STOCHASTICITY FROM EXTERNAL MAGNETIC-FIELD MEASUREMENTS

To determine whether or not magnetic field lines inside a tokamak plas ma ace stochastic we need the Fourier coefficients of any perturbing r adial field inside the plasma. Usually what is measured with magnetic pick-up coils is the root mean square poloidal field outside the plasm a. Although no unique transformation is available, we present a model which allows an interpretation of the measured (external) root mean sq uare field in terms of the internal Fourier harmonics. The results are applied to particular TEXT discharges, and suggest a link between mag netic stochasticity and an increasing (more positive) radial electric field, as measured with a heavy ion beam probe.