DUSTY PLASMA

The void of understanding

What is a plasma?

-contains dust particles, which accumulate charge over time

DUSTY/COMPLEX plasma

a lla

d IIdt

dQ

LAB SETUP

FORCES ACTING ON DUST

ELECTROSTATIC•Electrons have a much higher thermal velocity than ions, due to their small mass.•Initially, ions and electrons are concentrated in the centre of the plasma.•Because of their larger thermal velocity, electrons diffuse to the outer chamber faster than ions.•Charge gradient = confining electric field directed radially outwards.

ION DRAG FORCE• Due to the transfer of momentum between ions and dust particles• One of two ways

direct collision Coulomb Collision (where a charged particle’s

momentum is changed because it is in the vicinity of another charged particle)

The VOID Organized structure

Dust free regionSharp, well defined

boundary

CONTRACTION

DUSTFALL

What is happening??•The plasma is turned off – no changing electric field between electrodes•Electron temperature decreases rapidly (to approx. 300K) because they are no longer being given energy by the electric field •REMEMBER

•So dust particles become much less negative, as ion flux is closer in magnitude to electron flux onto dust particle (from about -1000e to -15e)• This occurs in the first 500 microseconds of the afterglow (ie. less than the time from one frame to the next)

a l

lad II

dt

dQ

BUT WAIT!!!!!

F=EQD

OBSERVATIONS SUGGESTForces change •The electric force (inwards) exceeds the ion drag force outwards.

Equilibrium upset •Thus, the equilibrium of the forces on the void boundary is perturbed and the void collapses.

Stability regained •We observe that the void regains stability again before falling, suggesting that an equilibrium is once again established.

Electric Force

FE = QdE

Dust charge decreases, so electric force

decreases.

So the charge on the dust particle

remains constant, and an equilibrium is again established.

Ions and electrons

diffuse and are lost at the

same rate

Ambipolar

diffusion

WHY DOES IT STOP

CONTRACTING?

Questions still to be answeredWhy does the

boundary of the void become less distinct

over time?

What happens to the void when diffusion

switches from ambipolar to free?

Why does the void seem to collapse at the top of the

chamber ? What is the significance of when this

happens?

These boxes are here

Why study dusty plasmas?

ONE: they’re everywhere! 99% of the universe is dusty plasma!

TWO: Dusty plasma plays a significant part in the semiconductor industry, and has been responsible for contamination of wafers in the past.

THREE: allows us to study systems of self-organization – that is, where a system becomes more complex in its organization without the help of an outside source.

THANKYOU!!!!!Dr. Alex Samarian

Brett LaydonDick Hunstead

The Physics Department