We present results from a novel experimental platform [1] which utilizes the radiation pulse form a wire-array Z-pinch to drive radiative ablation from solid targets positioned ~ 3 cm from the axis of the pinch. The experiments were performed on the MAGPIE generator at Imperial College (1.4 MA, 240 ns) [2]. They used critically massed aluminium or tungsten wire arrays to generate driving radiation with a typical on target brightness temperature of 10 eV. Experiments were diagnosed with a state-of-the-art diagnostic suite [3, 4] including laser interferometry; collective optical Thomson scattering; optical fast-frame imaging; Faraday rotation imaging; B-dot (inductive) probes; and X-Ray spectroscopy. The experimental measurements were compared with simulations performed using the codes Chimera [5], Helios-CR, and PrismSPECT [6].

This experimental platform is advantageous in the sense that plasmas are produced with well posed initial conditions, and have a simple overall morphology. To date experiments have been performed in double and single target configurations with a variety of low-Z and mid-Z target materials. Most of the work done to date induced plasma ablation in a background magnetic field (B~5 T, supported by the Z-pinch current pulse). We have, however, recently demonstrated the possibility of reducing this field to ~0.1 T by adjusting the return current path, with a negligible change to the character of the driving radiation pulse.

Experiments performed with a single target demonstrated a simple (quasi-1D) morphology, making it possible to study the atomic properties of the ablated plasma flows. Our initial results indicate that photoionization plays an important role in determining the charge-state distribution of ablated plasma flows. In this talk we will argue that the experimental platform is relevant to studies of atomic-kinetics in accretion-disk environments.

In experiments where a pair of ablating driven targets were fielded, we were able to study the collisions between two radiatively driven flows. We were also able to study ablated plasma jets using conical or wedge-shaped target geometries. The properties of the stagnated plasma layers, generated in collisions, were observed to depend strongly on both the ion-species in the plasma and the presence/absence of an ambient magnetic field. In this talk we argue that these observations imply the experiments are relevant to the study of radiative cooling instabilities in young stellar objects (YSOs) [7].

Supported by the US DOE under Award Nos. DE-SC0020434 and DE-NA0003764, and by the US DTRA under Award No. HDTRA1-20-1-0001

[1] J. Halliday et al. Phys, Plasmas; 29 042107 (2022). https://doi.org/10.1063/5.0084550

[2] I. Mitchell et al. Rev. Sci. Instrum. 67, 1533 (1996). https://doi.org/10.1063/1.1146884

[3] G. Swadling et al. Rev. Sci. Instrum. 85, 11E502 (2014). https://doi.org/10.1063/1.4890564

[4] L. Suttle et al. Rev. Sci. Instrum. 92, 033542 (2021). https://doi.org/10.1063/5.0041118

[5] K. McGlinchey et al. Phys. Plasmas 25, 122705 (2018). https://doi.org/10.1063/1.5064504

[6] MacFarlane et al. J. Quant. Spectrosc. Radiative Transfer 99, 381 (2006). https://doi.org/10.1016/j.jqsrt.2005.05.031

[7] R. Markwick et al. Phys. Plasmas 29, 102901 (2022). https://doi.org/10.1063/5.0095166

Magnetic reconnection –the abrupt change in magnetic field topology accompanied by the explosive release of heat and kinetic energy –is an important process in astrophysical plasmas. In high-energy-density astrophysical environments, strong radiative cooling can modify the reconnection process, by rapidly removing the magnetic energy dissipated in the current sheet. The MARZ (Magnetically Ablated Reconnection on Z) collaboration investigates radiatively-cooled reconnection in the laboratory on the Z pulsed-power machine, as part of the Z Fundamental Science Program.

We perform 2D and 3D resistive magnetohydrodynamic simulations of reconnection in a pulsed-power-driven dual exploding wire array using the code GORGON. We model the radiation using tabulated emissivity and opacity calculated using the SpK atomic model code, and model the radiation transport with either a probability-of-escape model or diffusive multi-group model.

The simulated wire arrays generate magnetized supersonic (M_s 4−5) and super-Alfvénic (M_A∼1.5) flows which collide to form a current sheet with an initial Lundquist number∼400. The simulations show that radiative cooling leads to faster reconnection rates compared to the non-radiative case, consistent with the colder temperature and the strong radiatively-driven compression of the current sheet. Plasmoids, which originate within the sheet earlier in time, are also quenched by radiative cooling. Finally, there is reduced magnetic flux pile-up outside the sheet, resulting in lower magnetic field and density of the inflows into the layer.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

We present results from the first experimental investigation of pulsed-power-driven magnetic reconnection in a strongly-radiatively cooled regime. The experiment is designed to provide cooling rates much faster than the hydrodynamic transit rate, which is required to observe the radiative collapse of the reconnection layer. A dual inverse wire array load driven by the Z machine (20 MA, 300 ns rise time) generates oppositely-directed highly-collisional, super-Alfvénic plasma flows with anti-parallel magnetic fields. The flows interact in the mid-plane to generate an elongated current sheet, that exhibits strong XUV and X-Ray emission. Inductive probes measure peak magnetic fields of 25-30 T in the inflow to the current sheet, which is roughly 10x larger than in previous pulsed-power-driven reconnection experiments. Visible spectroscopy, which exhibits well-defined Al-II and Al-III emission lines, is used to characterize the inflows to the reconnection layer. Experimental spectra are compared to the output of collisional-radiative models and radiation transport simulations to estimate the density and temperature of the inflows.

A filtered X-ray diode, which collects >1 keV photons from the reconnection layer, exhibits a sharp peak in intensity (50 ns FWHM), indicating radiative collapse of the layer. X-ray spectroscopy of the reconnection layer shows well-defined He-like and Li-like k-shell emission lines, consistent with temperatures > 100 eV. Radiation transport calculations further indicate that the X-ray spectrum corresponds to localized hotspots of enhanced temperature embedded within a relatively colder layer. Time-resolved X-ray images confirm this picture, and show an elongated current sheet with brightly-emitting fast-moving hotspots (up to 35 km/s near the center of the reconnection layer). Resistive MHD simulations in 2D and 3D indicate that hotspots are consistent with plasmoids, which appear as localized regions of enhanced emission.

* SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

Magnetic reconnection is a process whereby antiparallel magnetic field lines undergo a rapid change in topology, releasing significant energy in the process. This occurs in the interaction region between two adjacent exploding wire arrays, driven by strong currents from Z facility at Sandia National Laboratories. Ablated surface plasmas in each array carry opposingly directed magnetic fields, which can undergo a change in magnetic topology when the flows collide and compress each other. At Z’s high currents the reconnection layer can reach high enough densities and temperatures to radiatively cool and ultimately collapse to a very small region.

To investigate the above system using simulation, it is critical to resolve the reconnection layer (of spatial order 100um) while also capturing dynamics of the wire arrays (of order 10cm across). Resolving features with at least six orders of magnitude difference is intractable for uniform grid simulations in all three dimensions. This talk describes a static mesh refinement capability implemented in the 3D extended-MHD code Gorgon, to address such resolution-limited systems.