arXiv subjects

J. L. Peebles

Publications and source records attributed to J. L. Peebles.

4 recordsLinked to original sources

arXiv2026

Suppression of Electromagnetic Pulses from Laser-Target Interactions by Strong Magnetic Fields

Laser-target interactions generate intense electromagnetic pulses (EMP) that can interfere with measurements and damage equipment. In this paper we show that applying a magnetic field to nanosecond pulse laser-target interactions decreases the magnitude of EMP. We demonstrate this effect in two experiments with different geometries (spherical vs. planar), laser intensities (${\sim}10^{13}$ vs. ${\sim} 10^{15}$~W/cm$^2$) and applied field strength (12~T vs. 0.1~T) that both observed suppression of EMP in the ${\sim} 1$~GHz band (by factors of $0.65\times$ and $0.32\times$ respectively). We then observe the opposite effect at high intensities with a picosecond pulse: for planar experiments with laser intensities ${\sim}10^{19}$~W/cm$^2$ and magnetic fields of 6--10~T, the magnitude of EMP is increased by a factor of $1.75\times$. These results provide a benchmark for models of EMP generation, but suggest that magnetic fields are not a viable solution for mitigating EMP on the high intensity laser facilities where it is most damaging.

physics.plasm-ph↗

arXiv2024

Distortions in Charged-Particle Images of Laser Direct-Drive Inertial Confinement Fusion Implosions

Energetic charged particles generated by inertial confinement fusion (ICF) implosions encode information about the spatial morphology of the hot-spot and dense fuel during the time of peak fusion reactions. The knock-on deuteron imager (KoDI) was developed at the Omega Laser Facility to image these particles in order to diagnose low-mode asymmetries in the hot-spot and dense fuel layer of cryogenic deuterium--tritium ICF implosions. However, the images collected are distorted in several ways that prevent reconstruction of the deuteron source. In this paper we describe these distortions and a series of attempts to mitigate or compensate for them. We present several potential mechanisms for the distortions, including a new model for scattering of charged particles in filamentary electric or magnetic fields surrounding the implosion. Particle-tracing is used to create synthetic KoDI data based on the filamentary field model that reproduces the main experimentally observed image distortions. We conclude that the filamentary scattering model best matches the observed image distortions. Finally, we discuss potential impacts of filamentary fields on other charged-particle diagnostics.

physics.plasm-ph↗

arXiv2022

Diagnosing Magnetic Fields in Cylindrical Implosions with Oblique Proton Radiography

Two experiments on the OMEGA Laser System used oblique proton radiography to measure magnetic fields in cylindrical implosions with and without an applied axial magnetic field. Although the goal of both experiments was to measure the magnitude of the compressed axial magnetic field in the core of the implosion, this field was obfuscated by two features in the coronal plasma produced by the compression beams: an azimuthal self-generated magnetic field and small length scale, high-amplitude structures attributed to collisionless effects. In order to understand these features, synthetic radiographs are generated using fields produced by 3-D HYDRA simulations. These synthetic radiographs reproduce the features of the experimental radiographs with the exception of the small-scale structures. A direct inversion algorithm is successfully applied to a synthetic radiograph, but is only partially able to invert the experimental radiographs in part because some protons are blocked by the field coils. The origins of the radiograph features and their dependence on various experimental parameters are explored. The results of this analysis should inform future measurements of compressed axial magnetic fields in cylindrical implosions.

physics.plasm-ph↗

arXiv2019

Direct-drive measurements of laser-imprint-induced shock velocity nonuniformities

Perturbations in the velocity profile of a laser-ablation-driven shock wave seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been measured. Direct measurements of these velocity perturbations were recorded using a two-dimensional high-resolution velocimeter probing plastic material shocked by a 100-ps picket laser pulse from the OMEGA laser system. The measured results for experiments with one, two, and five overlapping beams incident on the target clearly demonstrate a reduction in long-wavelength ($>$25 um) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These experimental measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint for laser direct drive inertial confinement fusion research since they highlight the significant (factor of 3) underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation, such as multiphoton ionization are neglected.

physics.plasm-ph↗