The effect of dust on a plasma

When dust grains are added to a plasma, initially the more swiftly moving electrons are collected (absorbed) by the dust, giving it a negative charge, and then the slower ions are collected. Since the dust particles are electrically floating, the electron and ion fluxes to the dust must equalize, and this condition fixes the dust floating potential to some negative value. If a large amount of dust is introduced into the plasma, the collection of electrons can result in a reduction of the electron density. This can be observed on a Langmuir probe in the plasma in which dust particles were allowed to fall through the magnetized plasma column of a Q machine. The dust transit time is sufficiently long so that the grains achieve their equilibrium charge. This is shown in the Langmuir current-voltage characteristic shown below.

In this Langmuir characteristic, the electron current is shown as the positive probe current and the ion current is shown as the negative current. The probe voltage was swept from - 10 to + 10 volts and the current was measured at each voltage. The TOP curve was taken in the absence of dust in the plasma. The BOTTOM curve was taken with the dust initially ON, but turned OFF when the probe voltage was roughly 9 V. When the dust was ON, the electron saturation current was reduced by rougly 20% due to the collection of electrons on the dust. The dust grains are so massive, that they do not produce a measurable current. When the dust was turned off, the electron current returned to the "DUST-OFF" value.

Further details can be found in two papers.  paper-1    paper-2