Theory of pellet cloud oscillation striations

The Rayleigh-Taylor instability is proposed as a mechanism to explain the occurrence of 'oscillating striations', the regular fluctuations in the emitted light intensity from a pellet ablation cloud. The instability is driven by the E x B rotation of the ablation column about its symmetry axis parallel to the B field. The radial E field comes from a radially varying floating potential, arising as a result of attenuation of the field-aligned hot plasma electrons streaming through the inhomogeneous ablation cloud density. Based on a model ellipsoidal density profile, the angular rotation speed was found to be nearly uniform. A radial eigenmode, eigenvalue equation was solved for the m = I shift mode, which predicts frequencies and threshold instability conditions consistent with experimental observations. implications for fast confined alpha diagnostics using pellet injection are discussed briefly.