On-axis Parallel Ion Speeds near Mechanical and Magnetic Apertures in a Helicon Plasma Device
Authors: Xuan Sun, S.A. Cohen, Earl E. Scime, and Mahmood Miah
Date of PPPL Report: January 2006
Published in: Physics of Plasmas 12,103509 (2005) 8pp
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 0.1-5 cm 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 pre-sheath region upstream of the mechanical aperture, the convective ion speed increased over a collisional distance from stagnant at 4 cm 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 magnetic-field strength and mechanical-aperture electrical bias are also presented.