We present results from the Rotating Plasma Experiment (RPX) [1,2], a novel laboratory platform developed on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise-time), designed to probe physics relevant to astrophysical accretion disks and jets. RPX drives differentially rotating high-energy-density plasma flows using the slightly off-radial inward-convergence of 8 magnetized plasma jets [3], from an ablating aluminium wire array Z pinch.

The data show that rotating plasmas have a hollow density structure and are radially confined by the ram pressure of the ablation flows. A combination of axial thermal and magnetic pressure launches an axial, highly collimated, supersonic jet with a velocity ~ 100 km/s (M > 5). The axial jet also rotates, transporting angular momentum, as it remains collimated by a hot (T_i ~ 250 eV) surrounding plasma halo. The flow velocity stratification is such that angular frequency decreases with radius, as the opposite happens to specific angular momentum. The calculated squared epicyclic frequency (Rayleigh determinant) of the flow is estimated to be k² ~ r^-2.8 > 0. This implies that the flows at RPX are quasi-Keplerian and share stability properties of gravitationally-driven accretion disks in astrophysics, opening a new frontier to laboratory modelling of these objects.

References

[1] V. Valenzuela-Villaseca, et. al., Accepted at Phys. Rev. Lett. (2023). Preprint: arXiv 2201.10339v1

[2] M. Bocchi, et al., ApJ 767, 84 (2013)

[3] D. D. Ryutov, Astrophys. Space Sci. 336, 21 (2011)

Using our new tilted wire array platform, we present results from our experiments on the MAIZE facility (~500 kA peak current, 150 ns rise time) to study pulsed-power driven magnetic reconnection with an embedded guide field. Taking an existing dual exploding wire array load, we rotate the two arrays in opposite directions, such that when the oppositely directed plasma flows collide there is both an anti-parallel reconnecting component of the magnetic field, and an out-of-plane “guide” field component. Between these arrays, a current sheet is formed from the interaction of oppositely-directed magnetic fields (~2 T) advected by carbon plasma flows moving at ~50-100 km/s. We study three tilt angles: 0, 22.5, and 45 degrees, with corresponding guide field to reconnecting field ratios of 0, 0.4, and 1. Line-integrated electron density measurements of the reconnection layer in these configurations were made using a simultaneous end-on and side-on Mach-Zehnder interferometry system (1064 nm, 2 ns, 40 uJ), which measured peak line-integrated densities of ~6e17 cm^-2 inside the reconnection layer (in the absence of a guide field). Measurements were taken at different times after current start on different shots to study the evolution of the layer. An optical fast-framing camera and a four-frame XUV MCP detector observed the plasma dynamics. Oppositely-wound B-dot probe pairs were fielded at different radial distances from the wire arrays, to measure both the advected magnetic field (~2 T) and the ablation flow velocity (~50-100 km/s) via a time-of-flight technique.

This work is supported by the NSF and the DOE NNSA through grant PHY-2108050. MAIZE facility support was provided by the NNSA Stewardship Sciences Academic Programs under DOE Cooperative Agreement DE-NA0003764.

The Pulsed Power Plasmas (P3) group at UC San Diego develops experimental and diagnostic platforms for a range of HEDP and related plasma studies including inertial fusion, laboratory astrophysics and basic plasma physics. These studies are supported by simulation work carried out in collaboration with academic, national laboratory and private partners. Pulsed power drivers focus on 1-us timescale devices, where the relatively slow varying plasma parameter can be studied in detail over large (mm³) volumes. Shocks generated by supersonic and magneto-supersonic flows can be designed to be stationary in the laboratory frame, and jet and flow systems are driven for timescale many times the hydrodynamic scale.

We will present data from a new Faraday rotation system examining plasmas driven by our 1us, 200kA device, Bertha. The laser system operates at 1064ns with a 1J, 6ns pulse allowing discrimination of the Faraday signal against the load emission using high extinction polarizers. Data are compared to calculations from both short circuit and exploding wire loads. We will also present new data on the development of our 1.3us, 750kA driver, Rama. This uses a novel solid-state triggering system developed at Imperial College London, allowing all 6 of the device’s capacitors to be triggered independently. Pulse-shaping capabilities may be of interest for a variety of plasma experiments, particularly in plasma jets produced from an unsteady, episodic source.

A photoionized plasma experimental platform has been established for Zebra in which a supersonic neon gas jet is driven and backlit by the broadband x-ray flux from the collapse of a wire-array implosion and characterized by a suite of optical and x-ray diagnostics. A mm-scale cylindrical volume of photoionized plasma can be produced during the 20-30 ns time interval of the x-ray flux emission, in which the gas jet is effectively “frozen” in space. Supersonic gas flows are collimated and do not require sealing windows or tamping layers, which removes unnecessary x-ray flux attenuation and enables close proximity to the pinch ~6 mm, thus optimizing plasma x-ray heating and ionization. The plethora of diagnostic measurements are used to inform, constrain, and test theory models. The time-history of the x-ray flux is measured with PCD’s while its spectral distribution is determined over an order-of-magnitude wavelength range with a soft x-ray spectrometer equipped with a spherical diffraction grating. Atomic density radial spatial profiles of the neutral gas jet are extracted from Mach-Zehnder interferometry. The radiation drive characteristics and atomic density of the neutral gas jet inform simulations of the experiment. Simulation results are then benchmarked with electron density maps extracted from dual color air-wedge shearing laser interferometry. Analysis of the transmission x-ray spectroscopy data provides the charged state distribution and electron temperature of the plasma which further test and constrain modeling results. We will discuss the experimental platform and measurements, the modeling and data analysis, and the astrophysical impact¹.

¹ R. C. Mancini et al Phys. Rev. E 101, 051201 (2020)

This work is supported by DOE NNSA HEDLP Grant DE-NA0004038.