Study of Ablation and Implosion Phases in Cylindrical and Star Wire Arrays
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An advanced set of laser probing diagnostics was applied for the investigation of implosion dynamics and magnetic fields in cylindrical, star, and nested wire arrays at the Nevada Terawatt Facility. Plasma diagnostics at a wavelength of 532 nm provide a five-frame optical probing of the z-pinch including shadowgraphy, Faraday rotation diagnostics, interferometry, and schlieren diagnostics. The Faraday rotation was applied for the investigation of magnetic fields and currents as well as structures in the plasma column of the precursor in wire array z-pinches. Faraday images and their complimentary shadowgrams reveal the presence of magnetic fields (B) that have opposite directions between both sides of the precursor, inside which the current was flowing since the early stage of the implosion of cylindrical and conical wire arrays. The current in the precursor plasma column was estimated to be 0.05-0.15 MA. Measurement of the electron plasma density with regular laser interferometery meets the zero-number fringe issue on the axis of the z-pinch. We suggested a new diagnostic to record a continuous history of the interferograms and the individual evolution of the fringes. In this case, the plasma density could be measured by deriving the shift of the fringes on the slit of a streak camera. Implosion stage was investigated in wire arrays with five-frame laser probing. Bubble-like implosion was identified as a mechanism of the mass transport in wire arrays. Development of bubbles on the breaks of the wire cores and evolution of bubbles were studied. Implosions with speeds of around 200-500 km/s were recorded in the wire arrays and the dynamics of the implosion plasma bubbles, current reconnection, as well as a shock in the precursor were observed. The z-pinch imaging and Faraday rotation diagnostics with 1-MA wire arrays were analyzed. A bubble-like mass transport is observed in all types of wire arrays (cylindrical, nested, linear, etc). A new type of "star" wire array was designed and studied. These loads consist of multiple nested, low-wire-number, cylindrical arrays aligned azimuthally such that the wires appear as "rays" extending from the axis of symmetry. In low wire-number star arrays, the imploding plasma starts on the edge wires, cascades from wire to wire, accelerating toward the center, and forms moving plasma columns with a smooth leading edge. The cascading mode of implosion was confirmed by several optical plasma diagnostics, including five-frame laser probing of z-pinch in three directions, an optical streak camera, and a time-gated CCD. In star wire arrays, smoothing of plasma instabilities was observed in the last phase of the implosion. The hydrodynamic regime of collision mitigates the instabilities, improves the homogeneity of the imploding plasma, and increases the radiated power in the star-like wire array. Star wire array generates a short high power x-ray pulse. Star wire arrays could be a good alternative to single cylindrical and double nested wire arrays in HEDP experiments.