A non-rotating helical magnetic field perturbation grows in the JET plasma when the electron density falls below a minimum value. The growth of this perturbation usually leads to a disruption. The helical phase of the perturbation is reproducible for a given configuration of the applied poloidal and toroidal magnetic fields. Small asymmetries in the applied magnetic fields are shown to cause the growth of the perturbation. Various sources of such field errors in JET have been identified. The existence of a minimum density for a stable plasma can place a severe constraint on the operation of JET. The magnitude of the minimum density depends upon various plasma parameters. Neutral beam injection can be used to reduce the minimum density
The interaction between plasma rotation, stochastic fields and tearing mode excitation by external perturbation fields