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Description
A Helicon wave heated hydrogen plasma is created by applying 800 W RF power to a Nagoya-III antenna at 13.56 MHz frequency, which excites m = ±1 azimuthal mode in the plasma. A permanent ring magnet provides the axial field required for the helicon wave excitation. The plasma expands from the source chamber to the expansion chamber in a diverging magnetic field. Diagnosis of the wave field through a b-dot probe confirms the helicon wave excitation in the hydrogen plasma. The dc magnetic field is varied from 40 G to 85 G in the source region by changing the ring magnet position to study the effect of the diverging field in the expansion chamber. The field in the expansion chamber reduces down to < 10 G within a distance of 6 cm from the source boundary. The axial profile of the density attains a maximum value under the antenna and decays as we move away from the antenna into the expansion chamber. In the expansion chamber a second peak in density, weaker than the first one, is observed for a particular magnetic field configuration but is absent for other configurations. The phase and amplitude of the axial component of the wave field is measured and a wave damping is observed for the magnetic field configuration in question. This wave damping corresponds to the resonance cone absorption of the helicon wave. No such damping is observed for other cases where there is no dramatic increase in the downstream density.
