Stability and Power Coupling in Dynamic Screw Pinch Plasmas
Campbell, Paul
2020
Abstract
Fast z-pinches are formed when large axial currents run through cylindrical metal shells, or liners, to produce a Lorentz force that implodes the system. This implosion process is susceptible to magnetohydrodynamic instabilities, such as the magneto-Rayleigh-Taylor instability (MRTI). These instabilities are undesirable since many experiments rely on a sufficiently symmetric implosion. The study of MRTI is of particular relevance to magnetized fusion concepts like magnetized liner inertial fusion (MagLIF), which are degraded by this instability. To reduce MRTI growth in solid-metal liner implosions, the use of a dynamic screw pinch (DSP) has been proposed [P. F. Schmit et al., Phys. Rev. Lett. 117, 205001 (2016)]. In a DSP configuration, a helical return-current structure surrounds the liner, resulting in a helical magnetic field that drives the implosion. In this dissertation, the first experimental tests of a solid-metal liner implosion driven by a DSP are presented [P. C. Campbell et al., Phys. Rev. Lett. 125, 035001 (2020)]. Using the 1-MA, 100–200-ns COBRA pulsed-power driver, three DSP cases were tested (with peak axial magnetic fields of 2T, 14T, and 20T) along with a standard z-pinch (SZP) case (with a straight return-current structure and thus zero axial field). These experiments demonstrated enhanced stability in thin-foil liner implosions. When compared to theory [A.L. Velikovich et al., Phys. Plasmas 22, 122711 (2015)], these results agree reasonably well. The strongest DSP case tested showed a factor of three reduction in instability amplitude at stagnation. Specifically, at a convergence ratio of 2, the MRTI amplitudes for the SZP case and for the 14-T and 20-T DSP cases were, respectively, 1.1+/-0.3 mm, 0.7+/-0.2 mm, and 0.3+/-0.1 mm. While the convergence ratio of the experiments was low, relative to other imploding liner experiments, the trends in the data were clear; when the DSP generates stronger axial magnetic fields, the instability amplitude decreases. Measurements using micro B-dot probes showed that the return current structures in the DSP cases generated axial magnetic field values in line with the values predicted by electromagnetic simulations. Measurements taken inside the imploding liners showed a significant amount of flux injection and subsequent flux compression. Throughout the short-pulse experiments on COBRA, the 14-T and 20-T DSP cases stagnated 10--40 ns earlier than the SZP cases, which is most likely due to the added magnetic pressure from the axial field that is present in the DSP case. The load current on COBRA was measured with a Rogowski coil in the power feed. After peak current, the Rogowski measurement would often terminate during the falling edge of the current pulse in the SZP experiments, while in the 14-T DSP experiments, it would often continue well after the current pulse had returned to zero. Preliminary particle-in-cell (PIC) simulations suggest that, after peak current, electrons sourced near the liner are directed down into the power feed towards the Rogowski coil in the SZP configuration, while simulations of the 14-T DSP configuration suggest these electrons are ejected radially outward through the gaps between the DSP return-current posts and thus away from the Rogowski coil. The lack of electron interaction with the Rogowski coil may explain why the load current measurements persist for longer in the DSP experiments. This observation could have important implications for power delivery in magnetically driven implosions in general.Deep Blue DOI
Subjects
Plasma Physics Pulsed Power Z-pinches
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