Dielectric-metallic phase transition in magnetic-flux compression experiments

Fast magnetic flux compression in a Z-pinch like geometry is known to suffer from hydrodynamic instabilities that develop during the cylindrical implosion. One way of overcoming this problem is to use the dielectric-metallic phase transition in a shock-compressed semiconductor crystal or metallic powder, a technique initiated by Japanese and Russian researchers working with explosive-driven cylindrical flux compressors. At Loughborough University, an electric gun technique in a Z-pinch like geometry is for the first time being used for either novel dynamic transformers or magnetic flux compression purposes. In the magnetic flux compression mode, an imploding aluminium foil (plasma)/mylar flyer package compresses an initial magnetic field. A cascaded system has also been developed, in which the flyer package interacts with a cylindrical column of aluminium powder to achieve higher efficiency magnetic flux compression. The paper presents results from the initial phase of a detailed experimental study, undertaken to measure accurately the propagation velocity of the phase transition produced in plane geometry by an electric-gun driven flyer. Novel designs of velocity monitoring transducers are described and results from cylindrical implosion magnetic-flux compression experiments using both single stage implosion and the smoother wave technique are presented and compared