Destruction of drift surfaces in tokamaks through small magnetic field perturbations

In tokamak discharges magnetic field perturbations usually seem too small to cause a global destruction of flux surfaces. Perturbations, however, can modify globally the shape of the flux surfaces and thus influence particle orbits (e.g. through enhanced field curvature and gradients drifts, etc.). This influence was investigated especially for cases of field perturbations which are toroidally asymmetric (and ''non-periodic''). In these cases particle trajectory invariants (such as toroidal angular momentum, adiabatic invariants, etc.), which are basic for conventional plasma transport theories, are not necessarily available. It turns out that, although the magnetic field does form flux surfaces, the drift surfaces do not in general exist, what can be constructed as ''anomalous transport''. To quantify these effects accurate magnetic fields are computed using new mapping techniques. Particles are followed both directly and in drift approximation. As special cases perturbations are chosen imposed on a tokamak plasma by neutral beam injection and by faults in the toroidal field coils. It is estimated that the derived coefficients (χ, D) are in the range of the observed values for rather modest perturbation amplitudes. (Author)