Partially ionized gas -contains IONS, ELECTRONS and neutral particles.

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Presentation transcript:

Partially ionized gas -contains IONS, ELECTRONS and neutral particles.

-contains dust particles, which accumulate charge over time WHERE CAN THEY BE FOUND?

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 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)

F=EQ D

Forces 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. Ion Drag Force Fid ¢ Qd² Dust charge decreases, ion drag force decreases much more rapidly than electric force.

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 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 to look pretty.

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.