Recent photon Doppler velocimetry (PDV) measurements on electrically-thick aluminum conductors driven by intense current pulses on the Sandia Mykonos linear transformer driver (LTD) revealed discrepancies between magnetohydrodynamic (MHD) simulations and experimental observations of metal surface compression velocity, compression duration, and melt duration [1], motivating an investigation into whether magnetically insulated transmission line (MITL) plasma introduces unmodeled perturbations that measurably affect downstream load dynamics and electrothermal instability (ETI) growth [2]. The Mykonos MITL Plasma Impact on Load Surface Hydrodynamics and Electrothermal Instability (MyMASHETI) campaign was conducted on the Mykonos LTD to systematically investigate how perturbed power-flow hardware affects ETI growth on the downstream aluminum load surface. A series of aluminum barbell loads of various diameters were fielded, and multiple diagnostics were fielded to study the load surface including ICCD imaging, PDV, and x-pinch backlighting. Shot-by-shot comparison of ICCD images between control and modified cathode button contacts revealed a one-to-one correlation between upstream hardware condition and changes in load surface emissions. Fully refurbished transmission lines with clean, uniform contacts produced more spatially uniform ETI whereas shots with localized perturbed contacts revealed non-uniform ETI. This preliminary result suggests that the energetic electron emission associated with contact conditions in the transmission line introduces a measurable perturbation to the ETI growth during the early period of load expansion from the high pulsed current. This investigation highlights the potential importance of MITL physics for understanding and designing pulsed-power experiments.

Aluminum is a ubiquitous component in dynamic compression, pulsed power, and other high energy density physics studies. Its high-pressure behavior and phase diagram are extensively studied standards in shockwave physics. While theoretical calculations and multiphase equations of state have been benchmarked to velocity measurements of loading and unloading waves, pressure and density under shock, and other mechanical data, experimental temperature data under these conditions have not been reported. We conducted a series of experiments shocking and releasing aluminum 6061 and 1100 samples into lithium fluoride windows. We measured temperature at the sample-window interface under steady compression and subsequent isentropic release. These results allow us to constrain the temperature of the solid Hugoniot and the boundary between the liquid and face-centered cubic solid phases.

Developing predictive capability for high-current-density systems is vital for the design of magnetically driven implosions, high-current-density magnetically insulated transmission lines (MITLs), and other devices. MITLs, essential for cost-effective fast pulsed power, generate plasma that interacts with loads in fascinating ways. Recently, we observed the impact of plasma from a perturbed MITL on the evolution of a mm-diameter aluminum (Al) rod pulsed by the 900-kA, 80-ns rise-time SNL Mykonos linear transformer driver. An azimuthal variation was imposed on the cathode contact in an otherwise axisymmetric MITL. Electromagnetically downstream, high-resolution $(3 \mu ~\mathrm{m})$, time-gated (3 ns) optical images observed azimuthal variation of the load surface brightness, including from electrothermal instability (ETI), that followed the upstream variation. This is consistent with charged particles from intentionally sharp edges at specific azimuthal locations flowing to the load surface, via the $E \mathrm{x} B$ drift, at constant azimuthal angle, locally altering surface heating.

Radiometric studies of opaque materials under shock compression probe a sample-window interface that may be very different than the bulk state. Subtle experimental details, particularly an initial sample-window gap vs an adhesively bonded stack, have tremendous influence on measured temperature. This work experimentally tests gapped vs bonded targets and theoretically investigates observed differences in the measured temperature. The results indicate that perturbations created by a thin adhesive are smaller than those suggested by a previous work, whereas sample-window gaps alter the measured state more significantly than that previously realized. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0277627

Photonic Doppler Velocimetry (PDV) is a fiber-based measurement amenable to a wide range of experimental conditions. Interference between two optical signals-one Doppler shifted and the other not-is the essential principle in these measurements. A confluence of commercial technologies, largely driven by the telecommunication industry, makes PDV particularly convenient at near-infrared wavelengths. This discussion considers how measurement time scales of interest relate to the design, operation, and analysis of a PDV measurement, starting from the steady state through nanosecond resolution. Benefits and outstanding challenges of PDV are summarized, with comparisons to related diagnostics.

