Sheath expansion of plane probe by ion-beam reflection

The mechanism of the appearance of a knee on the voltage-current curve of a plane Langmuir probe with an incident ion beam is investigated experimentally. The two-dimensional shape of the sheath formed in front of the probe is measured by an emissive probe. The knee is found to be due to the enhanced expansion of an electron sheath by beam reflection, which increases the effective area of the probe. A potential profile similar to a double layer is formed in front of the probe. A weak axial magnetic field, which reduces a radial electron flow and which makes inverted V-shaped two-dimensional potential contours, quenches the knee.