Stabilization of Liner Implosions via a Dynamic Screw Pinch

Magnetically driven implosions are susceptible to magnetohydrodynamic instabilities, including the magneto-Rayleigh-Taylor instability (MRTI). 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. Here, we present the first experimental tests of a solid-metal liner implosion driven by a DSP. Using the 1-MA, 100–200-ns COBRA pulsed-power driver, we tested three DSP cases (with peak axial magnetic fields of 2 T, 14 T, and 20 T) and a standard $z$-pinch (SZP) case (with a straight return-current structure and thus zero axial field). The liners had an initial radius of 3.2 mm and were made from 650-nm-thick aluminum foil. Images collected during the experiments reveal that helical MRTI modes developed in the DSP cases, while nonhelical (azimuthally symmetric) MRTI modes developed in the SZP case. Additionally, the MRTI amplitudes for the 14-T and 20-T DSP cases were smaller than in the SZP case. Specifically, when the liner had imploded to half of its initial radius, the MRTI amplitudes for the SZP case and for the 14-T and 20-T DSP cases were, respectively, $1.1\pm 0.3\mathrm{mm}$, $0.7\pm 0.2\mathrm{mm}$, and $0.3\pm 0.1\mathrm{mm}$. Relative to the SZP, the stabilization obtained using the DSP agrees reasonably well with theoretical estimates.

Figure 1

CAD models of the return-current structures tested in these experiments. (a) Straight SZP return-current structure, including an illustration of the power feed. (b)–(d) Twisted DSP return-current structures, with predicted peak axial fields of 2 T, 14 T, and 20 T, respectively.

Figure 2

Example experimental data, including drive currents, liner implosion trajectories, and the axial magnetic field, $B_z(t)$, measured on the mid-field DSP shot that produced the short-pulse current trace shown. Fitting curves for the experimentally measured liner radii are also plotted.

Figure 3

(a) Visible self-emission images for each of the four experimental cases tested. (b) XUV image from a SZP experiment near stagnation. (c) XUV image from a mid-field DSP experiment near stagnation.

Figure 4

A plot of the average instability amplitudes as a function of the normalized distance moved, $\widehat{d}\equiv 1-r_{\ell }(t)/r_{\ell }(0)$. Linear fits for each shot are plotted as dashed lines.

Figure 5

Several SZP and mid-field DSP drive currents from the shot series. These plots illustrate the effects that the DSP configuration has on late-time load-current measurements and possibly on power flow after peak current.