  We present the first observation of an instability in an expanding ultracold
plasma. We observe periodic emission of electrons from an ultracold plasma in
weak, crossed magnetic and electric fields, and a strongly perturbed electron
density distribution in electron time-of-flight projection images. We identify
this instability as a high-frequency electron drift instability due to the
coupling between the electron drift wave and electron cyclotron harmonic, which
has large wavenumbers corresponding to wavelengths close to the electron
gyroradius.
