Laser Cooling of Ions in a Neutral Plasma

[摘要] In this thesis, we present results from the first successful application of laser-cooling to a neutral plasma.Specifically, we laser-cool an ultracold neutral plasma (UNP) generated from the photoionization of a cold trapped gas of strontium atoms.After photoionization, the ions heat up to a temperature of~500 mK through a process known as disorder induced heating (DIH).After laser-cooling the plasma for 135 μs, we observe a temperature of 50 mK.One main driver of interest in UNP systems is that, even after DIH, the thermal energy scale of the ions (k_{B}T) is less than the interaction energy scale (E_{c}=e^{2}/4πε_{0}a, where a=(3/4πn)^{1/3}$ is the distance between nearest neighbors).This places UNPs in the `strongly coupled' regime, defined by Γ=E_{c}/k_{B}T> 1$.Other plasmas in this regime include dense astrophysical systems like white dwarf stars (Γ>10) and laser-produced plasmas relevant for inertial confinement fusion experiments.Plasmas in this regime are not well-described by conventional plasma theory.UNPs are amenable to measurements of quantities that are important for modeling dynamics of more complex strongly coupled systems.These measurements can also be used to test new theories of strongly coupled plasma dynamics.However, DIH limits Γ to values of 3 or lower in UNPs, which has historically limited their effectiveness as a test of these theories.It has also limited the use of UNPs as a tool for obtaining greater understanding of strongly coupled plasma physics in general. Through laser-cooling, we are able to increase Γ to 11, the highest recorded in a UNP system.This brings UNPs deep enough into the strongly coupled regime to serve as a very stringent test of strongly coupled plasma theory.Moreover, this is comparable to values predicted to exist in white dwarf stars, and thus UNPs can be used to directly study the physics relevant to these exotic astrophysical systems. The application of laser-forces to neutral plasmas opens the door to a number of interesting possibilities beyond laser-cooling.For example, these forces may be used to generate a shear-flow, from which one could obtain measurements of viscosity, or a thermal gradient, from which one could obtain measurements of thermal conductivity.We also show that laser forces can inhibit the expansion of the plasma, which should motivate future studies regarding the possibility of optical confinement of a plasma.

[发布机构] Rice University

[效力级别] Plasma