arXiv subjects

J. Griff-McMahon

Publications and source records attributed to J. Griff-McMahon.

11 recordsLinked to original sources

arXiv2026

Laboratory measurements of energy partitioning and anomalous electron heating in magnetized, perpendicular collisionless shocks

We present laboratory results on energy partitioning from supercritical, magnetized collisionless shock experiments ($\rm{M_A} \sim 8$, $\rm{M_{ms}}\sim 4$). We report the first observation of fully-developed laboratory shocks that evolve for more than seven upstream ion gyration periods and have a downstream region that extends more than four shocked ion gyroperiods. Thomson scattering measurements are used to measure electron and ion temperatures, plasma density, and flow speeds. We directly measure a compression ratio of $3.6\pm0.3$, consistent with shock jump conditions. A foot ahead of the shock exhibits super-adiabatic electron and ion heating. The downstream electron temperature has an $\approx 30\%$ excess above adiabatic and collisional electron-ion heating, implying significant collisionless anomalous electron heating. We find a downstream electron-ion temperature ratio $T_e^{(d)}/T_i^{(d)} = 0.8 \pm 0.3$, consistent with spacecraft observations but outside the range of predictions from theory and numerical simulations.

physics.plasm-ph↗

arXiv2026

Expansion-Driven Self-Magnetization of High-Energy-Density Plasmas

Understanding plasma self-magnetization is one of the fundamental challenges in both laboratory and astrophysical plasmas. Self-magnetization can modify the plasma transport properties, altering the dynamical evolution of plasmas. Multiple high-energy-density (HED) experiments have observed the formation of ion-scale magnetic filaments of megagauss strength, though their origin remains debated. Here, we conduct 2D collisional particle-in-cell (PIC) simulations with a laser ray-tracing module for a fully self-consistent simulation of the plasma ablation, expansion, and magnetization. The simulations use a planar geometry, effectively suppressing the Biermann magnetic fields, to focus on anisotropy-driven instabilities. The laser intensity is varied between $10^{13}$ and $10^{14}$ W/$\rm cm^2$, which is relevant to HED and inertial fusion experiments where collisions must be considered. We find that above a critical intensity, the plasma rapidly self-magnetizes via an expansion-driven Weibel process, producing plasma beta of 100 ($β= 8πk_B n_eT_e/B^2$) and Hall parameter $ω_{\rm ce}τ_{e}>1$ within the first few hundred picoseconds. The magnetic field is sufficiently strong to modify plasma heat transport, and simulations with artificially suppressed magnetic field show noticeably different temperature profiles.

physics.plasm-ph↗

arXiv2026

Laser ion acceleration from concave targets by subpicosecond pulses

Laser-driven proton acceleration provides a powerful route for generating ultrashort, high-charge proton beams. Many applications, including secondary neutron sources and inertial fusion, benefit from tight proton beam focusing. Concave targets offer a robust solution, yet the scaling of proton focusing with laser and target parameters remains poorly understood. Here, we present a numerical study of laser-driven proton acceleration and focusing from hemispherical targets using the fully kinetic, relativistic Particle-In-Cell code EPOCH. We focus on the sub-picosecond laser-pulse regime (duration $\lesssim 10^2$ fs), centering on the laser parameters of our recent experiment at the CSU ALEPH laser facility. We investigate the proton acceleration mechanisms, characterize proton focusing, and assess how focal spot parameters scale with laser and target parameters. We identify Target Normal Sheath Acceleration as the dominant mechanism, supplemented by a secondary post-acceleration stage near the geometrical center of the hemisphere. We demonstrate that both the proton focal spot size and focal plane position scale approximately linearly with the hemisphere radius, with the focal plane consistently located downstream of the geometrical center. The opening angle of the concave target mainly affects the proton beam waist. Energy-dependent proton focusing is interpreted as a consequence of the evolving curvature of the accelerating structure, which departs from the target curvature. Evidence for self-similar proton focusing is found in the regime of nearly uniform target irradiation.

physics.plasm-ph↗

arXiv2026

Evolution of laser-driven magnetic fields from proton tomography

Self-generated magnetic fields are commonly produced in high-power laser-plasma interactions. These fields can inhibit plasma heat-flow which makes them important in inertial fusion and controlled laboratory astrophysics experiments. In this work, we characterize the time evolution of self-generated magnetic fields using multi-view proton tomography at two timings. Tomographic reconstructions of the magnetic field show a clear transition from fields located close to the target at early time to more extended coronal fields at later time. The tomographic inversion and mesh radiography also enable a direct measurement of the magnetic-flux evolution. Comparisons with extended-MHD simulations show only moderate agreement in field structure, but good agreement in magnetic flux. This suggests that the field generation model is largely correct under these conditions, while the magnetic transport model requires additional development to reproduce the observed field structure.

physics.plasm-ph↗

arXiv2024

Particle-in-cell simulations of expanding high energy density plasmas with laser ray tracing

The design and analysis of high energy density (HED) laser experiments typically rely on radiation hydrodynamics simulations. However, some laser-plasma interaction regimes are not collisional and cannot be adequately modeled with hydrodynamics. For example, strongly driven magnetic reconnection and magnetized collisionless shock experiments possess extended hydrodynamic or even kinetic properties, necessitating first-principles kinetic simulations. In this paper, we present the benchmarking and first results obtained with a laser-ray-tracing and inverse Bremsstrahlung absorption module implemented in the particle-in-cell code PSC. The simulation results are compared to radiation hydrodynamic simulations using the FLASH code as well as analytical estimates. We successfully benchmark the energy deposition model and overall hydrodynamic evolution of the systems. We also consider possible kinetic effects that may be expected from laser-target ablation in the HED regime, including non-local transport and two-temperature effects.

