Implosion and Radiation Dynamics of Cylindrical and Planar Wire Array Z-pinch Plasma
AuthorWilliamson, Kenneth Martin
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The wire array z-pinch is an accessible source of high energy density plasma and among the most efficient sources of x-ray radiation with application to astrophysical plasma study, radiation physics, and inertial confinement fusion (ICF). This dissertation presents the results of analysis from two load geometries with potential application to ICF studies: the cylindrical and planar configurations. Experiements were performed on the 1.0 - 1.7 MA Zebra pulsed power generator at the University of Nevada, Reno's Nevada Terawatt Facility and on the 1.0 MA COBRA pulsed power generator at Cornell University. Fundamental plasma parameters were assessed from radiation emission 10 eV > hν > 30000 eV that was imaged, diffracted, and recorded with spatial and temporal resolution. Implosion dynamics were further studied using optical shadowgraphy and streak imaging. A new diagnostic was designed and fielded specifically to study the time-evolution of the extreme ultraviolet (EUV) spectral emission during the ablation and implosion phases.The complex load geometry of double planar and nested cylindrical wire arrays with combined wire materials of different ablation properties were utilized to control implosion timing and radiation pulse shapes. Nested wire arrays induce some fractional division of the load current in each array as a function of the wire number and array diameter. An ablation dominated implosion regime was achieved with combined materials of significantly different ablation properties on the COBRA generator. In particular, Al (inner)/ Stainless Steel (outer) nested cylindrical wire array loads imploded in a nonconventional mode where the inner array imploded on axis before the outer array moved from its original position significantly affecting the resulting radiation output. Compact cylindrical arrays, loads with a diameter less than 8mm, produced yields comparable to the highest observed with other configurations at the Zebra generator. However, this resulted from the optimization of the allowable interwire gap allowed by the decreased diameter, not the value of the diameter itself. Yields scaled significantly lower with current for the 3-6mm-diameter compact wire arrays than the ideal quadratic relationship. This effect was attributed to the greatly increased mass required optimize the implosion time.Planar loads experience an uneven current distribution. Much higher fraction current is expected to flow through the lower-inductance edge wires of the load causing a cascading implosion. Within a single planar load this cascade occurs within the wire plane inhibiting precursor formation and early radiation output. Within a double planar array the plasma is injected into the interior of the array causing unique multi-step precursor dynamics influenced by the penetration of the global magnetic field. This multi-step precursor formation in DPWAs with suitable global magnetic field shielding can be used to shape the resulting radiation by off-axis mass redistribution. Plasma parameters derived from EUV spectra indicated a plasma electron temperature and density of 60 eV and 10-19 cm-3.