Identification of microtearing modes below the ion gyroscale in the National Spherical Torus Experiment

Gyrokinetic calculations indicate microtearing modes below the ion gyroscale are linearly unstable in a National Spherical Torus Experiment (NSTX) plasma. The modes are robustly unstable with respect to simulation parameters, radial location and discharge time. The modes exist at higher wavenumbers and exhibit narrower electric potential mode structures than conventional microtearing modes, but both modes extend to similar normalized radial wavenumbers. Mode growth rates increase with higher electron temperature gradients and higher collisionality. Finally, microtearing modes below the ion gyroscale are the most unstable modes near the magnetic axis, but electron temperature gradient modes are the most unstable modes in the outer plasma region.