Theory of the ubiquitous mode

Abstract

Standing modes along the magnetic field lines and with frequencies between the mean ion transit and electron bounce frequencies are driven unstable by a combination of effects including inverse Landau damping, collisional de-trapping of electrons and magnetic curvature drifts in a magnetically confined plasma. The mode phase velocity, depending on the plasma parameters and on the mode perpendicular wavelength, is either in the direction of the electron diamagnetic velocity (electron drift mode) or in the direction of the ion diamagnetic velocity (ubiquitous mode). An intermediate region exists where the mode becomes fluid-like and is driven by the magnetic curvature drift. This fluid-like instability is shown to appear for wavelengths longer than the ion gyroradius and is expected to affect significantly the rate of particle and thermal energy transport in high-temperature toroidal-confinement devices.