arXiv subjects

T. M. Johnson

Publications and source records attributed to T. M. Johnson.

3 recordsLinked to original sources

arXiv2024

Development of a platform for experimental and computational studies of magnetic and radiative effects on astrophysically-relevant jets at OMEGA

Accurate modeling of astrophysical jets is critical for understanding accretion systems and their impact on the interstellar medium. While astronomical observations can validate models, they have limitations. Controlled laboratory experiments offer a complementary approach for qualitative and quantitative demonstration. Laser experiments offer a complementary approach. This article introduces a new platform on the OMEGA laser facility for high-velocity (1500 km/s), high-aspect-ratio ($\sim$36) jet creation with strong cylindrical symmetry. This platform s capabilities bridge observational gaps, enabling controlled initial conditions and direct measurements

physics.plasm-ph↗

arXiv2022

Observation of electromagnetic filamentary structures produced by the Weibel instability in laser-driven plasmas

We present experimental observations of electron-scale structures in an expanding high-energy-density (HED) plasma generated with a modest intensity $\sim 2 \times 10^{14}$ W/cm$^2$, $\sim1$ ns laser. The observed structures have wavelengths ($\sim 150-220 μm$) and growth rates ($\sim 0.4-1.0$ ns$^{-1}$) consistent with an electron-driven Weibel instability where the anisotropy in the electron distribution is small, $A\sim 0.002$. This instability is found to be a better match to the observed phenomena than other typical field-generation mechanisms found in HED plasmas, including counter-streaming ion Weibel and magnetothermal instabilities. These observations experimentally demonstrate for the first time that the electron Weibel instability must be considered alongside other magnetic field generation and amplification mechanisms in expanding ablation plasmas, which are ubiquitous in HED research. They also provide physics insight into the generation of magnetic fields in large-scale astrophysical plasmas. Additionally, inspection of the magnetic power spectrum shows a possible scaling match to analytic gyrokinetic predictions, $|B_k|^2\propto k^{-16/3}$, at scales below the electron Larmor radius.

physics.plasm-ph↗

arXiv2022

Experimental Evidence of Plasmoids in High-$β$ Magnetic Reconnection

Magnetic reconnection is a ubiquitous and fundamental process in plasmas by which magnetic fields change their topology and release magnetic energy. Despite decades of research, the physics governing the reconnection process in many parameter regimes remains controversial. Contemporary reconnection theories predict that long, narrow current sheets are susceptible to the tearing instability and split into isolated magnetic islands (or plasmoids), resulting in an enhanced reconnection rate. While several experimental observations of plasmoids in the regime of low- to intermediate-$β$ (where $β$ is the ratio of plasma thermal pressure to magnetic pressure) have been made, there is a relative lack of experimental evidence for plasmoids in the high-$β$ reconnection environments which are typical in many space and astrophysical contexts. Here, we report the observation of strong experimental evidence for plasmoid formation and dynamics in laser-driven high-$β$ reconnection experiments.

physics.plasm-ph↗