Simulation of ultracold plasmas using the Monte Carlo method

Luis A. Peña Ardila

James L. Hart, Scott Bergeson, Ross L. Spencer

Gouthaman S. Balaraman

In the last decade, ultracold plasmas have been created in the laboratory by photo-ionizing laser-cooled atoms. To understand the overall dynamics of ultracold plasmas, one needs to understand Rydberg collisional processes at ultracold temperatures. The two kinds of problems addressed in this thesis are: study of Rydberg atomic processes at ultracold temperatures, and a study of the overall dynamics of \nthe ultracold plasmas. \nTheoretical methods based on quantal-classical correspondence is used to understand Rydberg atomic processes such as radiative cascade, and radiative recombination. A simulation method suitable for ultracold collisions is developed and tested. \nThis method is then applied to study collisional-Stark mixing in Rydberg atoms. \nTo study the dynamics of the ultracold plasmas, a King model for the electrons in plasmas is proposed. The King model is a stationary, finite sized electron distribution for the electrons in a cloud of fixed ions with a Gaussian distribution. A Monte-Carlo method is developed to simulate the overall dynamics of the King distribution.

H. R. Sadeghpour, Thomas Pohl, D. Vrinceanu

Rydberg atom formation is a source of heating in plasmas. The rate of three-body recombination in an ultracold neutral plasma was measured and temperature for the plasma was extracted. With large-scale Monte Carlo and particle-in-cell simulations, we have ab initio calculated the rate of excitation, de-excitation, ionization (and recombination) in electron-Rydberg atom collision and investigated the short-time dynamics of three-body recombination in an ultracold neutral plasma. Comparison with observed rates is quite good. Particular attention is paid to the low-frequency microfield effect on Rydberg state cut-off in the plasma.

James D. Hanson, F. Robicheaux

Thomas Pohl, D. Vrinceanu, H. R. Sadeghpour

We present extensive Monte Carlo calculations of electron-impact-induced transitions between highly excited Rydberg states and provide accurate rate coefficients. For moderate energy changes, our calculations confirm the widely applied expressions in P. Mansbach and J. Keck [Phys. Rev. 181, 275 (1969)] but reveal strong deviations at small energy transfer. Simulations of ultracold plasmas demonstrate that these corrections significantly impact the short-time dynamics of three-body Rydberg atom formation. The improved rate coefficients yield quantitative agreement with recent ultracold plasma experiments.