On-axis parallel ion speeds near mechanical and magnetic apertures in a helicon plasma device

Using laser-induced fluorescence, measurements of parallel ion velocities were made along the axis of a helicon-generated Ar plasma column whose radius was modified by spatially separated mechanical and magnetic apertures. Ion acceleration to supersonic speeds was observed 0.1–5cm downstream of both aperture types, simultaneously generating two steady-state double layers (DLs) when both apertures were in place. The DL downstream of the mechanical aperture plate had a larger potential drop, ΔϕDL=6–9kTe, compared to the DL downstream of the magnetic aperture, ΔϕDL∼3kTe. In the presheath region upstream of the mechanical aperture, the convective ion speed increased over a collisional distance; from stagnant at 4cm from the aperture to the 1.4 times the sound speed at the aperture. The dependence of the free- and trapped-ion-velocity-distribution functions on the magnetic-field strength and mechanical-aperture electrical bias are also presented.