Spiral structure in magnetically-confined electron sheet beams

One of the outstanding features in the propagation of thin electron beams along an axial magnetic field is the transformation of the beam into discrete vortex-like current bundles whenever a threshold determined by either the beam current or distance of propagation is surpassed. The morphological changes observed in thin sheet beams, in which the beam ends are free to move, are particularly interesting. Initially, a sheet beam rotates some 30 - 400 about axis as a rigid body. However, because of the Alfven-Kyhl charge concentration-ripple instability, increasing the current further results in the formation of vortices along the length of the sheet, rather than rigid body rotation (figure). Another threshold is reached in current in which the central vortices suddenly rotate two or three times about each other, producing a spiral structure. This action is accompanied by the unraveling of vortices nearer the ends woth a concomittant release of magnetic field energy in the form of electromagnetic radiation