Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physics of Plasmas

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

Patrick McQuillen, Jose Castro, Trevor Strickler, S. J. Bradshaw, T. C. Killian

Published Apr 1, 2013• 11 citations
Publisher

Abstract

We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma.

DOI
10.1063/1.4802813

Related papers

Creating and studying ion acoustic waves in ultracold neutral plasmas

T. C. Killian, Patrick McQuillen, Thomas M. O’Neil +1 more

We excite ion acoustic waves in ultracold neutral plasmas by imprinting density modulations during plasma creation. Laser-induced fluorescence is used to observe the density and velocity perturbations created by the waves. The effect of expansion of the plasma on the evolution of the wave amplitude is described by treating the wave action as an adiabatic invariant. After accounting for this effect, we determine that the waves are weakly damped, but the damping is significantly faster than expected for Landau damping.

arXiv:1201.0786•Physics of Plasmas•Mar 23, 2012•24 citations

Wave steepening and shock formation in ultracold neutral plasmas

M. K. Warrens, Nina P. Inman, Grant M. Gorman +3 more

We present observations of wave steepening and signatures of shock formation during expansion of ultracold neutral plasmas formed with an initial density distribution that is centrally peaked and decays exponentially with distance. The plasma acceleration and velocity decrease at large distance from the plasma center, leading to central ions overtaking ions in the outer regions and the development of a steepening front that is narrow compared to the size of the plasma. The density and velocity change dramatically across the front, and significant heating of the ions is observed in the region of steepest gradients. For a reasonable estimate of electron temperature, the relative velocity of ions on either side of the front modestly exceeds the local sound speed (Mach number M≳1). This indicates that by sculpting steep density gradients, it is possible to create the conditions for shock formation, or very close to it, opening a new avenue of research for ultracold neutral plasmas.

arXiv:2409.11319•Physics of Plasmas•Nov 1, 2024•2 citations

Collective effects in ultracold neutral plasmas

Castro Nieto, José Antonio

This thesis describes the measurements of collective effects in strongly coupled ultra-cold neutral plasmas (UNPs). It shows the implementation of experimental techniques that perturb either the density or velocity distribution of the plasma and it describes the subsequent excitation, observation and analysis of the aforementioned collective phenomena. UNPs are interesting in that they display physics of strongly coupled systems. For most plasma systems, collective effects are well described with classical hydrodynamic or kinetic descriptions. However, for strongly coupled systems, the Coulomb interaction energy between nearest neighbors exceeds the kinetic energy, and these descriptions must be modified as the plasma crosses over from a gas-like to liquid-like behavior. Strongly coupling can be found in exotic plasma systems found astrophysics, dusty plasmas, non-neutral trapped ion plasmas, intense-laser/matter interactions and inertial confinement fusion experiments. Compared to other strongly coupled plasmas, UNPs are ideal for studying collective effects in this regime since they have lower timescales, precisely controllable initial conditions and non-invasive diagnostics. Previous studies of UNPs concentrated on plasma expansion dynamics and some collective effects such as disorder induced heating, but little work had been done in relaxation or collision rates and collective modes in UNPs. This thesis presents a method for measuring collision rates by perturbing the velocity distribution of the plasma, observing plasma relaxation and measuring the relaxation rate. It also presents a new technique for observing collective modes in the plasma by perturbing the initial density of the plasma and how this results in the excitation of ion acoustic waves and a measurement of its dispersion relation. Finally, this thesis presents how this last technique can be used to create a gap in the center of the plasma and how this leads to hole propagation and plasma streaming and presents a characterization of both phenomena. The result of these experiments will be valuable for predicting the behavior of collective effects in other strongly coupled plasmas and for comparison with theories that describe them.

