David A. Hammer

Vacuum gap breakdown uniformity in coaxial systems at high voltages and currents remains a critical topic for pulsed power applications. This is especially relevant as new drivers for inertial fusion energy are developed, which will generate significantly higher electrical power than present machines. We present experimental investigations performed on the COBRA generator at 1 MA, examining two methods to force uniform breakdown and current density distribution in a coaxial system: cathode surface coatings and the application of external axial magnetic fields. A series of coatings, gold, copper, and graphite (Aerodag-G), were applied to aluminum cathode heads, each introducing distinct work functions and surface morphologies. It was found that copper and gold coatings delayed breakdown initiation relative to bare aluminum, while graphite coatings accelerated plasma formation and produced the first observed fully filled coaxial gap, correlating to a symmetric current density profile. Surface scanning electron microscope imaging and plasma behavior indicated that uniformly spaced, conductive surface microstructures enhanced material-assisted breakdown and rapid gap fill. Separately, external magnetic fields of up to 1.9 T were applied during breakdown, demonstrating that increasing magnetic field strength enhanced azimuthal plasma spread. Optical diagnostics revealed that plasma expansion velocities increased with magnetic field strength and that an axial field induced retrograde motion of breakdown sites, consistent with J x B-driven plasma dynamics. These combined approaches suggest that surface engineering and an applied magnetic field can control breakdown symmetry and current density distribution in coaxial vacuum gaps, offering direct relevance to designs such as MagLIF on Sandia's Z-machine.

Pulsed-power generators concentrate electrical energy in time and space, producing plasmas with extreme properties. During the final stage of current focusing, a current-carrying column can be compressed by its own magnetic field through the pinch effect. Among various pinch configurations, the x-pinch—formed by crossing two or more wires in an x-shape—has been extensively studied as a micrometer-scale, sub-nanosecond x-ray source. Here, we explore a novel aspect of x-pinches, highlighting their potential as a point-like source of MeV protons. The x-pinch experiments were performed on the XP generator, delivering a peak current of 500 kA with a rise time of 70 ns. A hybrid x-pinch configuration, comprising two solid conical electrodes connected by a single polymer fiber, was employed. A critical requirement for reproducible acceleration of protons to MeV energies was the use of low linear-mass fibers. Using nylon and polyethylene fibers with diameters of 25–40 μm, protons with energies of up to 3.9 MeV were observed in the radial direction, i.e., perpendicular to the fiber axis. Additional source parameters, particularly the effective source size and the anisotropy of the radial emission, were characterized using various CR-39-based ion detectors. Finally, the applicability of the x-pinch as a point-like proton source was demonstrated by x-pinch-driven proton radiography of an exploding wire.

High-energy-density physics research has focused on examining new frontiers of plasma behavior in the extreme conditions found in fusion plasmas and astrophysical systems, amongst others. This has increasingly necessitated the use of large-scale experimental facilities to achieve such conditions, which are typically located at national laboratories. During a highly productive couple of decades, significant advances have been made in inertial fusion performance, including achieving ignition at NIF, powerful x-ray sources for astrophysics studies, and contributions to basic science issues. A great deal of this progress has been based on physics scaling studies, novel experimental and numerical research, and diagnostic development carried out at a university scale. In this article, we will focus on pulsed power high-energy-density research and highlight how university-scale programs have been a fundamental part of progress. Contributions have comprised those from both large collaboratives, such as the NNSA Centers of Excellence, and innovative single PI programs, and training in both physics and pulsed power engineering remains a focus.

We present an analytical 1D axisymmetric model describing the evolution of the dynamic Z pinch. This model is capable of predicting the trajectories of the imploding sheath's magnetic piston and preceding shock front, along with the velocity, pressure, density, and magnetic field profiles, for any time-dependent current, spatially varying initial density profile, and weak initial axial field. The implosion is divided into stages, with each stage described by a set of coupled ordinary differential equations derived from the ideal MHD equations. Comparisons with experimental data from the COBRA pulsed-power facility are quite promising and imply this model could prove useful in designing and analyzing future pulsed-power experiments.

