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Physics of Plasmas

Plasma oscillations in spherical Gaussian shaped ultracold neutral plasma

Tianxing Chen, Ronghua Lu, Li Shuai Guo, Shensheng Han

Published Apr 1, 2016• 0 citations
Publisher

Abstract

The collective plasma oscillations are investigated in ultracold neutral plasma with a non-uniform density profile. Instead of the plane configuration widely used, we derive the plasma oscillation equations with spherically symmetric distribution and Gaussian density profile. The damping of radial oscillation is found. The Tonks–Dattner resonances of the ultracold neutral plasma with an applied RF field are also calculated.

DOI
10.1063/1.4947542

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The collective plasma oscillations are investigated in ultracold neutral plasma with a non-uniform density profile. Instead of the plane configuration widely used, we derive the plasma oscillation equations with spherically symmetric distribution and Gaussian density profile. The damping of radial oscillation is found. The Tonks–Dattner resonances of the ultracold neutral plasma with an applied RF field are also calculated.
Cold plasma theory is used to calculate the response of an ultracold neutral plasma to an applied rf field. The free oscillation of the system has a continuous spectrum and an associated damped quasimode. This quasimode dominates the driven response and is resonant in the tail of the density distribution. Recent experiments used the plasma response to an applied rf field to determine the plasma density in an expanding ultracold plasma. The comparison between experiment and theory indicates that this method accurately determines the expansion velocity and underestimates the initial plasma density by a factor of 3 in weakly collisional plasmas.
Using both fluid and kinetic descriptions, with repulsive forces between nearby atoms included, we discuss the basic oscillations and waves of a cloud of ultracold atoms confined in a magneto-optical trap. The existence of a hybrid mode, with properties similar to both plasma and acoustic waves, is described in detail. Tonks-Dattner resonances for confined hybrid modes in a spherical cloud are discussed, and the prediction of a nonlinear coupling between the dipole resonance and the hybrid modes is considered. Landau damping processes and quasilinear diffusion in velocity space are also discussed.
Calculations of the spectrum of eigen oscillations of an inhomogeneous ultracold plasma are presented both in the absence and in the presence of charge imbalance. The collective modes of these plasma oscillations are recorded in experiments as absorption resonances of a radio frequency electric field. It is shown that in the presence of friction of the electronic component, a discrete spectrum of plasma eigen oscillations is formed. The dependence of the frequency of these resonances on the plasma expansion time in the presence of a charge imbalance is obtained. There is good agreement with the experiments of different authors.
We report the observation of plasma oscillations in an ultracold neutral plasma. With this collective mode we probe the electron density distribution and study the expansion of the plasma as a function of time. For classical plasma conditions, i.e., weak Coulomb coupling, the expansion is dominated by the pressure of the electron gas and is described by a hydrodynamic model. Discrepancies between the model and observations at low temperature and high density may be due to strong coupling of the electrons.
Ultracold neutral plasmas (UNPs) are created in a vacuum by photoionizing laser-cooled atoms. The expansion of UNPs begins as a non-equilibrium system that quickly relaxes to a local equilibrium state [1] . We aim to quantify and compare the transport processes that govern the relaxation to equilibrium in the expanding UNP. Using a 1D-1V kinetic code in spherical coordinates, we model the radial expansion of an UNP. We compare our simulation results against analytic expressions to determine where the expansion of the UNP is adiabatic or isothermal. We carry out simulations for various initial density distributions such as an exponentially decaying density distribution [2] . We ultimately aim to extend this approach to a 1D-3V multi-component kinetic code that incudes collisions [3] .