An experimental and theoretical investigation of a Helicon plasma source has been completed as a precursor to the development of an automatic system for controlling the operation of such sources. The number density in an argon plasma has been observed over a parameter space defined by the input variables of filling pressure (1–100 μbar), magnetic field (0–450 G) and rf power (0–1250 W). Four distinct modes of operation have been identified, namely: electrostatic, inductive, Helicon and high pressure Helicon modes. A global model of the source has been modified to include the effect of radial confinement by the axial field. Comparisons with the experimental results show that the model generally can predict the plasma density to within 15%–20% for both the inductive and Helicon modes. It is found that the ratio of the ion cyclotron radius compared with the radius of the plasma is a useful measure of the amount of confinement. An alternative measure based on the product of the ion cyclotron frequency and the collision time for momentum transfer does not lead to satisfactory agreement between the global model and the experimental observations. The modified global model appears to describe the behavior of the plasma well enough to allow the development of a conventional control system that can be used within each of the modes. The model, however, is unable to predict the locations of the mode jumps within the parameter space.