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A spatially resolved air-wedge shearing interferometer and shadowgraph diagnostic provides measurements of electron density with a resolution of [Formula: see text]40 [Formula: see text]m. A [Formula: see text]100-ns-long, monoenergetic electron bunch at 19.8 MeV and a current of 1.4 kA ([Formula: see text] [Formula: see text]) heats 100-[Formula: see text]m-thick aluminum (Al) foils in a 1-mm-spot to [Formula: see text] eV. A 5-ns-long, [Formula: see text]60 mJ, frequency doubled Nd:YAG laser probes the dense Al plasma. Electron densities up to [Formula: see text] are resolved; the maximum resolvable density is limited by opacity, transmission, and spatial fringe density achievable with the detector. This diagnostic provides measurements of the total phase shift, transmission, and electron density. Several measurements at different time slices provide the ability to determine the velocity of the leading edge of the shadowgraph and compare it to the motion of different density shells. These measurements are also compared to radiation hydrodynamics simulations. A rough quantitative agreement is shown between the hydro simulations and the measurements; there are differences in the exact density distributions.

The objectives of this tutorial are as follows: 1) to help students and researchers develop a basic understanding of how pulsed-power systems are used to create high-energy-density (HED) matter; 2) to develop a basic understanding of a new, compact, and efficient pulsed-power technology called linear transformer drivers (LTDs); 3) to understand why LTDs are an attractive technology for driving HED physics (HEDP) experiments; 4) to contrast LTDs with the more traditional Marx-generator/pulse-forming-line approach to driving HEDP experiments; and 5) to briefly review the history of LTD technology as well as some of the LTD-driven HEDP research presently underway at universities and research laboratories across the globe. This invited tutorial is part of the Mini-Course on Charged Particle Beams and High-Powered Pulsed Sources, held in conjunction with the 44th International Conference on Plasma Science in May of 2017.

This work discusses and compares two proposed practical approaches for realizing graphene-based reconfigurable terahertz metamaterials, namely: graphene-only plasmonic structures, and graphene/metal hybrid structures. From rigorous theoretical analysis, full-wave electromagnetic numerical simulations, as well as supporting experiments, several reconfigurable structures are analyzed and compared in terms of their: (i) Quality-factor, (ii) Extinction-ratio, (iii) Unit-cell dimensions, and (iv) Resonance-frequency tunability-range. From this analysis it is observed that at terahertz frequencies, although typically possessing larger unit-cell dimensions and being limited by a restricted resonance-frequency tunability-range, reconfigurable metamaterials based on graphene/metal hybrid structures can provide much larger quality-factors, extinction levels, and, when reconfigured, smaller extinction-level degradation than graphene-only plasmonic structures. As a result, when analyzed in the context of reconfigurable terahertz metamaterials, graphene might result attractive as a reconfigurable media providing tunability to otherwise passive metallic structures rather than as a reconfigurable plasmonic material per-se.

X-pinch radiography is a simple but effective technique for diagnosing imploding cylindrical plasmas [1]. Two or more crossed wires are placed in the return current path, which causes these wires to “pinch” and emit a burst of x-rays. These x-rays create a radiographic image that can provide a view into the structure of the imploding plasma column. The work presented here discusses the hardware upgrades that were necessary to implement this diagnostic on the MAIZE facility, a 1-MA, 100-ns rise time pulsed power machine at the University of Michigan. We will also present preliminary results from using this diagnostic during imploding cylindrical foil experiments, where the magnetic field uniformity at the surface of the foil has been varied from one experiment to the next.

Warm dense matter (WDM) is typically produce for experimental studies by high power lasers or intense heavy ions beams. Both techniques produce WDM that is confined only by material inertia. However, the study of bulk material properties (i.e. viscosity) benefits from experiments conducted on longer time scales. Pulsed-power drivers use magnetic fields to compress matter into WDM regimes. The magnetic field also provides the confinement time necessary to relax into this state at the mesoscale. Predictions made by numerical simulations (PERSEUS) have shown that a dielectric layer reduces initial instabilities in cylindrical samples.1 A mega-ampere pulsed power generator can confine WDM up to 10 Mbars. This research studies the impact of the dielectric layer on initial instabilities using MAIZE, a pulsed power generator located at the University of Michigan, with 1 MA peak current and 100 ns rise time into an impedance-matched load. We investigate the impact of an insulating polyurethane layer between coated and uncoated Al (1 mm OD) rods to determine how well the insulating layer dampens early-time expansion. A 12 frame laser backlighter is used to capture the expansion of the Al rod at 20 ns frame intervals. Peak currents of 300–400 kA were observed. Evaluation of the rod's expansion between uncoated and coated is presented.

