Waves propagating along a converging-diverging rf magnetoplasma having the
characteristics of a bounded m=0 helicon mode are reported and characterised.
The discharge features a 30 cm separation between the region of radiofrequency
energy deposition by a single loop antenna and the region of maximum magnetic
field applied by a pair of coils. With 200 W of rf input power, the resulting
plasma exhibits a strong axial plasma density gradient peaking at the magnetic
mirror throat where an Ar II blue-core is observed. Two dimensional B-dot probe
measurements show that the rf magnetic fields are closely guided by the
converging-diverging geometry. The wave is characterised as a m=0 mode
satisfying the helicon dispersion relation on-axis with radial boundary
conditions approximately matching the radii of the plasma column. Analysis of
the wave phase velocity and wave axial damping failed to identify collisionless
or collisional wave-plasma coupling mechanisms. Instead, the wave axial
amplitude variations can be explained by local wave resonances and possible
reflections from localised rapid changes of the refractive index. A
Venturi-like effect owing to the funnel-shaped magnetoplasma and conservation
of the wave energy may also explain some level of amplitude variations.Comment: The following article has been submitted to Plasma Sources Science
and Technolog