Positive charging of dust grains in an afterglow plasma *

In a glow-discharge plasma, a dust grain normally has a negative charge, but it can reverse and become positive in an afterglow. We find that this positive charge, which develops in the first few milliseconds of the afterglow, is enhanced by a dc electric field.  An experiment was performed with dust grains that were electrically levitated until abruptly switching off the power that sustained the plasma, causing them to fall. Acceleration measurements yielded the residual charge of grains. The value of the positive charge, which was thousands of elementary charges, increased with the dc electric field. This trend is explained by the grains collecting ions that drift at a mobility-limited velocity, after electrons have vanished. A greater electric field drives ions to a greater kinetic energy, so that they charge grains to a larger positive potential.