The Z machine is a current driver producing up to 30 MA in 100 ns that utilizes a wide range of diagnostics to assess accelerator performance and target behavior conduct experiments that use the Z target as a source of radiation or high pressures. We review the existing suite of diagnostic systems, including their locations and primary configurations. The diagnostics are grouped in the following categories: pulsed power diagnostics, x-ray power and energy, x-ray spectroscopy, x-ray imaging (including backlighting, power flow, and velocimetry), and nuclear detectors (including neutron activation). We will also briefly summarize the primary imaging detectors we use at Z: image plates, x-ray and visible film, microchannel plates, and the ultrafast x-ray imager. The Z shot produces a harsh environment that interferes with diagnostic operation and data retrieval. We term these detrimental processes "threats " of which only partial quantifications and precise sources are known. We summarize the threats and describe techniques utilized in many of the systems to reduce noise and backgrounds.

The evolution of metal carrying intense current is of fundamental importance to high-energy-density physics and applications. However, accurate numerical modeling of electrically pulsed conductors is challenging due to large uncertainties in the equation of state (EOS) and electrical conductivity when metal transforms from liquid to vapor. To benchmark MHD calculations and inform the choice of EOS and conductivity tables, extremely sensitive measurements have been made of the motion of the surface of electrically driven metal rods, using Photonic Doppler velocimetry (PDV). The rods were diamond-turned from 5N aluminum (99.999% pure AI) to diameters of $800-801\ \mu\mathrm{m}$ , with surface roughness (Ra) 17–245 nm. The Sandia Mykonos linear transformer driver pulsed them to 860 kA with 71 ns risetime (10-90%), in vacuum. PDV data were compared with MHD simulations to diagnose the density-temperature phase-space trajectory of the reflective surface, including magnetic compression of the solid, the duration of the solid-liquid phase transition on the surface, and the subsequent motion until plasma formation. Using PDV, radially inward motion of the solid aluminum current-carrying surface was resolved experimentally for the first time. PDV subsequently observed rapid outward acceleration of the surface, in correspondence with MHD simulations showing surface melting. Then PDV observed a decrease in the radial acceleration of the surface, in correspondence with computations showing the surface had finished melting. The difference in time between these latter two events provides the first experimental measurement of the duration of surface melting on a current-driven rod. An important application of the new measurements and improved MHD simulations is more accurate knowledge of the conditions in which high current instabilities, such as electrothermal and magnetically driven instabilities, are observed to develop.

Precise temperature determination is a significant challenge in extreme environments of dynamic compression studies. How can radiance measurements taken in high-pressure shock experiments constrain temperature in a meaningful and physically consistent way? Experiments maintaining sample compression against a transparent window can be tailored to present a uniform measurement area with uncertain spectral emissivity. We compare several methods to analyze radiance collected at multiple wavelengths, applying statistical methods and physical principles to improve temperature inference. With proper radiance collection and analysis, dynamic temperature uncertainties become comparable to thermomechanical ambiguities of the emitting surface.

Optical fiber diagnostics are extensively used in pulsed power experiments, such as the Sandia Z machine. However, radiation produced in a pulsed power environment can significantly affect these measurements. Catastrophic fiber darkening may be mitigated with shielding, but no flexible material can stop all radiation produced by the machine and/or target. Radiation-induced refractive index modulations are particularly challenging for optical interferometry. Several approaches for radiation-tolerant photonic Doppler velocimetry are discussed here.

Complex plasma-material interactions are found in many fusion and high energy density experiments. These can dramatically alter the performance of a fusion device, as contaminated fusion fuel does not achieve as high of a temperature and thus fusion yield, due to radiative losses. Magneto-inertial fusion schemes, such as Sandia's MagLIF concept, have metal components that carry increasingly high current density in the vicinity of the plasma. Electrothermal instability of the metal can mix the metal into the plasma, both directly and by seeding magnetohydrodynamic (MHD) instabilities. Accurate numerical modeling of electrically driven conductors is currently challenging due to uncertainties in the equation of state (EOS) and electrical conductivity, especially during the metal-insulator transition. To supply data for comparison to MHD modeling, photonic Doppler velocimetry (PDV) was used to measure the surface motion of mm-diameter (6061 and 5N Al, 5N Cu, 4N Ni, and Ti) rods driven to 0.8 MA in 100 ns by the Sandia Mykonos generator. The high quality of the data permitted the initial magnetic compression of rods to be measured for the first time. Uncoated pure (5N) Al rods compressed 40 nm radially before expanding. Subsequently, the reflective surface experiences several changes in acceleration during the current rise. Last, taking advantage of PDV's sensitivity to multiple simultaneous velocities, the time dependence of the distribution of velocities in the reflective material is being investigated to compare with computer simulations of electrothermal and MHD instabilities. The experimental measurements are being used to benchmark MHD calculations, and thereby inform the choice of EOS and conductivity tables for modeling.

