The dust direct current self-bias potential in a time varying plasma sheath

Abstract

We introduce an important physical effect, namely, the response of the dust particle charge to an oscillating electric field in a plasma sheath. The ac field may lower the average dust potential well below the corresponding floating potential Uf in a dc sheath. The resulting dust potential will be called the dust dc self-bias potential Udc and is generally a sensitive function of the ac potential amplitude and of the ratio ωch/ω between the so-called dust charging frequency ωch and the ac frequency ω. The difference between Udc and Uf maximizes for ωch≪ω and approaches 0 for ωch≫ω. We propose that the dust dc self-bias effect is important in all plasmas having a rapid potential variation and always has to be taken into account when calculating the charge and the force on dust particles in such plasmas. Since levitation of dust particles above surfaces normally requires negative dust particles, the dust dc self-bias effect may explain why some experiments indicate that levitation of dust particles is easier in rf discharges than in dc discharges.