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Bulletin of the American Physical Society

Ion friction at small values of the Coulomb logarithm

Robert Sprenkle, Scott Bergeson

Published May 29, 2019• 0 citations
OpenAlex

Abstract

We create a dual-species ultracold neutral plasma (UNP) by photo-ionizing Yb and Ca atoms in a dual-species magneto-optical trap. Unlike single-species UNP expansion, these plasmas are well outside of the collisionless (Vlasov) approximation. We observe the mutual interaction of the Yb and Ca ions by measuring the velocity distribution for each ion species separately. We model the expansion using a fluid code including ion-ion friction and compare with experimental results to obtain a value of the Coulomb logarithm of Λ= 0.04.

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•Jan 1, 2013•0 citations
We create a dual-species ultracold neutral plasma (UNP) by photo-ionizing Yb and Ca atoms in a dual-species magneto-optical trap. Unlike single-species UNP expansion, these plasmas are well outside of the collisionless (Vlasov) approximation. We observe the mutual interaction of the Yb and Ca ions by measuring the velocity distribution for each ion species separately. We model the expansion using a fluid code including ion-ion friction and compare with experimental results to obtain a value of the Coulomb logarithm of Λ= 0.04.
We report measurements and simulations of the time-evolving rms velocity distribution in an ultracold neutral plasma. A strongly coupled ultracold neutral Ca+ plasma is generated by photoionizing laser-cooled atoms close to threshold. A fraction of these ions is then promoted to the second ionization state to form a mixed Ca+-Ca2+ plasma. By varying the time delay between the first and the second ionization events, a minimum in ion heating is achieved. We show that the Coulomb strong-coupling parameter Γ increases by a factor of 1.4 to a maximum value of 3.6. A pure Ca2+ plasma would have Γ=6.8, moving these strongly coupled systems closer to the regime of liquid-like correlations.
We measured the velocity distribution of ions in the ultracold neutral plasma (UNP) during its early evolution by the technique of velocity map imaging. The ion temperature during the phase of ion equilibrium was obtained experimentally through this method. The Coulomb coupling parameter of ions in the UNP after disorder-induced heating was also determined to be 2.1, which agreed well with the prediction from a charged particle tracing simulation. In addition, the ion expansion during the ion equilibration phase was observed. Notably, the experimentally observed expansion speed is larger than the value obtained from the self-similar expansion model, indicating the involvement of additional mechanisms, besides the electron thermal pressure, in driving the ion expansion. We have also discussed the contributions of ion–ion correlations and charge imbalance to the plasma expansion.
Using high-resolution laser-induced fluorescence spectroscopy, ion dynamics in\nan expanding ultracold neutral plasma (UNP) have been studied in unprecedented\ndetail. The evolution of the ion temperature is of great interest because the ions in\nan ultracold neutral plasma are strongly coupled, meaning their Coulomb interaction\nenergy exceeds the thermal energy. This leads to novel plasma properties that are\nhard to describe theoretically. Understanding all the factors that contribute to ion\ntemperature evolution is critical for designing schemes to cool the ions and achieve\neven stronger Coulomb coupling.\nThis work has also observed several phenomena that have not previously been\nstudied in ultracold plasmas. Ion adiabatic cooling has been observed in a UNP for\nthe first time, resulting in ion temperatures as low as 100mK and up to a tripling of\nthe Coulomb coupling parameter. The importance of electron-ion energy transfer is\ndemonstrated with the first observations of electron-ion collisional heating. Inclusion\nof this effect into existing numerical models provides better agreement with experimental\nresults. Results of molecular dynamics simulations of equilibrating plasmas\nhave been compared to early time experimental data, which provides a new method\nof density determination that is much less sensitive to experimental systematics and\nis accurate to within 10%. This improved certainty in the density allows accurate\ncomparison between data and a theoretical model of the plasma dynamics. The\nagreement is excellent, but on a hydrodynamic time scale a small amount of extra\nheat (on order of 100mK), not ascribable to electron-ion collisions or other sources in\nthe model, is detected in the ion component. Potential sources are discussed within\nas well as proposals for further studies and improvements.
Abstract Ultracold neutral plasmas are strongly coupled Coulomb systems that are generated by photoionizing lasercooled atoms close to the ionization threshold. The strong coupling parameter Γ is limited at times later than ∼100 ns by disorder‐induced heating. A recent simulation predicted that higher values of Γ can be realized in ultracold neutral plasmas if the plasma ions are excited to higher ionization states. In this paper we present recent results from an experiment that increases the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Using laser‐induced fluorescence we map out the ion velocity distribution of the Ca+ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses. We compare our results to MD simulations, which estimate that Γ increases from approximately 2.5 to 3.6. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ultracold neutral plasmas (UNPs) are created in a vacuum by photoionizing laser-cooled atoms. The expansion of UNPs begins as a non-equilibrium system that quickly relaxes to a local equilibrium state [1] . We aim to quantify and compare the transport processes that govern the relaxation to equilibrium in the expanding UNP. Using a 1D-1V kinetic code in spherical coordinates, we model the radial expansion of an UNP. We compare our simulation results against analytic expressions to determine where the expansion of the UNP is adiabatic or isothermal. We carry out simulations for various initial density distributions such as an exponentially decaying density distribution [2] . We ultimately aim to extend this approach to a 1D-3V multi-component kinetic code that incudes collisions [3] .
Ultracold neutral plasmas (UNPs) are highly correlated, charged-particle systems studied in standard atomic-molecular optical physics laboratories. Advances in ultracold physics techniques have led to the development of magneto-optical traps that permit the localization of micro-Kelvin temperature atomic samples (10⁸ - 10¹⁰ cm -³ that can be photoionized with a defined excess laser energy. Magneto-optical trapping of molecules proves difficult, and to date, lags in reaching conditions attainable for atoms. Molecules possess additional degrees of freedom where energy can flow during laser trapping and cooling. Alternatively, elastic collisions with inert gases offer another route to low temperatures. Super-sonic expansions of target molecules seeded in a noble gas can reduced the translational temperature to values as low as 0.1 K. Here I report on the formation of a molecular ultracold plasma in a molecular beam of nitric oxide seeded in an inert gas. Our method does not directly form a plasma, but instead, I create a Rydberg gas that spontaneously evolves to a plasma. I have verified the transition to a collective plasma state by applying pulsed-fields to the field-free flight region, and observe a signal that persists well beyond classical field amplitudes required to ionize individual Rydberg states. Electric field screening is a collective effect occurring over a characteristic distance, quantified by the Debye length, λ_D. Molecular ultracold plasmas described here exhibit Debye lengths smaller than the overall size, λ_D<<σ. Plasmas of this nature offer an exotic environment for which to study fundamental phenomena. While plasmas are generally thought of as hot, UNPs exist at low enough temperatures that electrostatic interaction energy between particles is greater than their kinetic energy. At this point, charged particles begin to find themselves in local potential energy minima. Over the entire sample this translates to long-range order, or correlation. It is of great interest for researchers studying UNPs to maximize particle correlation, which requires UNPs to remain very cold, and dense through their lifetime.