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A large-diameter (50 cm) high-density plasma is produced in a few mTorr argon by inductive RF discharge using a conventional external antenna or a plasma-immersed internal antenna. A power transfer efficiency, ie, the ratio of net power deposited into plasma to total power into the matching circuits, is measured as a function of the electron density based on a test antenna method. The measured density dependence of the power efficiency is well described by an equivalent circuit where both inductive and capacitive couplings are included with stochastic power deposition process taken into account. The internal antenna, for the conditions studied, has higher power efficiency than the external antenna and enables a stable discharge at low pressures without density jump. The density jump observed in the external antenna discharge is attributed to the mode transition between a capacitive discharge and an …