Characteristics and scaling of tungsten-wire-arrayz-pinch implosion dynamics at 20 MA

Abstract

We present observations for 20-MA wire-array z pinches of an extended wire ablation period of 57%+/-3% of the stagnation time of the array and non-thin-shell implosion trajectories. These experiments were performed with 20-mm-diam wire arrays used for the double- z -pinch inertial confinement fusion experiments [M. E. Cuneo, Phys. Rev. Lett. 88, 215004 (2002)] on the Z accelerator [R. B. Spielman, Phys. Plasmas 5, 2105 (1998)]. This array has the smallest wire-wire gaps typically used at 20 MA (209 microm ). The extended ablation period for this array indicates that two-dimensional (r-z) thin-shell implosion models that implicitly assume wire ablation and wire-to-wire merger into a shell on a rapid time scale compared to wire acceleration are fundamentally incorrect or incomplete for high-wire-number, massive (>2 mg/cm) , single, tungsten wire arrays. In contrast to earlier work where the wire array accelerated from its initial position at approximately 80% of the stagnation time, our results show that very late acceleration is not a universal aspect of wire array implosions. We also varied the ablation period between 46%+/-2% and 71%+/-3% of the stagnation time, for the first time, by scaling the array diameter between 40 mm (at a wire-wire gap of 524 mum ) and 12 mm (at a wire-wire gap of 209 microm ), at a constant stagnation time of 100+/-6 ns . The deviation of the wire-array trajectory from that of a thin shell scales inversely with the ablation rate per unit mass: f(m) proportional[dm(ablate)/dt]/m(array). The convergence ratio of the effective position of the current at peak x-ray power is approximately 3.6+/-0.6:1 , much less than the > or = 10:1 typically inferred from x-ray pinhole camera measurements of the brightest emitting regions on axis, at peak x-ray power. The trailing mass at the array edge early in the implosion appears to produce wings on the pinch mass profile at stagnation that reduces the rate of compression of the pinch. The observation of precursor pinch formation, trailing mass, and trailing current indicates that all the mass and current do not assemble simultaneously on axis. Precursor and trailing implosions appear to impact the efficiency of the conversion of current (driver energy) to x rays. An instability with the character of an m = 0 sausage grows rapidly on axis at stagnation, during the rise time of pinch power. Just after peak power, a mild m = 1 kink instability of the pinch occurs which is correlated with the higher compression ratio of the pinch after peak power and the decrease of the power pulse. Understanding these three-dimensional, discrete-wire implosion characteristics is critical in order to efficiently scale wire arrays to higher currents and powers for fusion applications.

Measurements of magneto-Rayleigh–Taylor instability growth during the implosion of initially solid metal liners

“…Wire-array implosions are dominated by the ablation of about half the initial mass into the array interior before the implosion begins, a consequence of the large skin depth of the current and the small diameter of the wires. [9][10][11] By contrast, the thickness of the cylindrical tube liners proposed for magnetized liner inertial fusion 6 exceeds the skin depth of the proposed current pulse, and no significant prefilling of the interior volume is expected. The remaining published controlled studies of MRT growth were PHYSICS OF PLASMAS 18, 056301 (2011) done on multimicrosecond generators in which the imploding liners have significant material strength and remain in liquid or solid states for much of the implosion.…”

Theoretical and numerical research of wire array Z-pinch and dynamic hohlraum at IAPCM

“…These plasmas are accelerated toward the array axis by magnetic pressure and accumulate there to form M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT a precursor column, while the cold-dense wire cores are stable in their original positions and keep ablating. Experimental results show that the basic processes of wire-array ablation on the 20-MA Z machine [32] are similar to that on the smaller current facilities such as ANGARA [33], MAGPIE [34] and COBRA [35]. The ablation rate of wire core is axially nonuniform, and the ablated plasmas will carry a significant fraction of current.…”

Overview of the dynamic-hohlraum x-ray source at Sandia National Laboratories.

“…3A). The difference likely lies with the details of the wire-array dynamics not included in the simulations, and in particular the generation of trailing mass as the array mass is increased at these lower array diameters [27,36]. Additionally, the power exiting the bottom REH gradually decreases relative to the top as the diameter increases.…”

“…Measurements suggest the disruption may extend as much as 3 mm above or below the cathode or anode surface, [43] and is related to the axial radiation asymmetry discussed in Section VIII. The power decrease may also be caused, in part, by closure of the AK gap defined by the upper and lower pinch electrodes by trailing mass that shunts the current at large radii [27,36]. Figure 7A illustrates the development of the mass-driven shock from the impact of the wire arrays on the pedestal target (Fig.…”