the invention relates to a method for providing a direct estimation of the power delivered to the plasma in a tcp source . according to one embodiment , an apparatus is described . the apparatus includes a vacuum chamber and an electrical transformer to induce an electromagnetic field within the vacuum chamber . the transformer includes a primary winding , a secondary winding formed by the plasma loop , and a separate secondary winding implemented to measure the voltage along the plasma loop . the apparatus also includes a current transformer to measure the current flow through the plasma loop .