Measurement of the hot electrical conductivity in the PBX-M tokamak

A new method for the analysis of tokamak discharges in which the plasma current is driven by a combination of high power RF waves and a DC electric field is presented. In such regimes, which are the most usual in RF current drive experiments, it is generally difficult to separate the different components of the plasma current, i.e. purely ohmic, purely non-inductive and cross terms. If the bilinear (in wave power and electric field) cross-term is the dominant one, an explicit relation between the loop voltage drop and the injected power can be found. This relation involves two parameters, the purely RF current drive efficiency and the hot (power dependent) electrical conductivity. These can be simultaneously determined from a simple two parameter fit, if the loop voltage drop is measured at several RF power levels. An application to lower hybrid current drive experiments in the Princeton Beta Experiment (PBX-M) tokamak is presented. It is shown that the method also allows independent evaluation of the average power absorption fraction and the n || upshift