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

Whipple, JGR 1977. Ne = Ni, quasi-neutrality

E. Whipple, JGR 1977. Ne = Ni, quasi-neutrality

f d v f d v

f n A E kT

f n n A E kT n A E B E

nn kT kT

BB

E BE B

e i

e

i i i

3 3

0 0

0 1 0 1 1

0

1

1

0

0

2

exp( / )

( ) exp( / ) ( ) ( )

sinh exp

n

kTe ||E

Theory: Parallel E-fields

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 RingsJupiter’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.)