arXiv subjects

M. J. MacDonald

Publications and source records attributed to M. J. MacDonald.

4 recordsLinked to original sources

arXiv2026

Informing spectral models for dense plasmas with K-edge absorption measurements of warm dense copper

Warm dense matter remains a challenging regime to characterize experimentally and to model with predictive accuracy. Recent experimental platforms have been developed to generate, characterize, and diagnose uniform warm dense matter, enabling detailed comparisons with models. Here, we present experiments conducted at the OMEGA laser facility that compress and heat a buried layer target to warm dense matter conditions, where the targets are heated to temperatures of approximately 20 eV and compressed to densities of 25 g/cm^3. We probe the warm dense plasma using x-ray absorption spectroscopy, using the K-edge and bound-bound absorption features to constrain the temperature and charge state distribution of the plasma. We compare these measurements with two types of models: collisional-radiative models with detailed electronic structure and ad-hoc density effects, and a multi-ion model based on density functional theory in combination with excited-state projector augmented-wave potentials. Neither approach fully reproduces the observed data, We show that the broad structure and position of the K-edge region can be modeled using density functional theory in combination with excited-state projector augmented-wave potentials. The density functional theory results are contrasted with a collisional-radiative model approach that incorporates ad-hoc density effects, which show incomplete agreement with the experimental observations, highlighting a need for improved density-dependent atomic modeling in warm dense plasmas.

physics.plasm-ph↗

arXiv2026

Ionization and temperature measurements in warm dense copper using x-ray absorption spectroscopy

We detail experimental results inferring ionization and temperature for warm dense copper plasmas at several times solid density (15 to 25 g/cm$^3$) and temperatures of 10 to 21 eV. Experiments performed at the OMEGA Laser Facility generate uniform warm dense matter conditions via symmetric shock compression of a buried copper layer. The plasma is probed with a laser-generated x-ray source to collect the K-shell x-ray absorption spectrum. Fitting bound-bound absorption contributions from constituent charge states of copper provides an estimated $\overline{Z}$ of approximately 4 to 7 for these warm dense copper plasmas. We find that these partially ionized plasmas have K-edge shifts of 12 to 30 eV and bound-bound resonance 1s$\rightarrow$3p absorption shifts of 4 to 26 eV with respect to the cold K-edge. This study provides necessary experimental data to improve ionization and opacity models in the warm dense matter regime.

hep-ex↗

arXiv2021

Experiments conducted in the burning plasma regime with inertial fusion implosions

An experimental program is currently underway at the National Ignition Facility (NIF) to compress deuterium and tritium (DT) fuel to densities and temperatures sufficient to achieve fusion and energy gain. The primary approach being investigated is indirect drive inertial confinement fusion (ICF), where a high-Z radiation cavity (a hohlraum) is heated by lasers, converting the incident energy into x-ray radiation which in turn drives the DT fuel filled capsule causing it to implode. Previous experiments reported DT fuel gain exceeding unity [O.A. Hurricane et al., Nature 506, 343 (2014)] and then exceeding the kinetic energy of the imploding fuel [S. Le Pape et al., Phys. Rev. Lett. 120, 245003 (2018)]. We report on recent experiments that have achieved record fusion neutron yields on NIF, greater than 100 kJ with momentary fusion powers exceeding 1PW, and have for the first time entered the burning plasma regime where fusion alpha-heating of the fuel exceeds the energy delivered to the fuel via compression. This was accomplished by increasing the size of the high-density carbon (HDC) capsule, increasing energy coupling, while controlling symmetry and implosion design parameters. Two tactics were successful in controlling the radiation flux symmetry and therefore the implosion symmetry: transferring energy between laser cones via plasma waves, and changing the shape of the hohlraum. In conducting these experiments, we controlled for known sources of degradation. Herein we show how these experiments were performed to produce record performance, and demonstrate the data fidelity leading us to conclude that these shots have entered the burning plasma regime.

physics.plasm-ph↗

arXiv2017

Relativistic electron streaming instabilities modulate proton beams accelerated in laser-plasma interactions

We report experimental evidence that multi-MeV protons accelerated in relativistic laser-plasma interactions are modulated by strong filamentary electromagnetic fields. Modulations are observed when a preplasma is developed on the rear side of a $μ$m-scale solid-density hydrogen target. Under such conditions, electromagnetic fields are amplified by the relativistic electron Weibel instability and are maximized at the critical density region of the target. The analysis of the spatial profile of the protons indicates the generation of $B>$10 MG and $E>$0.1 MV/$μ$m fields with a $μ$m-scale wavelength. These results are in good agreement with three-dimensional particle-in-cell simulations and analytical estimates, which further confirm that this process is dominant for different target materials provided that a preplasma is formed on the rear side with scale length $\gtrsim 0.13 λ_0 \sqrt{a_0}$. These findings impose important constraints on the preplasma levels required for high-quality proton acceleration for multi-purpose applications.

physics.plasm-ph↗