arXiv subjects

Bridget O'Mara

Publications and source records attributed to Bridget O'Mara.

3 recordsLinked to original sources

arXiv2026

The Effect of Magnetization on Electron Heating in Low-Density Ultracold Neutral Plasmas

Ultracold neutral plasmas provide a useful system for studying extreme parameter regimes plasma physics in an accessible laboratory setting. The parameter space of plasma physics can be characterized in part by coupling strength and degree of magnetization. The range of achievable strong coupling is determined in part by the lowest possible temperatures that can be achieved. This work examines the early-lifetime electron heating of moderately coupled, strongly magnetized plasmas. This heating is dominated by disorder-induced heating and heating due to Rydberg atom formation. By using experimentally informed simulations, it is found that disorder-induced heating has a large influence in electron temperature well into the plasma lifetime. Additionally, the dependence of the minimum achievable electron temperature on magnetization and initial electron energy is examined. In this work, we find electron temperatures as low as $0.52^{+.10}_{-.05}\ \mathrm{K}$ (for electron density, $n_{e}$, of $6.1 \times 10^{12}\ \mathrm{m^{-3}}$), which determines the maximum coupling strength for the measured experimental conditions.

physics.plasm-ph↗

arXiv2026

Experimental Tests of Radio-Frequency Heating Saturation in Ultracold Neutral Plasmas

For non-resonant radio-frequency (RF) fields, electron heating in sufficiently collisional plasmas can be driven primarily by inverse bremsstrahlung absorption. When the quiver velocity v_osc approaches the electron thermal velocity v_th, theory often predicts sublinear scaling of the heating rate with RF power, indicating saturation. We experimentally test this prediction in ultracold neutral plasmas by finding RF pulses of different amplitude and duration that produce the same electron heating. Despite v_osc being comparable to v_th, we measured no observable saturation. We compare our results to linear response theory (LRT) and a binary collision theory (BCT). The predicted saturation in both theories is sensitive to how common assumptions about cutoff parameters are applied, and agreement with experimental results is much better if quiver-velocity-dependent cutoffs in LRT and BCT are used. Additionally, under our conditions of moderate coupling and magnetization, we find no evidence that RF heating distorts the electron velocity distribution from Maxwell-Boltzmann, indicating saturation from the Langdon effect is suppressed.

physics.plasm-ph↗

arXiv2016

Velocity Amplitudes in Global Convection Simulations: The Role of the Prandtl Number and Near-Surface Driving

Several lines of evidence suggest that the velocity amplitude in global simulations of solar convection, U, may be systematically over-estimated. Motivated by these recent results, we explore the factors that determine U and we consider how these might scale to solar parameter regimes. To this end, we decrease the thermal diffusivity $κ$ along two paths in parameter space. If the kinematic viscosity $ν$ is decreased proportionally with $κ$ (fixing the Prandtl number $P_r = ν/κ$), we find that U increases but asymptotes toward a constant value, as found by Featherstone & Hindman (2016). However, if $ν$ is held fixed while decreasing $κ$ (increasing $P_r$), we find that U systematically decreases. We attribute this to an enhancement of the thermal content of downflow plumes, which allows them to carry the solar luminosity with slower flow speeds. We contrast this with the case of Rayleigh-Benard convection which is not subject to this luminosity constraint. This dramatic difference in behavior for the two paths in parameter space (fixed $P_r$ or fixed $ν$) persists whether the heat transport by unresolved, near-surface convection is modeled as a thermal conduction or as a fixed flux. The results suggest that if solar convection can operate in a high-$P_r$ regime, then this might effectively limit the velocity amplitude. Small-scale magnetism is a possible source of enhanced viscosity that may serve to achieve this high-$P_r$ regime.

astro-ph.SR↗