Feedback stabilisation of resistive tearing modes in tokamaks

The likely consequences of disruptions in a device the size of ITER are sufficiently severe that it is worth investigating a variety of schemes for disruption control. The majority (but not all) of tokamak disruptions are preceded by an m=2, n=1 tearing mode, and it has been demonstrated that control of these can delay or prevent a disruption. It has long been known that radially localised axisymmetric current perturbations can stabilise resistive tearing modes by changing the stability of the current profile J0 (r), but it has also been proposed that localised heating or current drive inside magnetic islands (e.g. at the O-point) could be used to control the non-linear evolution of the island. This is addressed in greater detail here, for m=2, n=1 modes, considering realistic deposition profiles; the poloidal and toroidal extent of the deposition zone, as given by an ECRH antenna pattern for example; the use of the scheme in a feedback loop and, finally, the application to error-field-driven tearing modes. The profile control effect is referred to as 'DC' current drive, the island heating scheme as 'phased' current drive. This paper will consider Electron Cyclotron Current Drive, although ECCD is not the only possible scheme. (author) 18 refs., 3 figs