arXiv subjects

D. A. Hammer

Publications and source records attributed to D. A. Hammer.

9 recordsLinked to original sources

arXiv2026

The Hall Term and Anomalous Resistivity Effects in Neon Gas-Puff Z-Pinches

In this paper, we compare experimental and numerical simulation results to benchmark the PERSEUS code against gas-puff $Z$-pinch implosions on COBRA. We then use the code to investigate the structure of the plasma sheath. To this end, we study the morphology of the implosion, focusing on non-magnetohydrodynamical (MHD) effects such as electron drifts governed by the Hall term within the growing magneto-Rayleigh-Taylor instability (MRTI). The spatial wavelength of MRTI is better reproduced when both the Hall term and an anomalous resistivity driven by the electron drift are included. Additionally, cathode-anode gap polarity effects are more accurately captured when the Hall term is turned on. The plasma sheath structure, which includes both the accelerating piston driven by the magnetic pressure and the shockwave ahead of it, matches interferometric measurements in width only when a current-driven anomalous resistivity model is used. This anomalous resistivity is assumed to be driven by the lower-hybrid-drift instability, which generates small-scale turbulence with typical wavelengths < 30μm.

physics.plasm-ph↗

arXiv2025

Resistive diffusion and radiative cooling effects in magnetized oblique shocks

Magnetized oblique shocks are of interest in various plasmas, including in astrophysical systems, magneto-inertial confinement fusion experiments, and in 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.

physics.plasm-ph↗

arXiv2025

Wavenumber Calibration for an Imaging Refractometer

An imaging refractrometer 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 refractrometer can be directly calibrated in terms of beam deflection angles using ray transfer matrix analysis. This paper describes a novel way to calibrate the refractrometer 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.

physics.plasm-ph↗

arXiv2024

Randomization of a Laser Wavefront by the Turbulent Gas-Puff Z-Pinch Plasma Column

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 COBRA current. Utilizing an imaging refractometer, we show 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 electron density gradients; hence, the measured phase randomization of the individual plane waves comprising the laser field implies 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 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.

physics.plasm-ph↗

arXiv2023

Plasma flows during the ablation stage of an over-massed pulsed-power-driven exploding planar wire array

We characterize the plasma flows generated during the ablation stage of an over-massed exploding planar wire array, fielded on the COBRA pulsed-power facility (1 MA peak current, 250 ns rise time). The planar wire array is designed to provide a driving magnetic field (80-100 T) and current per wire distribution (about 60 kA), similar to that in a 10 MA cylindrical exploding wire array fielded on the Z machine. Over-massing the arrays enables continuous plasma ablation over the duration of the experiment. The requirement to over-mass on the Z machine necessitates wires with diameters of 75-100 $μ$m, which are thicker than wires usually fielded on wire array experiments. To test ablation with thicker wires, we perform a parametric study by varying the initial wire diameter between 33-100 $μ$m. The largest wire diameter (100 $μ$m) array exhibits early closure of the AK gap, while the gap remains open during the duration of the experiment for wire diameters between 33-75 $μ$m. Laser plasma interferometry and time-gated XUV imaging are used to probe the plasma flows ablating from the wires. The plasma flows from the wires converge to generate a pinch, which appears as a fast-moving ($V \approx {100}$ kms$^{-1}$) column of increased plasma density ($\bar{n}_e \approx 2 \times 10^{18}$ cm$^{-3}$) and strong XUV emission. Finally, we compare the results with three-dimensional resistive-magnetohydrodynamic (MHD) simulations performed using the code GORGON, the results of which reproduce the dynamics of the experiment reasonably well.

physics.plasm-ph↗

arXiv2018

Mega-ampère to mega-gauss: the generation of intense magnetic fields using fast pulsed-power drivers

Intense magnetic fields modify the properties of extremely dense matter via complex processes that call for precise measurements in very harsh conditions. This endeavor becomes even more challenging because the generation of mega-gauss fields in a laboratory is far from trivial. This paper presents a unique and compact approach to generate fields above 2 mega-gauss in less than 150 ns, inside a volume close to half a cubic centimeter. Magnetic insulation, keeping plasma ablation close to the wire surface, and mechanical inertia, limiting coil motion throughout the current discharge, enable the generation of intense magnetic fields where the shape of the conductor controls the field topology with exquisite precision and versatility, limiting the need for mapping exactly magnetic fields.

physics.plasm-ph↗

arXiv2017

A compact linear accelerator based on a scalable microelectromechanical-system RF-structure

A new approach for a compact radio-frequency (RF) accelerator structure is presented. The new accelerator architecture is based on the Multiple Electrostatic Quadrupole Array Linear Accelerator (MEQALAC) structure that was first developed in the 1980s. The MEQALAC utilized RF resonators producing the accelerating fields and providing for higher beam currents through parallel beamlets focused using arrays of electrostatic quadrupoles (ESQs). While the early work obtained ESQs with lateral dimensions on the order of a few centimeters, using printed circuits board (PCB), we reduce the characteristic dimension to the millimeter regime, while massively scaling up the potential number of parallel beamlets. Using Microelectromechanical systems scalable fabrication approaches, we are working on further reducing the characteristic dimension to the sub-millimeter regime. The technology is based on RF-acceleration components and ESQs implemented in PCB or silicon wafers where each beamlet passes through beam apertures in the wafer. The complete accelerator is then assembled by stacking these wafers. This approach has the potential for fast and inexpensive batch fabrication of the components and flexibility in system design for application specific beam energies and currents. For prototyping the accelerator architecture, the components have been fabricated using PCB. In this paper, we present proof of concept results of the principal components using PCB: RF acceleration and ESQ focusing. Ongoing developments on implementing components in silicon and scaling of the accelerator technology to high currents and beam energies are discussed.

physics.acc-ph↗

arXiv2004

Effect of Bilayer Thickness on Membrane Bending Rigidity

The bending rigidity $k_c$ of bilayer vesicles self-assembled from amphiphilic diblock copolymers has been measured using single and dual-micropipet techniques. These copolymers are nearly a factor of 5 greater in hydrophobic membrane thickness $d$ than their lipid counterparts, and an order of magnitude larger in molecular weight $\bar{M}_n$. The macromolecular structure of these amphiphiles lends insight into and extends relationships for traditional surfactant behavior. We find the scaling of $k_c$ with thickness to be nearly quadratic, in agreement with existing theories for bilayer membranes. The results here are key to understanding and designing soft interfaces such as biomembrane mimetics.

cond-mat.soft↗

arXiv1998

Active Stark Atomic Spectroscopy

Active Stark Atomic Spectroscopy (ASAS) method can be used to determine a high electric field in the diode of an ion or electron accelerator as a function of position and time, including the positions of anode and cathode plasma emission surfaces (in order to obtain the effective accelerating gap). As possible probe beams, we suggest the use of lithium and sodium atoms. The diagnostic provides a means to measure diode quantities spectroscopically with excellent spatial resolution.

physics.atom-ph↗