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The axial wavelengths and azimuthal mode structures of a cylindrical helicon plasma source have been investigated over a broad range of input conditions (pressure 1-16 µbar, field 0-450 G, rf power 0-2500 W @ 13.56 MHz) in an argon plasma. Several distinct modes of operation, separated by discontinuous jumps, have been identified: an electrostatic mode, an m = 0 wave mode and three m = 1 helicon wave modes. Two different types of mode jump have also been identified. A cavity mode transition has been identified in which a mode jump occurs when the wavelength of standing waves governed by the length of the plasma vessel leads to a node in the wave field at the centre of the antenna. A radial mode transition has also been identified in which the favourable condition that the axial wavelength should be equal to twice the length of the antenna for efficient coupling of the rf power is maintained at high powers by the plasma making a transition from the first to the second radial mode. A procedure based on these observations is suggested for predicting where mode transitions are likely to occur within the parameter space of a helicon source.

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