The outer core of the Earth is composed primarily of liquid iron, and the inner core boundary is governed by the intersection of the melt line and the geotherm. While there are many studies on the thermodynamic equation of state for solid iron, the equation of state of liquid iron is relatively unexplored. We use dynamic compression to diagnose the high‐pressure liquid equation of state of iron by utilizing the shock‐ramp capability at Sandia National Laboratories’ Z‐Machine. This technique enables measurements of material states off the Hugoniot by initially shocking samples and subsequently driving a further, shockless compression. Planetary studies benefit greatly from isentropic, off‐Hugoniot experiments since they can cover pressure‐temperature (P‐T) conditions that are close to adiabatic profiles found in planetary interiors. We used this method to drive iron to P‐T conditions similar to those of the Earth’s outer‐inner core boundary, along an elevated‐temperature isentrope in the liquid from 275 GPa to 400 GPa. We derive the equation of state using a hybrid backward integration – forward Lagrangian technique on particle velocity traces to determine the pressure‐density history of the sample. Our results are in excellent agreement with SESAME 92141, a previously published equation of state table. With our data and previous experimental data on liquid iron we provide new information on the iron melting line and derive new parameters for a Vinet‐based equation of state. The table and our parameterized equation of state are applied to provide an updated means of modeling the pressure, mass, and density of liquid iron cores in exoplanetary interiors.

The very short burn time and small size of burning plasmas created at advanced laser-fusion facilities will require high-spatial-resolution imaging diagnostics with fast time resolution. These instruments will need to function in an environment of extremely large neutron fluxes that will cause conventional diagnostics to fail because of radiation damage and induced background levels. One solution to this challenge is to perform an ultrafast conversion of the x-ray signals into the optical regime before the neutrons are able to reach the detector and then to relay image the signal out of the chamber and into a shielded bunker, protected from the effects of these neutrons. With this goal in mind, the OMEGA laser was used to demonstrate high-temporal-resolution x-ray imaging by using an x-ray snout to image an imploding backlighter capsule onto a semiconductor. The semiconductor was simultaneously probed with the existing velocity interferometry system for any surface reflector (VISAR) diagnostic, which uses an optical streak camera and provided a one-dimensional image of the phase in the semiconductor as a function of time. The phase induced in the semiconductor was linearly proportional to the x-ray emission from the backlighter capsule. This approach would then allow a sacrificial semiconductor to be attached at the end of an optical train with the VISAR and optical streak camera placed in a shielded bunker to operate in a high neutron environment and obtain time-dependent one-dimensional x-ray images or time-dependent x-ray spectra from a burning plasma.

Photonic Doppler Velocimetry (PDV) was used to investigate the exploding surface of coated metal driven by lineal current density increasing at 3×1015 A/m/s. Aluminum-6061 rods were coated with three thicknesses (5, 17, and 41 μm) of Parylene-N and driven to 850 kA in approximately 100 ns, with the metal thicker than the skin depth. According to PDV, the metal surface melts at 85±13 T and expands with a constant acceleration. Then, at a surface magnetic field of 140–180 T, the metal acceleration begins to increase in time. Later, plasma forms on, and is correlated with PDV signal loss from, uncoated aluminum rods, 5-μm-coated rods, and 17-μm-coated rods, while no evidence suggests that plasma forms on 41-μm-coated metal.

Temperature is a difficult thermodynamic variable to measure in dynamic compression experiments. Optical pyrometry is a general-purpose technique for measuring temperature from a radiant surface, but that surface is often the interface between distinct materials with temperatures that vary spatially along the loading direction. This leads to a fundamental problem: how does the measured interface temperature relate to this temperature profile along the compression axis? Numerical analysis of loading history and thermal diffusion at these interfaces shows that seemingly subtle changes in experiment geometry can lead to very different temperature profiles. We compare these results to laboratory temperature measurements of shock-compressed tin.

Photonic Doppler Velocimetry (PDV) is a fiber-based diagnostic for the extreme conditions created by high-speed impact, explosive detonation, electrical pulsed power, and intense laser ablation. PDV is a conceptually simple application of the optical Doppler effect, but measurements above 1 km/s only became practical at the beginning of the twenty-first century. This review discusses the evolution of PDV, its operational details, practical analysis, and outstanding challenges.