Modifying wire-array Z-pinch ablation structure and implosion dynamics using coiled arrays

Coiled arrays, a cylindrical array in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a 2-stream structure in the r,thetas plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement and plasma is axially transported from these positions such that the streams become aligned with sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of axial JxB forces that result from the interaction of the global magnetic field with radial components of a helical current path as well as additional current paths suggested by the simulations. With this ability to control where ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separation to prevent perturbations in the implosion sheath from merging. This produces a new, organised mode of implosion in which the global instability can be controlled and the perturbations correlated between all the wires in the array. For large wavelength 8-wire coiled arrays, this produced a dramatic increase in x-ray power, equaling the x-ray power of a 32-wire straight array. These experiments were carried out on the MAGPIE generator (IMA, 240ns) at Imperial College, London.