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arXiv:1808.05262 · Physical Review A

Heating and cooling of electrons in an ultracold neutral plasma using Rydberg atoms

Ethan Crockett, R. C. Newell, F. Robicheaux, Duncan Tate

Published Oct 24, 2018• 12 citations
arXivHTMLPDF

Abstract

We have experimentally demonstrated both heating and cooling of electrons in an ultracold neutral plasma (UNP) by embedding Rydberg atoms into the plasma soon after its creation. We have determined the relationship between the initial electron temperature, ${T}_{e,i}$, and the binding energy of the added Rydberg atoms, ${E}_{b}$, at the crossover between heating and cooling behaviors (that is, the binding energy of the atoms, which, when they are added to the plasma, neither accelerate nor slow down the plasma expansion). Specifically, this condition is $|{E}_{b}|\ensuremath{\approx}2.7\ifmmode\times\else\texttimes\fi{}{k}_{B}{T}_{e,i}$ when the diagnostic used is the effect of the Rydberg atoms on the plasma asymptotic expansion velocity. Additionally, we have obtained experimental estimates for the amount of heating or cooling which occurs when the Rydberg binding energy does not satisfy the crossover condition. The experimental results for the crossover condition, and the degree of heating or cooling away from the crossover, are in agreement with predictions obtained from numerical modeling of the interactions between Rydberg atoms and the plasma. We have also developed a simple intuitive picture of how the Rydberg atoms affect the plasma, which supports the concept of a ``bottleneck'' in the Rydberg state distribution of atoms in equilibrium with a coexisting plasma.

DOI
10.1103/physreva.98.043431

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We have experimentally demonstrated both heating and cooling of electrons in an ultracold neutral plasma (UNP) by embedding Rydberg atoms into the plasma soon after its creation. We have determined the relationship between the initial electron temperature, Te,i{T}_{e,i}Te,i​, and the binding energy of the added Rydberg atoms, Eb{E}_{b}Eb​, at the crossover between heating and cooling behaviors (that is, the binding energy of the atoms, which, when they are added to the plasma, neither accelerate nor slow down the plasma expansion). Specifically, this condition is ∣Eb∣\ensuremath≈2.7\ifmmode×\else\texttimes\fikBTe,i|{E}_{b}|\ensuremath{\approx}2.7\ifmmode\times\else\texttimes\fi{}{k}_{B}{T}_{e,i}∣Eb​∣\ensuremath≈2.7\ifmmode×\else\texttimes\fikB​Te,i​ when the diagnostic used is the effect of the Rydberg atoms on the plasma asymptotic expansion velocity. Additionally, we have obtained experimental estimates for the amount of heating or cooling which occurs when the Rydberg binding energy does not satisfy the crossover condition. The experimental results for the crossover condition, and the degree of heating or cooling away from the crossover, are in agreement with predictions obtained from numerical modeling of the interactions between Rydberg atoms and the plasma. We have also developed a simple intuitive picture of how the Rydberg atoms affect the plasma, which supports the concept of a ``bottleneck'' in the Rydberg state distribution of atoms in equilibrium with a coexisting plasma.
We report a systematic experimental and numerical study of the expansion of ultracold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities, v0{v}_{0}v0​, of ultracold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature, Te,0=mionv02/kB{T}_{e,0}={m}_{\mathrm{ion}}{v}_{0}^{2}/{k}_{B}Te,0​=mion​v02​/kB​ (where mion{m}_{\mathrm{ion}}mion​ is the Rb+{\mathrm{Rb}}^{+}Rb+ ion mass), as a function of the original Rydberg atom density and binding energy, Eb,i{E}_{b,i}Eb,i​. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range ${10}^{7}--{10}^{9} {\mathrm{cm}}^{\ensuremath{-}3}$, for states with principal quantum number n>40,Te,0n>40, {T}_{e,0}n>40,Te,0​ is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity kBTe,0{k}_{B}\phantom{\rule{0.16em}{0ex}}{T}_{e,0}kB​Te,0​ is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|\ensuremath{\lesssim}{k}_{B}{T}_{e,0}\ensuremath{\lesssim}2.5\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n\ensuremath{\lesssim}40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where Te,0{T}_{e,0}Te,0​ is determined by their Coulomb coupling parameter, ${\mathrm{\ensuremath{\Gamma}}}_{e}\ensuremath{\sim}0.01$.
We report a systematic experimental and numerical study of the expansion of ultra-cold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities, v0v_0v0​, of ultra-cold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature, Te,0=mionv02/kBT_{e,0} = m_{ion} v_0^2/k_BTe,0​=mion​v02​/kB​ (where mionm_{ion}mion​ is the Rb+^++ ion mass), as a function of the original Rydberg atom density and binding energy, Eb,iE_{b,i}Eb,i​. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range 107−10910^7 - 10^9107−109 cm−3^{-3}−3, for states with principal quantum number n>40n > 40n>40, Te,0T_{e,0}Te,0​ is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity kB Te,0k_B \, T_{e,0}kB​Te,0​ is strongly correlated with the fraction of atoms which ionize, and is in the range 0.6×∣Eb,i∣≲kBTe,0≲2.5×∣Eb,i∣0.6 \times |E_{b,i}| \lesssim k_BT_{e,0} \lesssim 2.5 \times |E_{b,i}|0.6×∣Eb,i​∣≲kB​Te,0​≲2.5×∣Eb,i​∣. On the other hand, plasmas from Rydberg samples with n≲40n \lesssim 40n≲40 evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where Te,0T_{e,0}Te,0​ is determined by their Coulomb coupling parameter, Γe∼0.01\Gamma_e \sim 0.01Γe​∼0.01.
We have studied the behavior of cold Rydberg atoms embedded in an ultracold plasma. We demonstrate that even deeply bound Rydberg atoms are completely ionized in such an environment, due to electron collisions. Using a fast pulse extraction of the electrons from the plasma we found that the number of excess positive charges, which is directly related to the electron temperature Te{T}_{e}Te​, is not strongly affected by the ionization of the Rydberg atoms. Assuming a Michie-King equilibrium distribution, in analogy with globular star cluster dynamics, we estimate Te{T}_{e}Te​. Without concluding on heating or cooling of the plasma by the Rydberg atoms, we discuss the range for changing the plasma temperature by adding Rydberg atoms.
By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.
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.