Ion Energy Distributions in a Permanent-Magnet-Expanded Plasma Containing an Electric Double Layer

Ion energy distribution functions are measured along the axis in a solenoid-free plasma expanded by permanent magnets (PMs). The source diameter, the working argon gas pressure, and the 13.56-MHz radio-frequency power are chosen as 6.5 cm, 0.35 mtorr, and 250 W, respectively. The divergent magnetic field of about 100 G in the source tube and of about a few gauss in the diffusion chamber is provided by the PMs. The contour image of the measured ion energy distributions clearly shows the electrostatic ion acceleration in the region where the local plasma potential rapidly drops over a few centimeters. The result also indicates a loss of the beam ions and an increase of the bulk ions in the diffusion chamber due to the charge exchange process between the beam ions and argon neutrals.