Carbon pellet cloud striations

Abstract

Fine-scale striations, with alternating rows of bright and dark zones, have been observed in the ablation clouds of carbon pellets injected into the TEXT tokamak. The striations extend along the magnetic field for about 1 cm with quite regular cross-field variations characterized by a wavelength of a few millimeters. It is proposed that the striations are the manifestation of the saturated state of growing fluctuations inside the cloud. The instability is generated by E*B rotation of the ablation cloud. The outward centrifugal force points down the ablation density gradient, inducing the Rayleigh-Taylor instability. The instability is not present for wave numbers along the field lines, which may explain why the striations are long and uniform in that direction. The E field develops inside the ablation cloud as a result of cold electron return currents that are induced to cancel the incoming hot plasma electron current streaming along the lines.<>