Microtearing modes and fast edge fluctuations in Alcator C-Mod plasmas

Magnetic fluctuations, observed in the Alcator C-Mod experiment, are modelled via the instability of microtearing modes with ratio of poloidal to toroidal mode number m/n = 4. These modes are predominantly electromagnetic in nature, although the electrostatic component is non-vanishing, are driven by the steep temperature gradient at the plasma edge and are shown to have a relatively broad spectrum for typical experimental parameters.