We perform stability analysis on the finite-difference time domain
method (FDTD) when extended to incorporate local space-time adaptive
mesh refinement (AMR). The neutrally stable Yee algorithm becomes
extremely sensitive to perturbations introduced by the interpolation
schemes employed at grid refinement interfaces. In this paper we
investigate the stability of a range of interpolation schemes using
Gustafsson-Kreiss-Sundstrom-Trefethen (GKS-T) mode and
reflection/transmission coefficients analysis on the infinite domain
with a single interface. This analysis allows detection of trapping
instabilities, exponentially growing modes, mode resonances with the
interface and mode-mode resonances. We also apply matrix stability
analysis for more complicated computational domains containing
multiple grid refinement interfaces.