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Chamber Clearing by Electrostatic Fields. L. Bromberg ARIES Meeting UCSD January 10, 2002. Motivation. Aerosols are created in chamber due to large heating flux on surface of liquid Aerosols are charged, due to presence of plasma afterglow following the pulse - PowerPoint PPT Presentation
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Motivation
• Aerosols are created in chamber due to large heating flux on surface of liquid
• Aerosols are charged, due to presence of plasma afterglow following the pulse
• Electric fields can be used to remove aerosol• Similar to electrostatic precipitators used commercially for
removing particulate matter from industrial flows
Structure of talk
• Charge state of aerosols exposed to plasma conditions
• Motion of charged aerosols under the presence of an electric field
• Implications to IFE chamber clearing
Aerosol charging
• In the presence of a plasma, a particulate charge varies as dq/dt = Ii+Ie
where Ie = - a2 e ve ne exp ( -e2 | Z | / 0 a Te)
Ii = a2 e vi ni (1 + e2 | Z | / 0 a Ti)
• Z is the average number of electronic charges in the particulate
• It is assumed that all radiation fields have decayed away• Good for times > 1 microsecond
Average electronic charge number vs Te for several densities
0
5000
10000
15000
20000
25000
30000
35000
0 2 4 6 8 10 12 14 16Te (eV)
Number of electronic charges on aerosol
ne : 5.e18 - 2.e19 /m3ne = 5 1018 - 2 1019 m-3
a = 10 m
Average electronic charge number vs aerosol size
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0.0E+00 2.0E-06 4.0E-06 6.0E-06 8.0E-06 1.0E-05 1.2E-05Aerosol diameter (m)
Elementary charge number
Te = 5 eV
Ti = 0.2 eV
Density and temperature evolution (assumption)
0.E+001.E+142.E+143.E+144.E+145.E+146.E+147.E+148.E+149.E+141.E+15
0 0.00001 0.00002 0.00003 0.00004 0.00005
time (s)
Density (1/m3)
0
1
2
3
4
5
6
7
8
Temperature (eV)
DensityTeTi
Average electronic charge number vs time
100
1000
10000
100000
0 0.00001 0.00002 0.00003 0.00004 0.00005time (s)
Elementary charges in aerosol
Time evolution of aerosol charging
• Aerosol charge equilibrates very fast with background
• ~ several microseconds at densities of 1015 /m3
• Aerosol charge not very dependent on initial state• Initial aerosol charge depends on radiation field and fast
electron/ion bombardment
Aerosol motion in the presence of an electric field
• The motion of a particulate in a gas under the effect of an applied field is given by: v = Zp E E is the applied electric field and Zp is the particulate mobility: Zp = qp Cc / 3 a where qp is the particulate charge Cc is the Cunningham correction factor is the gas viscosity a is the particulate radius
Cc = 1 + Kn ( 1.25 + 0.40 exp( -1.1/ Kn ) ) Kn = 2 l / a, l being the mean free path of the a gas molecule For Xe at 0.1 Torr, l = 2.62 x 10-4 m
Gas viscosity
• Using the kinetic theory of transport gases (assuming hard sphere collisions):
• = ( T / T0 ) 1/2
(independent of density!!!)
• For Xenon, • ~ 44 Pa s at 600K
At 2000 K, ~ 110 Pa s
y = 0.3282x0.7649
0102030405060708090
0 500 1000 1500Temperature (K)
viscosity (micro Pa s)
Aerosol cleaning
pressure (STP) Torr 0.1Electron temperature eV 5Viscosity, Xe Pa s 1.10E-04Mean Free path m 2.62E-04
Aerosol diameter m 3.00E-07Knudsen number 1747Cunningham factor 2883
Elementary charges 2.54E+02Mobility 3.77E-04Applied electric field V/m 1.00E+04
Aerosol velocity m/s 3.77E+00
Velocity of aerosols10 kV/m, 0.1 Torr Xe, 5 eV
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0.0E+00 2.0E-06 4.0E-06 6.0E-06 8.0E-06 1.0E-05 1.2E-05Aerosol diameter (m)
aerosol velocity (m/s)
Aerosol clearing with electric fields
• Velocity of aerosols is small, compared with the chamber size• ~ several m/s
• For a rep-rate of 10 Hz, the maximum distance that the aerosols will be able to move is 1 m!
• Relatively large electric fields are required (100 V/cm)• Difficult to generate inductively• Capacitive field generation requires electrodes in chamber
• Can the liquid wall itself be used as electrodes?
• Could be used to prevent aerosols from depositing in optics.
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