Transcript Electromagnetically Trapped Dusty Plasma Ring R. Sheldon, E. Thomas Jr, D. Gallagher, M.

Electromagnetically
Trapped Dusty
Plasma Ring
R. Sheldon, E. Thomas Jr, D. Gallagher, M. Adrian, M. Abbas,
P. Craven & E. Reynolds
Wheaton College / National Space Science & Technology
Center / Auburn U./ WVU
Oct 18, 2002
Motivation of this talk
• 1. It can’t be done.
• Argument: The gyroradius of a charged particle is given by mv/qB.
Since dust has ~106 atoms, the gyroradii, even for highly
magnetized plasma, is km!
• Reply: These are NOT dust + plasma, but dustyplasmas. The
plasma is magnetized and traps the dust electrically.
• What we need then is a net plasma “space charge” that
provides the potential well for the charged dust.
• So we have 3 reasons for trying to trap dust in a magnet:
• 1) To prove the naysayers wrong.
• 2) To examine the suitability for gossamer solar sails
• 3) To “paint” the plasma space charge as a diagnostic.
POLAR/CEPPAD observations
Theory: Parallel E-fields


Whipple, JGR 1977. Ne = Ni, quasi-neutrality
E. Whipple, JGR 1977.
Ne = Ni, quasi-neutrality
n

kTe

f e d 3v 

E ||
fi d 3v
f e  n0 A0 exp(  E / kT )
f i  (n0  n1 ) A0 exp(  E / kT )  n1 A1 E  B ( Ei ) ( i )
 n0
 
 
  exp

2 sinh 


kT
kT  
 n1
1
B0

B
E  B  
E  B0
Space Charge Spectroscopically
Lab Experiment
Lab Experiment
Langmuir Floating Probe
Inconclusive Results
Diode
Laser
Dust Tracer Experiment
Dusty Plasmas
• Charged dust, when combined with a plasma,
scatters light, and can form a "Coulomb crystal"
• Auburn University
University of Iowa
Dust Levitation
Plasma is confined to magnet plane.
Dust follows E// along magnetic field
lines. Thus magnetized plasma
provides the confinement for
unmagnetized dust grains.
Dust bunnies
Pencil Laser illumined spots
Scanning Laser illumined ring
Multiple dust rings
Space charge Conclusions
We demonstrate a good laboratory analog to field-aligned flows
observed in space and give conclusive evidence that space charge is
produced in dipole trapped plasmas, generating a quadrupolar electric
field.
Space charge is observed to develop in both laboratory and space that
carries important consequences for dipole trapped plasmas. Evidence
for this space charge is:
- Ionospheric beams of He+, O+ at 40keV coming simultaneous with
pancake trapped plasmasheet ions.
- Field-aligned discharges in laboratory system that require a large
voltage difference between the equator and the poles.
- Discharges are produced both by external injection of hot plasma, or
by DC-glow discharge from the central magnet. The second technique
is more symmetric and analogous to very rapidly spinning magnets.
- Dust charges negatively in a plasma discharge, and was found to
collect at the magnet equator, supported against the force of gravity by
electrostatic forces.
Therefore, simple MHD modelling of magnetospheres must be seen as
deficient in principle.
Gossamer solar sails
• The difficulty with solar sails is their flimsyness, which
dustyplasmas may solve.
• A magnetically supported dusty sail has numerous
advantages: robust against UV, solar flares, radiation:
impossible to tear, easy to store and deploy, no struts.
Comparable to 3ug/m2 sails.
• Problems: magnet is heavy. Only negatively charged dust
is trapped, but UV charges dust positive.
• Can we make “plasma magnets” with induced currents?
Can we change photoemissive properties to make dust that
charges negative in sunlight?
Dusty Rings
Jupiter’s micron dust rings as seen
from behind the planet.
Saturn Images. E-ring known to
be micron sized dust. (Radial
striations!)
Saturn’s rings
• This simple lab system (~$10k) provides a useful analog to
planetary magnetospheres. We have already seen analogs to
ring currents, aurora, cusp access, even substorms!
• Now it appears, we have an analog to Saturn’s rings.
Obviously, we are not doing Kepler’s laws and orbits. But
for micron-sized E-ring, we demonstrate important
dustyplasma effects that have not been incorporated in
previous models.
• Radial and latitudinal confinement of the ring
• Dust acoustic waves, spokes, etc.
• It’s pretty. (Maybe The Sharper Image would be interested.)