THE DUST DIRECT-CURRENT SELF-BIAS POTENTIAL IN A TIME-VARYING PLASMA SHEATH

We introduce an important physical effect, namely, the response of the dust particle charge to an oscillating electric field in a plasma she ath. The ac field may lower the average dust potential well below the corresponding floating potential U-f in a de sheath. The resulting dus t potential will be called the dust de self-bias potential U-dc and is generally a sensitive function of the ac potential amplitude and of t he ratio omega(ch)/omega between the so-called dust charging frequency omega(ch) and the ac frequency omega. The difference between U-dc and U-f maximizes for omega(ch)much less than omega and approaches 0 for omega(ch)much greater than omega. We propose that the dust de self-bia s effect is important in all plasmas having a rapid potential variatio n and always has to be taken into account when calculating the charge and the force on dust particles in such plasmas. Since levitation of d ust particles above surfaces normally requires negative dust particles , the dust de self-bias effect may explain why some experiments indica te that levitation of dust particles is easier in rf discharges than i n de discharges. (C) 1996 American Vacuum Society.