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Register a new userSqueezed State of an Electron Cloud as a Quasi-Neutral One-Component Plasma
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We present a one-dimensional model which gives a novel physical interpretation to the specific state of an ensemble of electrons continuously injected into an electrostatic potential well immersed in a strong applied magnetic field preventing radial expansion. When the space-charge field of the electrons accumulated in the potential well compensates the external electrostatic field, a force-free steady-state of the electron cloud forms. This state of equilibrium is known in another context as a squeezed state of an electron beam. It is shown that the spatial distribution of the electron number density in this steady-state correlates with the shape of the potential well. Perturbations of the steady-state propagate along the electron cloud in the form of Trivelpiece-Gould modes.
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Recently a filamentation instability was observed when a laser-generated pair cloud interacted with an ambient plasma. The magnetic field it drove was strong enough to magnetize and accelerate the ambient electrons. It is of interest to determine if and how pair cloud-driven instabilities can accelerate ions in the laboratory or in astrophysical plasma. For this purpose, the expansion of a localized pair cloud with the temperature 400 keV into a cooler ambient electron-proton plasma is studied by means of one-dimensional particle-in-cell (PIC) simulations. The clouds expansion triggers the formation of electron phase space holes that accelerate some protons to MeV energies. Forthcoming lasers might provide the energy needed to create a cloud that can accelerate protons.
We demonstrate magnetic confinement of an ultracold neutral plasma (UCNP) created at the null of a biconic cusp, or quadrupole magnetic field. Initially, the UCNP expands due to electron thermal pressure. As the plasma encounters stronger fields, expansion slows and the density distribution molds to the field. UCNP electrons are strongly magnetized over most of the plasma, while ion magnetization is only significant at the boundaries. Observations suggest that electrons and ions are predominantly trapped by magnetic mirroring and ambipolar electric fields respectively. Confinement times approach 0.5 ms, while unmagnetized plasmas dissipate on a timescale of a few tens of microseconds.
Orbital-free molecular dynamics simulations are used to benchmark two popular models for hot dense plasmas: the one component plasma (OCP) and the Yukawa model. A unified concept emerges where an effective OCP (eOCP) is constructed from the short-range structure of the plasma. An unambiguous ionization and the screening length can be defined and used for a Yukawa system, which reproduces the long range structure with finite compressibility. Similarly, the dispersion relation of longitudinal waves is consistent with the screened model at vanishing wavenumber but merges with the OCP at high wavenumber. Additionally, the eOCP reproduces the overall relaxation timescales of the correlation functions associated with ionic motion. In the hot dense regime, this unified concept of eOCP can be fruitfully applied to deduce properties such as the equation of state, ionic transport coefficients, and the ion feature in x-ray Thomson scattering experiments.
The Yukawa one-component plasma (OCP) is a paradigm model for describing plasmas that contain one component of interest and one or more other components that can be treated as a neutralizing, screening background. In appropriately scaled units, interactions are characterized entirely by a screening parameter, $kappa$. As a result, systems of similar $kappa$ show the same dynamics, regardless of the underlying parameters (e.g., density and temperature). We demonstrate this behavior using ultracold neutral plasmas (UNP) created by photoionizing a cold ($Tle10$ mK) gas. The ions in UNP systems are well described by the Yukawa model, with the electrons providing the screening. Creation of the plasma through photoionization can be thought of as a rapid quench from $kappa_{0}=infty$ to a final $kappa$ value set by the electron density and temperature. We demonstrate experimentally that the post-quench dynamics are universal in $kappa$ over a factor of 30 in density and an order of magnitude in temperature. Results are compared with molecular dynamics simulations. We also demonstrate that features of the post-quench kinetic energy evolution, such as disorder-induced heating and kinetic-energy oscillations, can be used to determine the plasma density and the electron temperature.
We report on the laser-driven generation of purely neutral, relativistic electron-positron pair plasmas. The overall charge neutrality, high average Lorentz factor ($gamma_{e/p} approx 15$), small divergence ($theta_{e/p} approx 10 - 20$ mrad), and high density ($n_{e/p}simeq 10^{15}$cm$^{-3}$) of these plasmas open the pathway for the experimental study of the dynamics of this exotic state of matter, in regimes that are of relevance to electron-positron astrophysical plasmas.
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