Expansion and electron temperature evolution in an ultracold neutral plasma
Killian, Thomas C.
Doctor of Philosophy
This work describes the evolution of an ultracold neutral plasma as it expands freely in vacuum. It presents a comprehensive study of the electron temperature evolution under different initial conditions. Ultracold neutral plasmas are created by photoionizing laser-cooled neutral atoms in ultrahigh vacuum. The ions are typically at a temperature of ∼ 1K while the electron temperature can be set from 1--1000 K. After photoionization, some of the highly energetic electrons escape from the cloud, leaving a net positive charge in the cloud. This creates a Coulomb well which traps the rest of the electrons, and a plasma is formed. Since the electrons have a lot of kinetic energy, they tend to leave the cloud, however, the Coulomb force from the ion pulls the electrons back into the cloud. This exerts a recoil force on the ions, and the whole plasma starts expanding radially outwards. Since the expansion is caused by the thermal pressure of the electrons, a study of the plasma expansion unravels the complicated electron temperature evolution, under different initial conditions. Many collisional processes become significant as a plasma expands. These physical processes tend to heat or cool the ions and electrons, leading to very different kinds of evolution depending on the initial conditions of the plasma. This work demonstrates three different regions of parameter space where the degree of significance of these physical processes is different during the ultracold neutral plasma evolution. The experimental results are verified by theoretical simulations, performed by Thomas Pohl, which untangle the complicated electron temperature evolution.
Atomic physics; Plasma physics