Ion acceleration in Z -pinch plasmas is traditionally linked to the axial direction of current. Here, we report the radial emission of MeV protons from a low-mass, hydrogen-containing X pinch. Proton energies up to 3 MeV are observed in a hybrid X pinch with a 30 μ m polyethylene fiber at a 400 kA peak current. The low-mass load, correlation of protons with hard x-rays, and timing of hard x-ray emission point to proton acceleration driven by current disruption following fiber disintegration. A new insight into the acceleration mechanism arises from the use of an interelectrode gap that is too short for magnetic fields to bend axially accelerated protons by π / 2 radians; thus, the observed radial emission implies a substantial radial acceleration component. The radial emission also enables X -pinch-driven proton radiography, the exceptional potential of which is demonstrated with exploding wires.

Magnetized oblique shocks are of interest in various plasmas, including astrophysical systems, magneto-inertial confinement fusion experiments, and aerospace applications. Through experiments on the COBRA pulsed power facility (Cornell University, 1 MA peak current, 100 ns rise time), we investigate oblique shock formation in a system with a magnetic field and where both radiative cooling and resistive diffusion are important. Compared to previous pulsed power experiments, which have investigated quasi-parallel oblique shocks, here, we consider perpendicular-type shocks, which can support magnetic field compression. In our experiments, supersonic, super-Alfvénic, collisional plasma flows, generated using an aluminum exploding wire array, are deflected by angled obstacles to generate oblique shocks. The shocks are imaged using laser shadowgraphy and Mach–Zehnder interferometry, while optical Thomson scattering provides measurements of the flow velocity and temperature. The shocks exhibit shallower shock angles and higher density compression, when compared to canonical Rankine–Hugoniot predictions. These results are best described by a model that includes both resistive diffusion and radiative cooling, consistent with the values of the cooling parameter and the resistive diffusion length in the experiment.

An imaging refractometer can be used to describe the properties of a high-energy density plasma by analyzing the transverse intensity distribution of a laser beam that has passed through the plasma. The output of the refractometer can be directly calibrated in terms of beam deflection angles using ray transfer matrix analysis. This paper describes a novel way to calibrate the refractometer output in terms of the spatial wavenumbers of the transverse intensity distribution of the laser beam. This is accomplished by replacing the plasma with a gridded structure that modulates the transverse intensity of the beam, producing an intensity distribution with a known Fourier Transform. This calibration technique will generate a one-to-one mapping of deflection angle to wavenumber and will enable measurement of the size of Fourier space available to the system. The spectrum of wavenumbers generated when the laser beam passes through a high-energy density plasma may contain information about the types of density fluctuation that are present in the plasma.

In this paper, we present the first direct experimental evidence supported by numerical modeling of a turbulent plasma column formed during a gas-puff $z$-pinch implosion generated by the cobra current. Utilizing an imaging refractometer, we showed a significant decrease in spatial autocorrelation of the laser field and the appearance of a laser speckle pattern shortly before stagnation. The intensity distribution of the speckles measured during different shot campaigns while employing long and short cobra pulses follows the speckle statistics satisfactorily. The imaging refractometer signal is proportional to the integral over the electron density gradients; hence, the measured phase randomization of the individual plane waves composing the laser field implies a random density distribution. To validate this, the beam propagation method code simulates the laser beam propagation through different artificial density distributions with various average fluctuation scales and generates synthetic imaging refractometer data. The results reproduce similar trends in the experimental data, such as the increasing vertical width for the decreasing average spatial scale of the fluctuations and the decreasing spatial correlation length of the laser field. Therefore, during the gas-puff $z$-pinch implosion process, it is likely that the plasma flow is almost always turbulent with the average spatial scale of the turbulent density fluctuations decreasing towards stagnation.

High temporal resolution x-ray streak camera studies of micropinch formation in Cu hybrid x pinches reveal key plasma conditions. Analysis of Ne-like Cu lines indicate an average electron temperature of about 200 eV and 4.5×10^{28}m^{-3} electron density. The spectra suggest that the electron temperature jumps to about 1 keV, inferred from the continuum and the postcontinuum line emission that includes Li-like Cu lines. There is no sign of a rapid temperature change or a substantial surge in radiation emission during the 200 ps precontinuum x-ray burst, suggesting that the radiative collapse process does not play a major role in micropinch formation. Two-dimensional extended Magnetohydrodynamic (MHD) simulations, coupled to a collisional-radiative spectral analysis code, suggest the significance of the rapid radial implosion of high-temperature, low-density plasma, the axial outflow, and the dynamic plasma pressure in micropinch formation.