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Description
This work presents the first experimental
characterization of cavity modes trapped within a
plasma column in an axis-symmetric magnetic trap.
Trapped wave has been characterized by means of a
movable antenna as a function of the B_{min}/B_
{ECR} ratio and plasma parameters. The study
demonstrates that E.M waves can be trapped
between two R-cutoff layers, in proximity of the B
minimum position. Results suggest that the trapped
waves consist of Whistler waves, propagating
perpendicularly to the magnetic field and storing
large part of the E.M power introduced in the plasma
chamber. If R-cut-off is removed by increasing the
density, trapped waves are not detected anymore. A
typical Electron Energy Distribution Function
composed by two different electron populations is
measured in the layer where trapped waves are
revealed, suggesting that additional heating is
occurring. The existence of trapped waves in a
plasma column was already foreseen by several
studies concerning the development of kinetic
instability in plasmas and this experimental
characterization allows to shed more light on nonlinear
plasma coupling and to the generation of
supra-thermal electrons in ECRIS plasmas.