•Jan 1, 2011•1 citation

Ion Acoustic Waves in Ultracold Neutral Plasmas

Jose Castro, Patrick McQuillen, T. C. Killian

We photoionize laser-cooled atoms with a laser beam possessing spatially periodic intensity modulations to create ultracold neutral plasmas with controlled density perturbations. Laser-induced fluorescence imaging reveals that the density perturbations oscillate in space and time, and the dispersion relation of the oscillations matches that of ion acoustic waves, which are long-wavelength, electrostatic, density waves.

arXiv:1005.4670•Physical Review Letters•Aug 6, 2010•80 citations

Expansion and Shockwave Development in Ultracold Neutral Plasmas with an Initial Exponentially Decaying Density Distribution

Mackenzie Kate Warrens

Ultracold neutral plasmas (UNPs), which are formed by photoionizing a cloud of cold atoms, have been a powerful platform for studying a wide variety of plasma phenomena. Two areas of interest include expansion into vacuum and the development of shockwaves. Previous UNP experiments have used plasmas with an initial Gaussian density distribution. UNPs with a radially exponentially decaying density distribution, or exponential UNPs, can be formed by photoionizing atoms out of a purely magnetic trap. This dissertation studies the expansion in free space of exponential UNPs as well as the development and characterization of shockwaves in exponential UNPs. The free space expansion of an exponential UNP is compared to the well-known self-similar hydrodynamic expansion of a Gaussian plasma. Similar scaling laws describe the size evolution well for both cases. The evolution of the plasma size and velocity show a characteristic ion acoustic time scale, indicating that the plasma is generally well-described by a hydrodynamic description. While the exponential UNP is predominantly in the hydrodynamic regime, heating at the central peak for low-density, high-electron temperature suggests significant local non-neutrality at early times. This thesis presents the first observation of a shockwave in a UNP. The evolution of the density and velocity for exponential UNPs show signs of wave steepening. A significant density and velocity jump over a narrow region develops. These occur at the same location in the plasma. Additionally, a large spike in the ion temperature occurs at this front. The relative ion velocity across the front modestly surpasses the local sound speed. The initial conditions were varied to characterize the development of shockwaves. Varying the density and electron temperature did not impact shock formation for the ranges used. Changing the shape caused the shocks to disappear as the density gradient decreased. This suggests that the electron thermal pressure gradient plays a significant role in the shock formation. The establishment of shock formation in UNPs opens a new avenue of research for UNPs.

•Apr 24, 2025•0 citations

High-resolution ionization of ultracold neutral plasmas

P McQuillen, J Castro, T. C. Killian

Collective effects, such as waves and instabilities, are integral to our understanding of most plasma phenomena. We have been able to study these in ultracold neutral plasmas by shaping the initial density distribution through spatial modulation of the ionizing laser intensity. We describe a relay imaging system for the photoionization beam that allows us to create higher resolution features and its application to extend the observation of ion acoustic waves to shorter wavelengths. We also describe the formation of sculpted density profiles to create fast expansion of plasma into vacuum and streaming plasmas.

