Stochasticity from external magnetic field measurements

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