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 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 clearing steps Charging stepClearing step

Aerosol charging In the presence of a plasma, a particulate charge varies as dq/dt = I i +I e where I e = -  a 2 e v e n e exp ( -e 2 | Z | /  0 a T e ) I i =  a 2 e v i n i (1 + e 2 | Z | /  0 a T i ) 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 T e for several densities n e = m -3 a = 10  m

Average electronic charge number vs aerosol size Te = 5 eV Ti = 0.2 eV

Density and temperature evolution (assumption)

Average electronic charge number vs time

Time constant for evolution

Time evolution of aerosol charging Aerosol charge equilibrates very fast with background ~ several microseconds at densities of /m 3 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 = Z p E E is the applied electric field and Z p is the particulate mobility: Z p = q p C c / 3   a where q p is the particulate charge C c is the Cunningham correction factor  is the gas viscosity a is the particulate radius C c = 1 + Kn ( exp( -1.1/ Kn ) ) Kn = 2 / a, being the mean free path of the a gas molecule For Xe at 0.1 Torr, = 2.62 x m

Gas viscosity Using the kinetic theory of transport gases (assuming hard sphere collisions):  =   ( T / T 0 ) 1/2 (independent of density!!!) For Xenon,  ~ 44  Pa s at 600K At 2000 K,  ~ 110  Pa s

g/cm s = 0.1 Pa s

Aerosol cleaning

Velocity of aerosols

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.