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Measurement of the threshold ion number in the formation of an ultracold plasma

Syed Zaheer Ud Din, Yufan Li, Dongmei Zhao, X. Ma, Jei Yang

Published Oct 10, 2020• 1 citation
Publisher

Abstract

We present threshold ion number measurement in the formation of an ultracold plasma in a rubidium magneto-optical trap using the photoionization of laser-cooled atoms technique. The density and temperature of the trapped atoms in the magneto-optical trap were about 1010 cm-3 and 500 μK, respectively. The measured value of the lifetime of the ultracold plasma was exactly matched with the calculated value. The threshold ion number in the formation of ultracold plasma was obtained from the linear relationship between ion and electron numbers for different laser energies. The dependence of the threshold ion number on the depth of the potential well was investigated experimentally. The experimental results were explained by the Coulomb potential well model.

DOI
10.1117/12.2572798

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We present threshold ion number measurement in the formation of an ultracold plasma in a rubidium magneto-optical trap using the photoionization of laser-cooled atoms technique. The density and temperature of the trapped atoms in the magneto-optical trap were about 1010 cm-3 and 500 μK, respectively. The measured value of the lifetime of the ultracold plasma was exactly matched with the calculated value. The threshold ion number in the formation of ultracold plasma was obtained from the linear relationship between ion and electron numbers for different laser energies. The dependence of the threshold ion number on the depth of the potential well was investigated experimentally. The experimental results were explained by the Coulomb potential well model.
We created an ultracold plasma by photoionizing the laser-cooled and trapped rubidium atoms in a magneto-optical trap. In the externally applied direct current (DC) electric field environment, the electrons which escape from the potential well of the ultracold plasma were detected for different numbers of the ions and initial kinetic energies of the electrons. The results are in good agreement with the calculations, based on the Coulomb potential well model, indicating that the external DC field is an effective tool to adjust the depth of potential well of the plasma, and it is possible to create an ultracold plasma in a controlled manner.
In this paper, we report electronic temperature measurement in a Rb ultracold plasma formed in a magneto-optical trap. We used the ion evaporation rate and the model presented by Comparat et al. [Mon. Not. R. Astron. Soc. 361 , 1227 (2005)] to extract the electronic temperature in the plasma as a function of the energy difference between the laser photon and the ionization threshold. The obtained results are similar to those obtained in Cs; however, they do not seem to show any saturation. Comparison with other models may lead to improvements in the experiments and the theoretical models.
Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼10 10 cm −3 ) are investigated. The measured dependence on initial ion density is N 1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.
Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1-1000 K and the ion temperature is around 1 K. The density can approach 101110^{11}1011 cm−3^{-3}−3. Fundamental interest stems from the possibility of creating strongly-coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+^++ 2S1/2−>2P1/2{^2S_{1/2}} -> {^2P_{1/2}}2S1/2​−>2P1/2​ transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.
In this Letter we discuss our approach that makes possible creation of the steady-state ultracold plasma having various densities and temperatures by means of continuous two-step optical excitation of calcium atoms in the magneto-optical trap. A strongly coupled ultracold plasma can be used as an excellent test platform for studying many-body interactions associated with various plasma phenomena. The parameters of the plasma are studied using laser-induced fluorescence of calcium ions. The experimental results are well described by a simple theoretical model involving equilibration of the continuous source of charged particles by the hydrodynamical ion outflux and three-body recombination. The ultracold plasma with the peak ion density of 2.7×10^{6} cm^{-3} and the minimum electron temperature near 2 K has been prepared. Our steady-state approach in combination with the strong magnetic confinement of the plasma will make it possible to reach extremely strong coupling in such system.
We present the first experiment to observe an ultracold neutral plasma. The plasma, which was created by photoionization of laser cooled atoms, has charge densities as high as 2×109 cm−3, and the temperatures of electrons and ions are as low as 100 mK and 10 μK, respectively. The plasma has a lifetime of about 100 μs, much longer than predicted by recombination rates. When the laser that excites the atoms is tuned below the ionization limit we create a sample of very highly excited cold Rydberg atoms. At our highest densities and during a time of a few microseconds, in which the Rydberg atoms are essentially stationary, the ensemble evolves towards an unbound plasma-like state.