Discrete helical modes in imploding and exploding cylindrical, magnetized liners

Abstract

Discrete helical modes have been experimentally observed from implosion to explosion in cylindrical, axially magnetized ultrathin foils (Bz = 0.2 – 2.0 T) using visible self-emission and laser shadowgraphy. The striation angle of the helices, ϕ, was found to increase during the implosion and decrease during the explosion, despite the large azimuthal magnetic field (>40 T). These helical striations are interpreted as discrete, non-axisymmetric eigenmodes that persist from implosion to explosion, obeying the simple relation ϕ = m/kR, where m, k, and R are the azimuthal mode number, axial wavenumber, and radius, respectively. Experimentally, we found that (a) there is only one, or at the most two, dominant unstable eigenmode, (b) there does not appear to be a sharp threshold on the axial magnetic field for the emergence of the non-axisymmetric helical modes, and (c) higher axial magnetic fields yield higher azimuthal modes.

A Primer on Pulsed Power and Linear Transformer Drivers for High Energy Density Physics Applications

“…III, we find Z 0 = L/C ≈ 0.16 Ω, I peak = V 0 /Z 0 ≈ 900 kA, and τ peak = (π/2) √ LC ≈ 200 ns. These results agree reasonably well with experiments conducted on MAIZE [29], [108]- [112], especially given the simplicity of the LC model. Note that with V c = ±70 kV (|V 0 | = 2|V c | = 140 kV), the initial energy stored in the MAIZE capacitors is…”

The influence of the solid to plasma phase transition on the generation of plasma instabilities

“…Indeed, early stage results 29 show that the inclusion of the material’s physical properties modifies the ETI into an instability, which for future reference we choose to call it ETM instability, having growth rates at least one order of magnitude larger than those found in the literature. Moreover, the ETM instability could act as seeding mechanism for the outstanding helical instability structures observed in premagnetized liner experiments 30–35 . The further study of the ETM instability can enlighten the efforts for the understanding of the seeding mechanisms of such helical structures.…”

The Electrothermal Instability on Pulsed Power Ablations of Thin Foils

“…Figure 6.9 shows a remarkable reduction in sausage instability growth compared to the aluminum liner shown in Figure 6.8. While the sausage instability is expected to grow less for the denser tantalum liner than for aluminum, since sausage instability grows as 1/√ [39,43], the almost completely flat outer surfaces in Figure 6.9 imply the sausage instability was not seeded at all along most of the liner. Additionally, a few perturbations are visible along the bottom left side of the liner likely seeded by edge effects where the foil makes electrical contact with the support structure; these grow and merge similarly to the instability structures seen in while the tantalum liners show little, if any, growth, with amplitude that is on the order of the imaging resolution of the system.…”

“…Foils of varied materials were cut into 2.1 cm by 1.5 cm strips and wrapped into a liner geometry around an insulating support structure of total length 3.4 cm and diameter 6. Figure 3.15 [43]. The total length of the cavity is 3.05 m, corresponding to a 10 ns delay between pulses.…”

“…If (6.2) is a good indicator of the total growth of ETI, the largest reduction in seeding of plasma instabilities should result for a material with a low ratio of to . One such material is tantalum, which has a critical temperature of 10250 K [76] and a melting temperature of 3293 K. Liner ablations were performed on the solid plastic support structures to investigate the growth of the sausage instability [43] on a 500 nm tantalum liner compared to a typical 400 nm aluminum liner. Figures 6.8 and 6.9 show the framing camera images from these shots.…”