arXiv:1201.0168•Journal of Physics B Atomic Molecular and Optical Physics•Sep 14, 2011•18 citations
We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma.
We excite ion acoustic waves in ultracold neutral plasmas by imprinting density modulations during plasma creation. Laser-induced fluorescence is used to observe the density and velocity perturbations created by the waves. The effect of expansion of the plasma on the evolution of the wave amplitude is described by treating the wave action as an adiabatic invariant. After accounting for this effect, we determine that the waves are weakly damped, but the damping is significantly faster than expected for Landau damping.
We present observations of wave steepening and signatures of shock formation during expansion of ultracold neutral plasmas formed with an initial density distribution that is centrally peaked and decays exponentially with distance. The plasma acceleration and velocity decrease at large distance from the plasma center, leading to central ions overtaking ions in the outer regions and the development of a steepening front that is narrow compared to the size of the plasma. The density and velocity change dramatically across the front, and significant heating of the ions is observed in the region of steepest gradients. For a reasonable estimate of electron temperature, the relative velocity of ions on either side of the front modestly exceeds the local sound speed (Mach number M≳1). This indicates that by sculpting steep density gradients, it is possible to create the conditions for shock formation, or very close to it, opening a new avenue of research for ultracold neutral plasmas.
This thesis describes the measurements of collective effects in strongly coupled ultra-cold neutral plasmas (UNPs). It shows the implementation of experimental techniques that perturb either the density or velocity distribution of the plasma and it describes the subsequent excitation, observation and analysis of the aforementioned collective phenomena. UNPs are interesting in that they display physics of strongly coupled systems. For most plasma systems, collective effects are well described with classical hydrodynamic or kinetic descriptions. However, for strongly coupled systems, the Coulomb interaction energy between nearest neighbors exceeds the kinetic energy, and these descriptions must be modified as the plasma crosses over from a gas-like to liquid-like behavior. Strongly coupling can be found in exotic plasma systems found astrophysics, dusty plasmas, non-neutral trapped ion plasmas, intense-laser/matter interactions and inertial confinement fusion experiments. Compared to other strongly coupled plasmas, UNPs are ideal for studying collective effects in this regime since they have lower timescales, precisely controllable initial conditions and non-invasive diagnostics. Previous studies of UNPs concentrated on plasma expansion dynamics and some collective effects such as disorder induced heating, but little work had been done in relaxation or collision rates and collective modes in UNPs. This thesis presents a method for measuring collision rates by perturbing the velocity distribution of the plasma, observing plasma relaxation and measuring the relaxation rate. It also presents a new technique for observing collective modes in the plasma by perturbing the initial density of the plasma and how this results in the excitation of ion acoustic waves and a measurement of its dispersion relation. Finally, this thesis presents how this last technique can be used to create a gap in the center of the plasma and how this leads to hole propagation and plasma streaming and presents a characterization of both phenomena. The result of these experiments will be valuable for predicting the behavior of collective effects in other strongly coupled plasmas and for comparison with theories that describe them.
Ultracold neutral plasmas (UNPs), which are formed by photoionizing a cloud of cold atoms, have been a powerful platform for studying a wide variety of plasma phenomena. Two areas of interest include expansion into vacuum and the development of shockwaves. Previous UNP experiments have used plasmas with an initial Gaussian density distribution. UNPs with a radially exponentially decaying density distribution, or exponential UNPs, can be formed by photoionizing atoms out of a purely magnetic trap. This dissertation studies the expansion in free space of exponential UNPs as well as the development and characterization of shockwaves in exponential UNPs. The free space expansion of an exponential UNP is compared to the well-known self-similar hydrodynamic expansion of a Gaussian plasma. Similar scaling laws describe the size evolution well for both cases. The evolution of the plasma size and velocity show a characteristic ion acoustic time scale, indicating that the plasma is generally well-described by a hydrodynamic description. While the exponential UNP is predominantly in the hydrodynamic regime, heating at the central peak for low-density, high-electron temperature suggests significant local non-neutrality at early times. This thesis presents the first observation of a shockwave in a UNP. The evolution of the density and velocity for exponential UNPs show signs of wave steepening. A significant density and velocity jump over a narrow region develops. These occur at the same location in the plasma. Additionally, a large spike in the ion temperature occurs at this front. The relative ion velocity across the front modestly surpasses the local sound speed. The initial conditions were varied to characterize the development of shockwaves. Varying the density and electron temperature did not impact shock formation for the ranges used. Changing the shape caused the shocks to disappear as the density gradient decreased. This suggests that the electron thermal pressure gradient plays a significant role in the shock formation. The establishment of shock formation in UNPs opens a new avenue of research for UNPs.
Collective effects, such as waves and instabilities, are integral to our understanding of most plasma phenomena. We have been able to study these in ultracold neutral plasmas by shaping the initial density distribution through spatial modulation of the ionizing laser intensity. We describe a relay imaging system for the photoionization beam that allows us to create higher resolution features and its application to extend the observation of ion acoustic waves to shorter wavelengths. We also describe the formation of sculpted density profiles to create fast expansion of plasma into vacuum and streaming plasmas.