We present an in-depth analysis of micropinch formation dynamics in copper (Cu) and nickel (Ni) hybrid X-pinches using an X-ray streak camera with a ~18 ps temporal resolution. The experiments were carried out on a 400-kA peak current, 50 ns rise time pulsed-power machine. Our focus was on characterizing the L-shell radiation with energies below 1 keV, preceding the X-pinch continuum burst, which signifies micropinch formation. This enabled us to assess imploding plasma conditions just before stagnation. Through the analysis of Ne-like copper lines prior to the continuum, we determined an average electron temperature of ~200 eV and an electron density of 4.5e28 /m3. Interestingly, our data suggests that the electron temperature jumps to ~1 keV as inferred from the continuum and the post-continuum line emission. Contrary to expectations, we did not observe any rapid temperature change or significant surge in radiation emission during the 200 ps pre-continuum X-ray burst, which would have supported the radiative collapse process as a major contributor to micropinch formation. To further explore alternative hypotheses, we employed two-dimensional extended magnetohydrodynamic (MHD) simulations coupled with a collisional-radiative spectral analysis code. These simulations revealed several key factors that drive micropinch formation, including the rapid radial implosion and compression of high-temperature low-density plasma, the axial outflow of the cold wire core, the dynamo term in the generalized Ohm's law, and the dynamic pressure of the imploding Cu plasma during the final phase of the process. Our findings challenge the conventional understanding of micropinch formation and pave the way for future research on this fundamental plasma phenomenon.

X-pinches, formed by driving intense current through the crossing of 2 or more wires, provide an excellent platform for the study of “micro-pinches” due to their propensity to generate a single micro-pinch at a predetermined location in space (i.e., where the wires cross) [1,2]. Ideally, micro-pinches are areas of run-away compression to very small radii (~1 µm) leNickading to pressures on the order of ~1 Gbar for currents on the order of ~0.1 MA. However, the fraction of the total current that is driven through the dense micro-pinch plasma at small radii versus that being shunted through the surrounding coronal plasma at larger radii is not well known. To allow for the study of X-pinches and their current distribution on the 1-MA MAIZE facility, a Faraday rotation imaging diagnostic (1064 nm), as well as a corresponding modular load hardware, were developed. Presented is the status of these developments including preliminary experimental results characterizing deuterated-polyethylene-fiber X-pinches on the MAIZE LTD.

[1] S.A. Pikuz et al., Plasma Phys. Rep., 41, 291 (2015);
[2] S.A. Pikuz et al., Plasma Phys. Rep., 41, 445 (2015);

*This work was supported by the DOE Early Career Research Program under Grant DE-SC0020239 and by the NNSA SSAP under Cooperative Agreement DE-NA0003764.

Radiative collapse is a runaway process whereby a plasma reaches sufficiently high densities and temperatures such that radiative cooling drastically drops its thermal pressure, collapsing the plasma to very small regions. Excluding the effect of macro-instablities disrupting the convergence, collapse can be terminated by a transition to the optically thick regimes, or electron degeneracy pressure under stronger conditions.

This talk presents high-fidelity 2D MHD simulations of single wire explosion using fibres with varying Z. This system has been shown to go m=0 unstable and produce brightly emitting hot-spots at neck regions before they bifurcate and disrupt collapse. We revisit this system using upgraded models compared to previous simulations. These include tabulated equation of state and transport coefficients, improved radiative loss and transport models including opacities generated from an in-house atomics code, and a mesh refinement capability allowing high resolution on a large domain size. We find improved qualitative agreement between these simulations and experiments conducted at Imperial College London on the IMP generator.