (10 17 cm -3,
PROPAGATION: FIELD EMISSION
3 dec)
E-field (0.5 ~ 1.0 MV/cm)
S e (10 25 cm -3 s -1 , 3 dec)
[S photo ] (10 22 cm -3 s -1 , 3 dec)
The electric field is concentrated at the head of the streamer The streamer originates from the bubble top and propagates toward the grounded electrode
Its head becomes wider and wider since it gets closer to grounded electrode
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CONCLUDING REMARKS
The breakdown mechanism consists of two processes, initiation inside the bubble and propagation due to the electric field rarefaction
A large electric field, photo-ionization or field emission is needed to assist the initiation inside the bubble.
Electric field rarefaction may contribute to creating a low density channel, in which the streamer can propagate.
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