Electron Effects on the Formation of Ion Beams in Expanding Plasmas*

We report measurements of the electron energy probability function (EEPF) in an argon helicon plasma using the Druyvesteyn method. At low pressure, the EEPFs transition from a single Maxwellian energy distribution in the plasma core to a plasma with a fast electron component near the edge. This electron effect is studied as a function of pressure. Simultaneous measurements of the ion velocity distribution function (IVDF) using laser induced fluorescence (LIF) are also reported. Previous work determined that the rf power is strongly deposited at the skin depth resulting in a sharp decrease in the plasma potential and the formation of an ion hole. The effect of rf power on both the electrons (EEPFs) and ions (IVDFs) is presented. The spatial region surrounding the ion hole is studied in more detail. Current theories of ion beam and ion hole formation in expanding plasmas are incomplete and these measurements provide critical information regarding the spontaneous formation of these structures.