Evolution of the Parallel and Perpendicular Ion Velocity Distribution Function in Pulsed Helicon Plasma Sources Obtained by Time Resolved Laser Induced Fluorescence

The temporal evolution of parallel and perpendicular ion velocity distribution functions (ivdf) in a pulsed, helicon- generated, expanding, argon plasma is presented. The ivdf's temporal evolution during the pulse was determined with time resolved (1 ms resolution), laser induced fluorescence (LIF). The parallel ivdf measurements indicate that, in the expansion region of the plasma and for certain operational parameters, two ion populations exist: a fast moving population moving at supersonic speeds (1.1 Mach) resulting from acceleration in an electric double layer (EDL) and a slow moving population (0.7 Mach) generated by local ionization. After 100 ms, although present, the EDL is not fully developed and has not reached steady state. Measurements of the perpendicular ivdf indicate constant radial expansion, with ion speeds of $\sim$ 400 m/s, in the expansion region.