View
222
Download
0
Tags:
Embed Size (px)
Citation preview
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]
2nd 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
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
AGENDA
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_1
Plasmas sustained in liquids and bubbles in liquids are efficient sources of chemically reactive radicals, such as O, H, OH and H2O2.
Applications include pollution removal, sterilization and medical treatment.
The mechanisms for initiation of plasmas in liquids are poorly known.
PLASMAS IN LIQUIDS
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_2
Plasma Sources Sci. Technol. 17 (2008) 024010 Plasma Process. Polym. 6 (2009), 729
BREAKDOWN MECHANISM
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_3
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
MODELING PLATFORM: nonPDPSIM
Poisson’s equation:
Transport of charged and neutral species:
Electron Temperature (transport coefficient obtained from Boltzmann’s equation:
)( sj
jj Nq
jjj St
N
eeeiii
ie
e TTNKnEjt
n
2
5
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_4
MODELING PLATFORM: nonPDPSIM
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_5
Radiation transport and photoionization:
Electric field emission
',''
)()(3j
kijkjkkmk
imim
rdrrGrNA
rNrS
2
'
'4
''exp
,'ij
l
r
r
jjllk
ijrr
rdrN
rrG
i
j
kB
work
ke Tk
Eq
ATJ
21
0
03
2 exp
INITIATION: PASCHEN’S CURVE FOR BUBBLES
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_6
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.
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.
“Paschen’s law”, Wikipedia, Septemeber 21, 2011 (http://en.wikipedia.org/wiki/Paschen%27s_law)
CONFIGURATION
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_7
Sharp-Tip ElectrodeBubble ~ 75 um
Parallel ElectrodeBubble ~ 50 um
Parallel ElectrodeBubble ~ 20 um
Breakdown of liquids from pre-existing bubbles was numerically investigated.
MIN MAX MIPSE_SEP2011_8
INITIATION INSIDE BUBBLES
University of MichiganInstitute for Plasma Science & Engr.
Initiation processes inside the bubble within 0.1 ns
Initiation processes are associated with electron impact ionization, photo-ionization and field emission
MIN MAX MIPSE_SEP2011_9
SHARP-TIP ELECTRODE
University of MichiganInstitute for Plasma Science & Engr.
[e] (1018 cm-3, 3 dec)
E-field (5.0 ~ 7.0 MV/cm)
Se
(1027 cm-3s-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
[Sphoto] (1022 cm-3s-1, 3 dec)
MIN MAX MIPSE_SEP2011_10
PARALLEL ELECTRODE: PHOTO-IONIZATION
University of MichiganInstitute for Plasma Science & Engr.
[e] (1017 cm-3, 3 dec)
[EF] (0.8 ~ 1.8 MV/cm)
[Se] (1027 cm-3s-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
[Sphoto] (1022 cm-3s-1, 3 dec)
MIN MAX MIPSE_SEP2011_11
PARALLEL ELECTRODE: FIELD EMISSION
University of MichiganInstitute for Plasma Science & Engr.
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
[e] (1016 cm-3, 3 dec)
E-field (0.3 ~ 0.5 MV/cm)
Se
(1025 cm-3s-1, 3 dec)
[Sphoto] (1022 cm-3s-1, 3 dec)
PROPAGATION: E-FIELD RAREFACTION
University of MichiganInstitute for Plasma Science & Engr.MIPSE_SEP2011_12
“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.
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.
The streamer extends itself into the new low-density area. The loop continues until the streamer reaches the grounded electrode.
Plasma Process. Polym. 6 (2009), 729
E-fieldEnhancement
PhaseTransition
StreamerExtension
PROPAGATION: PHOTO-IONIZATION
University of MichiganInstitute for Plasma Science & Engr.
MIN MAX MIPSE_SEP2011_13
Gap = 1 mm
Vmax=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
PROPAGATION: PHOTO-IONIZATION
University of MichiganInstitute for Plasma Science & Engr.
MIN MAX MIPSE_SEP2011_14
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
[e] (1016 cm-3, 3 dec)
E-field (1.0 ~ 2.5 MV/cm)
Se
(1022 cm-3s-1, 3 dec)
[Sphoto] (1025 cm-3s-1, 3 dec)
PROPAGATION: FIELD EMISSION
University of MichiganInstitute for Plasma Science & Engr.
MIN MAX MIPSE_SEP2011_15
Gap = 2 mm
Vmax=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
PROPAGATION: FIELD EMISSION
University of MichiganInstitute for Plasma Science & Engr.
MIN MAX MIPSE_SEP2011_16
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
[e] (1017 cm-3, 3 dec)
E-field (0.5 ~ 1.0 MV/cm)
Se
(1025 cm-3s-1, 3 dec)
[Sphoto] (1022 cm-3s-1, 3 dec)
CONCLUDING REMARKS
University of MichiganInstitute for Plasma Science & Engr.
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.
MIPSE_SEP2011_17