Locked modes in DIII-D and a method for prevention of the low density mode

Locked n = 1 tearing modes are observed over a wide range of parameter space in DIII-D and other tokamaks. Much of the difficulty with low density operation is attributed to locked modes and they are also observed as precursors of density limit disruptions. From observations of a consistent locked mode toroidal phase, it appears that the modes are locking to a small field perturbation caused by slight irregularities in the location of one or more of the vertical field coils with respect to the toroidal field coil. By intentionally producing an n = 1 field with an external coil, it was possible to influence the onset of locked modes in low q, low density plasmas. The result is a significantly expanded or reduced stable operating parameter space, depending on the polarity and magnitude of the external perturbation applied and whether or not intrinsic field errors are reduced or increased. A simple theoretical model of the non-linear stability of a tearing mode under the effect of externally applied resonant field perturbations yields a critical island width for the onset of a locked mode. The island widths, computed by field line tracing, for combinations of intrinsic field errors and 'n = 1 coil' fields are in qualitative agreement with the critical island width for instability.