Ionization-driven low-frequency edge fluctuations in toroidal devices

A local linear stability theory of the interchange mode, the drift-resistive interchange mode and the drift-dissipative mode, in the presence of ionization and radiative drives, is examined. It is found that diffusive effects (which are responsible for providing a steady state in equilibrium) give a damping which overwhelms the ionization drive for the drift mode except for very long wavelengths. However, flute-type perturbations are found to be unstable owing to additional effects arising through the temperature dependence of the ionization cross section. Finally, the radial eigenvalue problem for simple drift-ionization instability is examined numerically and it is found that the mode is unstable for long wavelength perturbations only when the rate of diffusion damping of the perturbation is substantially smaller than the rate of diffusive damping of the equilibrium gradients.