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arXiv:2308.16473 · Physics of Plasmas

Suppressing electron disorder-induced heating of ultracold neutral plasma via optical lattice

H. Wang, Zonglin Yao, Haoyu Huang, Jianing Sun, Fuyang Zhou, Yong Wu, Jianguo Wang, Xiangjun Chen

Published May 1, 2024• 1 citation
arXivHTMLPDF

Abstract

Disorder-induced heating (DIH) prevents ultracold neutral plasma into the electron strong coupling regime. Here, we propose a scheme to suppress electronic DIH via optical lattice. We simulate the evolution dynamics of ultracold neutral plasma constrained by three-dimensional optical lattice using the classical molecular dynamics method. The results show that for experimentally achievable conditions, electronic DIH is suppressed by a factor of 1.3, and the Coulomb coupling strength can reach 0.8, which is approaching the strong coupling regime. Suppressing electronic DIH via optical lattice may pave a way for the research of electronic strongly coupled plasma.

DOI
10.1063/5.0202695

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Disorder-induced heating (DIH) prevents ultracold neutral plasma into the electron strong coupling regime. Here, we propose a scheme to suppress electronic DIH via optical lattice. We simulate the evolution dynamics of ultracold neutral plasma constrained by three-dimensional optical lattice using the classical molecular dynamics method. The results show that for experimentally achievable conditions, electronic DIH is suppressed by a factor of 1.3, and the Coulomb coupling strength can reach 0.8, which is approaching the strong coupling regime. Suppressing electronic DIH via optical lattice may pave a way for the research of electronic strongly coupled plasma.
We quantify the disorder-induced heating (DIH) of ultracold neutral plasmas (UCNPs) created from cold atoms in optical lattices with partial filling fractions, using a conservation of energy model involving the spatial correlations of the initial state and the equation of state in thermal equilibrium for a one-component plasma. We show, for experimentally achievable filling fractions, that the ionic Coulomb coupling parameter could be increased to a degree comparable to other proposed DIH-mitigation schemes. Molecular dynamics simulations were performed with compensation for finite-size and periodic boundary effects, which agree with calculations using the model. Reduction of DIH using optical lattices will allow for the study of strongly coupled plasma physics using low-density, low-temperature, laboratory-based plasmas, and lead to improved brightness in UCNP-based cold electron and ion beams, where DIH is otherwise a fundamental limitation to beam focal sizes and diffraction imaging capability.
Disorder-induced heating (DIH) is one of the main reasons reducing the coupling strength in ultracold plasma. We propose applying an optical lattice as periodic confinement before the ultracold atomic cloud is ionized to eliminate its effect. We demonstrate a numerical simulation for the dynamics of the ultracold plasmas using classical molecular dynamics method with open boundary. DIH is reproduced in the simulation with the random Gaussian initial distribution and is absent in the results with the ordered lattice initial distribution. We further find that the collisional heating from electrons is important for ultracold plasmas with chosen spatial correlations in the optical lattice. Carefully preparing the initial condition (e.g., the ion density, initial electron temperature, and so on), collisional heating for the ions would be significantly reduced and eventually negligible. This allows a much stronger coupling in ultracold plasma to be realized.
Ultracold neutral plasmas are created by photoionizing laser-cooled atoms in a magneto-optical trap (MOT). Due to their large electrical potential energies and comparatively small kinetic energies, ultracold plasmas fall into a regime of plasma systems which are called “strongly coupled.” A priority in the field of ultracold plasmas is to generate plasmas with higher values of the strong coupling parameter Γ, which is given as the ratio of the nearest-neighbor Coulomb potential energy to the average kinetic energy. The equilibrium strong coupling in ultracold plasmas is limited by the ultrafast relaxation of the ions due to spatial disorder in the initial system. This heating mechanism is called “disorder-induced heating” (DIH) and it limits the ion strong coupling in ultracold plasmas to order unity. This thesis describes experiments that explore ways to generate higher values of the strong coupling parameter in an ultracold neutral calcium plasma.One way to increase Γ is to mitigate the effects of DIH using electron screening. This thesis describes an experiment in which the initial electron temperature was systematically changed to determine the effect that electron screening has on the ion thermalization. At lower initial electron temperatures, corresponding to a higher degree of electron shielding, it was found that the screening slows the ion thermalization and reduces the equilibrium ion temperature by as much as a factor of two. However, electron screening also reduces the ion interaction strength by the same amount, which has the net effect of leaving the effective Γ unchanged.Another method for increasing the strong coupling of an ultracold plasma is to excite the plasma ions to a higher ionization state. Simulations predict that doubly ionizing the plasma ions can increase the strong coupling in an ultracold plasma by as much as a factor of 4, with the maximum value of Γ depending on the timing of the second ionization relative to the DIH process. This thesis describes an experiment designed to test these predictions in a Ca2+ plasma. Measurements of the change in the Ca+ ion temperature as a function of the timing of the second ionization pulses were made using laser-induced fluorescence. Results of these measurements show that the heating of the Ca+ ions due to the second ionization depends on the timing of the second ionization pulses, as predicted by MD simulations.
Ultracold neutral plasmas provide a useful system for studying extreme parameter regime 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 (DIH) and heating due to Rydberg atom formation. By using experimentally informed simulations, it is found that DIH 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 0.53 ± 0.05 K (for an electron density, ne, of 6.1×1012 m−3), which determines the maximum coupling strength for the measured experimental conditions.
Disorder-induced heating (DIH) is a nonequilibrium, ultrafast relaxation process that occurs when laser-cooled atoms are photoionized to make an ultracold plasma. Its effects dominate the ion motion during the first 100 ns of the plasma evolution. Using tools of atomic physics we study DIH with ns time resolution for different plasma densities and temperatures. By changing the frequency of the laser beam we use to probe the ions, we map out the time evolution of the velocity distribution. We can compare this to a fluorescence simulation in order to more clearly determine the relationship between the fluorescence signal and the velocity distribution. In this study we observe and characterize effects due to electron screening on the ions during the equilibration process.
Electron heating in a magnetized ultracold neutral plasma is modeled using classical molecular dynamics simulations. A sufficiently strong magnetic field is found to reduce disorder induced heating and three body recombination heating of electrons by constraining electron motion, and therefore heating, to the single dimension aligned with the magnetic field. A strong and long-lasting temperature anisotropy develops, and the overall kinetic electron temperature is effectively reduced by a factor of three. These results suggest that experiments may increase the effective electron coupling strength using an applied magnetic field.