A platform for characterizing the equation-of-state of the warm dense matter (WDM) regime is being developed on an intense, relativistic electron accelerator1,2. DARHT Axis-I generates a 100-ns-long electron pulse with a beam current of 1.7 kA and energy of 19.8 MeV that deposits energy into a thin metal foil heating it to a warm dense plasma. The collisional ionization of the target by the electron beam produces an anisotropic angular distribution of K- and L-shell radiation, in addition to a continuum of both scattered electrons and Bremsstrahlung up to the beam energy of 19.8 MeV. A bolometer3 and an array of diamond photo-conducting detectors will be calibrated on a 1-MA linear transformer driver at the University of Michigan4. These diagnostics will then be fielded on the Axis-I electron linac to characterize the background Bremsstrahlung and scattered electrons. Additionally, a gated X-ray imager is under development to acquire temporal and spatial measurements of X-rays. The goal of these diagnostics is to provide temperature and density measurements of the warm dense plasma for the first time with this heating technique.

Cylindrical liner implosions and ablations are susceptible to the magneto Rayleigh-Taylor (MRT) instability and general magnetohydrodynamic (MHD) instabilities, such as the $m=0$ “sausage” and the $m=1$ “kink” instabilities, where $m$ is the azimuthal mode number. Previous work has experimentally shown the effects of long pulse axial magnetic fields on these instabilities in both cylindrical liner ablations and implosions1. Simulations have also shown the effects of a time dependent helical magnetic field on the MRT instability using a dynamic screw pinch2, We have modeled and fabricated a helical return current path to set up such a field configuration (a predicted peak axial field of $B_{z}=2\mathrm{T}$ for a peak current of $I_{max}=600$ kA). This poster will present simulation results on the expected magnetic field profile as well as experimental measurements of the magnetic field values achieved.

The quality of warm dense matter samples created by magnetic compression can be largely affected by material ablation. When the ablated material carries currents, local instabilities can grow, which can lead to nonuniformities in the final magnetic pressure. Extending the previous work by Peterson et al. [Phys. Rev. Lett. 112, 135002 (2014)], Awe et al. [Phys. Rev. Lett. 116, 065001 (2016)], and Hutchison et al. [Phys. Rev. E 97, 053208 (2018)], the experiments reported here demonstrate that the expansion of the ablated material can be significantly reduced by using a simple aerosol spray technique. Coating the current-carrying surfaces with a 30–60-μm layer of polyurethane reduced the expansion of the ablated material by a factor of 2 and eliminated material ejections from sharp corners. This technique, tested at the Michigan Accelerator for Inductive Z-Pinch Experiments pulsed power facility at the University of Michigan with currents up to 400 kA, could allow the production of homogeneous warm dense matter samples on pulsed-power drivers. Because of the simplicity of this method, this work brings forth an important contribution to pulsed-power-driven experiments designed to study nuclear fusion, material properties, and radiation science.

We discuss recent work on graphene-based reconfigurable terahertz metamaterials. In particular two approaches towards constructing reconfigurable metamaterials are analyzed and compared, namely: (i) graphene only plasmonic structures, and (ii) graphene-metal hybrid metamaterials. Whereas in the first type of structures graphene has two simultaneous roles: (a) as a plasmonic medium, therefore defining the terahertz structural response of the metamaterial, and (b) as a reconfigurable medium, thus inducing changes in the metamaterial terahertz response, in the second type of structures the terahertz response is set primarily by the metallic pattern and graphene has just the role of constituting a reconfigurable medium. It is observed that whereas relaxation time is the most important parameter affecting the quality of the terahertz response in plasmonic metamaterials, for the analyzed graphene-metal hybrid metamaterial geometries, the conductivity swing in graphene is the main parameter affecting the terahertz response of the device.