Characterization of X-Pinches on the MAIZE Linear Transformer Driver

Abstract

An X-pinch, formed by driving an intense current pulse (>100 kA in ~100 ns) through the crossing of 2 or more wires, can result in the formation of a small radius, runaway compressional micro-pinch. A micro-pinch is characterized by a hot (>1 keV), current-driven (>100 kA), high-density plasma column (near solid density) with a small neck diameter (1–10 μm), a short axial extent (<1 mm), and a short duration (≲1 ns). With material pressures often well into the multi-Mbar regime, a micro-pinch plasma often radiates an intense, sub-ns burst of sub-keV to multi-keV x-rays. This dissertation presents two main areas of X-pinch characterization performed on the low-impedance (0.16 W), 1-MA, 100-ns MAIZE Linear Transformer Driver (LTD) at the University of Michigan. The first characterization involved the delivery of electrical current to small pinch radius. A micro-pinch compression to radii on the order of ~1 μm would lead to pressures on the order of ~1 Gbar for currents on the order of ~0.1 MA. However, the fraction of the total current driven through the dense micro-pinch plasma at small radii versus being shunted through the surrounding coronal plasma at larger radii is not well known. For this study, a Faraday rotation imaging diagnostic (1064 nm) capable of producing simultaneous high-magnification polarimetric and interferometric images was developed for the MAIZE facility. Designed with a variable magnification (1–10x), this diagnostic achieves a spatial resolution of approximately 35 μm, which is sufficient for resolving the ~100-μm-scale coronal plasma region immediately surrounding the dense core. Analysis of the interferograms shows a coronal density profile that remains low (≲10^17 cm^3) until close to the dense core (≲200 μm) where it quickly increases (≳10^18 cm^3). Extraction of Faraday rotation angles similarly showed low signals with polarization shifts of |⍺|≲ 3°. Combined, these profiles provide bounding measurements, up to minimal radii of ~100 μm, showing a strong indication that ~40–60% of the total current delivered by MAIZE is distributed in the coronal plasma region from 100–300 μm, thereby reducing magnetic drive pressures to maxima of ~100–150 kbar. The second characterization involved the use of deuterated wires (CD2) for the X-pinch loads. As the typical configurations of X-pinches produce characteristically small (~μm), sub-ns x-ray bursts for fast radiographic applications, the conversion of the metallic wire to a deuterated material produces (in addition to an x-ray burst) a burst of 2.45-MeV neutrons from nuclear fusion reactions occurring between pairs of deuterons. Experiments using a hybrid X-pinch configuration loaded with deuterated polyethylene fiber were performed on the MAIZE LTD to evaluate the main parameters of the neutron emission. The resulting neutron production was found to be highly correlated to x-ray emission peaks within .10 ns, with production duration of typically 15–20 ns. In the limited parameter sweep performed, the average yields were on the order of 10^7 neutrons, while the maximum yield obtained was as high as 1.4x10^8 neutrons. These yields are on par with those obtained using the gas-puff z-pinch system on MAIZE. This is true even though the X-pinch platform has a region of neutron production that is only a fraction of that in the gas-puff z-pinch platform—i.e., a 1-mm axial extent in the X-pinch platform versus a 10 mm axial extent in the gas-puff z-pinch platform. This highlights the importance of understanding micro-pinch physics.