The characterization of a helicon plasma source has been done in argon plasmas using a rf compensated Langmuir probe. It is now well known that helicon discharges exhibit three modes of operation: a capacitive mode (known as the E mode), which is a low-density mode (around 10{sup}9 cm{sup}-3), an inductive mode (known as the H mode) with higher densities (10{sup}10 cm{sup}-3), and a helicon-wave sustained mode (known as the W mode) with very high densities (10{sup}11-10{sup}12 cm{sup}-3). By increasing the injected power, the transitions between these modes can be observed and correspond to sudden increases of the plasma density and of the glow brightness. In this article the performances of the helicon source in terms of density and plasma potential are presented and discussed. Transitions between the different coupling modes are observed and explained thanks to two phenomenological models and a good agreement is found between the experimental values and the model results. It has been shown that the main parameter, which determines the coupling mode, is the plasma density, which depends on the If power and the gas pressure. So the transition between E and H modes is observed when the density reaches a critical value: for this value the skin depth δ is equal to the radius of the Pyrex tube. A similar result has been shown for the H- W transition: a minimum density, the value of which depends on the magnetic field amplitude, is necessary to observe the first longitudinal helicon-wave mode. This value is also directly linked with the geometry of the reactor and of the antenna.
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