physics.plasm-ph↗

arXiv2024

Proton Radiography Inversions with Source Extraction and Comparison to Mesh Methods

Proton radiography is a central diagnostic technique for measuring electromagnetic (EM) fields in high-energy-density, laser-produced plasmas. In this technique, protons traverse the plasma where they accumulate small EM deflections which lead to variations in the proton fluence pattern on a detector. Path-integrated EM fields can then be extracted from the fluence image through an inversion process. In this work, experiments of laser-driven foils were conducted on the OMEGA laser and magnetic field reconstructions were performed using both "fluence-based" techniques and high-fidelity "mesh-based" methods. We implement nonzero boundary conditions into the inversion and show their importance by comparing against mesh measurements. Good agreement between the methods is found only when nonzero boundary conditions are used. We also introduce an approach to determine the unperturbed proton source profile, which is a required input in fluence reconstruction algorithms. In this approach, a fluence inversion is embedded inside of a mesh region, which provides overconstrained magnetic boundary conditions. A source profile is then iteratively optimized to satisfy the boundary information. This method substantially enhances the accuracy in recovering EM fields. Lastly, we propose a scheme to quantify uncertainty in the final inversion that is introduced through errors in the source retrieval.

physics.plasm-ph↗

arXiv2024

Proton deflectometry analysis in magnetized plasmas: magnetic field reconstruction in one dimension

Proton deflectometry is increasingly used in magnetized high-energy-density plasmas to observe electromagnetic fields. We describe a reconstruction algorithm to recover the electromagnetic fields from proton fluence data in 1-D. The algorithm is verified against analytic solutions and applied to example data. Secondly, we study the role of source fluence uncertainty for 1-D reconstructions. We show that reconstruction boundary conditions can be used to constrain the source fluence profile, and use this to develop a reconstruction using a specified pair of boundary conditions on the magnetic field. From these considerations we experimentally demonstrate a hybrid mesh-fluence reconstruction technique where fields are reconstructed from fluence data in an interior region with boundary conditions supplied by direct mesh measurements at the boundary.

physics.plasm-ph↗

arXiv2024

X-ray imaging and electron temperature evolution in laser-driven magnetic reconnection experiments at the National Ignition Facility

We present results from X-ray imaging of high-aspect-ratio magnetic reconnection experiments driven at the National Ignition Facility. Two parallel, self-magnetized, elongated laser-driven plumes are produced by tiling 40 laser beams. A magnetic reconnection layer is formed by the collision of the plumes. A gated X-ray framing pinhole camera with micro-channel plate (MCP) detector produces multiple images through various filters of the formation and evolution of both the plumes and current sheet. As the diagnostic integrates plasma self-emission along the line of sight, 2-dimensional electron temperature maps $\langle T_e \rangle_Y$ are constructed by taking the ratio of intensity of these images obtained with different filters. The plumes have a characteristic temperature $\langle T_e \rangle_Y = 240 \pm 20$ eV at 2 ns after the initial laser irradiation and exhibit a slow cooling up to 4 ns. The reconnection layer forms at 3 ns with a temperature $\langle T_e \rangle_Y = 280 \pm 50$ eV as the result of the collision of the plumes. The error bars of the plumes and current sheet temperatures separate at $4$ ns, showing the heating of the current sheet from colder inflows. Using a semi-analytical model, we find that the observed heating of the current sheet is consistent with being produced by electron-ion drag, rather than the conversion of magnetic to kinetic energy.

physics.plasm-ph↗

arXiv2023

Measurements of Extended Magnetic Fields in Laser-Solid Interaction

Magnetic fields generated from a laser-foil interaction are measured with high fidelity using a proton radiography scheme with in situ x-ray fiducials. In contrast to prior findings under similar experimental conditions, this technique reveals the self-generated, Biermann-battery fields extend beyond the edge of the expanding plasma plume to a radius of over 3.5 mm by t=+1.4 ns, a result not captured in state-of-the-art magneto-hydrodynamics simulations. An analysis of two mono-energetic proton populations confirms that proton deflection is dominated by magnetic fields far from the interaction (>2 mm) and electric fields are insignificant. Comparisons to prior work suggest a new physics mechanism for the magnetic field generation and transport in laser-solid interactions.

physics.plasm-ph↗

arXiv2022

Atmospheric aerosol clearing by femtosecond filaments

Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments have been shown to clear foggy regions, improving transmission of subsequent pulses. However, the detailed fog clearing mechanism had yet to be determined. Here we directly measure and simulate the dynamics of ~5 micron radius water droplets, typical of fog, under the influence of optical and acoustic interactions characteristic of femtosecond filaments. We find that for filaments generated by the collapse of collimated near-infrared femtosecond pulses, the main droplet clearing mechanism is optical shattering by laser light. For such filaments, the single cycle acoustic wave launched by filament energy deposition in air leaves droplets intact and drives negligible transverse displacement, and therefore negligible fog clearing. Only for tightly focused non-filamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.

physics.optics↗

arXiv2020

Full path single-shot imaging of femtosecond pulse collapse in air turbulence

In a single shot, we measure the full propagation path, including the evolution to pulse collapse, of a high power femtosecond laser pulse propagating in air. This technique enables single-shot examination of the effect of parameters that fluctuate on a shot-to-shot basis, such as pulse energy, pulse duration, and air turbulence-induced refractive index perturbations. We find that even in lab air over relatively short propagation distances, turbulence plays a significant role in determining the location of pulse collapse.

physics.optics↗