CHARACTERIZATION AND MODELING OF A HELICON PLASMA SOURCE

An experimental and theoretical investigation of a Helicon plasma sour ce has been completed as a precursor to the development of an automati c system for controlling the operation of such sources. The number den sity in an argon plasma has been observed over a parameter space defin ed by the input variables of filling pressure (1-100 mu bar), magnetic field (0-450 G) and rf power (0-1250 W). Four distinct modes of opera tion have been identified, namely: electrostatic, inductive, Helicon a nd high pressure Helicon modes. A global model of the source has been modified to include the effect of radial confinement by the axial fiel d. Comparisons with the experimental results show that the model gener ally can predict the plasma density to within 15%-20% for both the ind uctive and Helicon modes. It is found that the ratio of the ion cyclot ron radius compared with the radius of the plasma is a useful measure of the amount of confinement. An alternative measure based on the prod uct of the ion cyclotron frequency and the collision time for momentum transfer does not lead to satisfactory agreement between the global m odel and the experimental observations. The modified global model appe ars to describe the behavior of the plasma well enough to allow the de velopment of a conventional control system that can be used within eac h of the modes. The model, however, is unable to predict the locations of the mode jumps within the parameter space. (C) 1998 American Vacuu m Society.[S0734-2101(98)01305-0].