The Evolution Dynamics Of Ultracold Neutral Plasmas

Ultracold neutral plasmas(UNPs)formed by photoionization of cold atoms are strongly coupled plasmas at a low temperature and a low density.The ultracold plasma is an advanced interdisciplinary research field of plasma physics,atomic physics,low temperature physics and condensed matter physics.In a strong Coulomb coupling system,the Coulomb potential energy between particles exceeds their average kinetic energy,and the particles develop spatial correlation during the evolution.In such a situation,the effective potential of Debye shielding and the classical hydrodynamic equations are no longer valid.Therefore,experimental measurements and numerical simulation studies of UNPs are helpful to test and refine the theoretical models of strongly coupled systems.In this dissertation,the evolution of electrons,ions and Rydberg atoms in UNPs is studied by experimental method and numerical simulation.There are three main topics,fast electron cooling in the electron equilibration,disorder-induced heating and plasma expansion in the ion equilibration,and the formation and evolution of Rydberg atoms in UNPs.In the electron equilibration,we observed a non-equilibrium evolution mechanism of fast electron cooling.The determination of electron equilibrium temperature is realized by measuring the quantitative relationship between the Rydberg atoms formed by the three-body recombination and the ions in the plasma.Combined with the simulation of molecular dynamics,both the experimental and theoretical results confirm that the electron cooling is a universal mechanism in UNPs and dominates the evolution behavior in electron equilibration for the initially weakly coupled electrons.In the ion equilibration,we observed ion disorder-induced heating and simultaneous plasma expansion.Using the velocity map imaging system,we have achieved the simultaneous measurement of ion temperature and expansion velocity of UNPs.The charged particle tracing method is applied to simulate the plasma evolution,and the results are in good agreement with the measurements.Both results show the same phenomenon of ion heating,and the ion equilibrium temperature is consistent with the theoretical value.It is also shown that the ions have always been strongly coupled during the ion equilibration,and the ion Coulomb coupling parameter is about 2 at equilibrium.Besides,the plasma expansion velocities of the experimental measurement and numerical simulation results exhibit similar characteristics.Compared with the self-similar expansion model,it is clear that the driving of the plasma expansion in the ion equilibration is the energy of the ion-ion spatial correlation.The understanding of the behavior of the Rydberg atoms in UNPs is essential for accurately describing the dynamics of UNPs.For this goal,we carried out researches on the formation and evolution of the Rydberg atoms in UNPs.By measuring Rydberg atoms through field ionization,we observed the evolution of the total number and the state distribution of Rydberg atoms.The evolution characteristics of the total number of Rydberg atoms are consistent with a theoretical model.And the evolution of the Rydberg state distribution also presents interesting phenomena.In brief,a new charged particle diagnostic scheme is proposed to measure electrons,ions and Rydberg atoms in UNPs.Combined with the charged particle tracing simulation,the fast electron cooling in the electron equilibration,the disorderinduced heating and simultaneous plasma expansion in ion equilibration,and the processes of Rydberg formation and evolution are studied.These results are helpful for detailed understanding of the evolution dynamics of UNPs and study of strongly coupled non-equilibrium plasma.