Modifying Wire-Array Z-Pinch Ablation Structure and Implosion Dynamics Using Coiled Wires

Coiled arrays, which are cylindrical arrays 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 two-stream structure in the r, theta 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 the 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 J x B forces that result from the interaction of the global magnetic field with a helical current path as well as additional current paths suggested by the simulations. With this ability to control where the 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 produced a new mode of implosion in which the global instability can be controlled, and perturbations correlated between all wires in an array. For large-wavelength eight-wire coiled arrays, this produced a dramatic increase in X-ray power, equaling that of a 32-wire straight array. These experiments were carried out on the mega ampere generator for plasma implosion experiments (1 MA, 240 ns) at Imperial College London,London,U.K.