Afterglow of an RF Plasma with Magnetic Field and Dust

We report experiments with the afterglow of a capacitively coupled radio-frequency gas-discharge plasma (CCP), with a magnetic field. An afterglow is the millisecond-long condition after the power is turned off, when electrons and ions are gradually transported to boundaries. This transport is expected to be altered by an axial magnetic field in a way that can be studied by recording the time-resolved collection of ions on a steadily biased cathodic electrode. This ion measurement will also yield an estimate of the volume-averaged ion density in the plasma prior to the afterglow. We will introduce silica microspheres into the plasma, and during the afterglow we will measure their charge. This measurement is made by video microscopy, to determine their acceleration in the presence of a DC electric field, as they fall toward a negatively biased horizontal lower electrode during the afterglow. Previous afterglow experiments without a B field have shown that such dust particles charge positively in the afterglow's cathodic sheath, where electrons are absent and ions drift in the argon gas. It is expected that magnetizing electrons will have the effect of altering this charge, due to the change in ion conditions corresponding to a change in ambipolar transport in the afterglow. Auburn's MDPX apparatus allows controlling the B field while allowing other parameters (self-bias, RF power, neutral gas pressure) to be kept fixed. Preliminary results will be reported.