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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The charge imbalance in ultracold plasmas

Ultracold plasmas are regarded as quasineutral but not strictly neutral. The results of charge imbalance in the expansion of ultracold plasmas are reported. The calculations are performed by a full molecular-dynamics simulation. The details of the electron velocity distributions are calculated without the assumption of electron global thermal equilibrium and Boltzmann distribution. Spontaneous evolutions of the charge imbalance from the initial states with perfect neutrality are given in the simulations. The expansion of outer plasma slows down with the charge imbalance. The influences of plasma size and parameters on the charge imbalance are discussed. The radial profiles of electron temperature are given for the first time, and the self-similar expansion can still occur even if there is no global thermal equilibrium. The electron disorder induced heating is also found in the simulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Damping of electron center-of-mass oscillation in ultracold plasmas

Applying a short electric field pulse to an ultracold plasma induces an electron plasma oscillation. This manifests itself as an oscillation of the electron center of mass around the ion center of mass in the ultracold plasma. In general, the oscillation can damp due to either collisionless or collisional mechanisms, or a combination of the both. To investigate the nature of oscillation damping in ultracold plasmas, we developed a molecular dynamics model of the ultracold plasma electrons. Through this model, we found that depending on the neutrality of the ultracold plasma and the size of an applied DC electric field, there are some parameter ranges where the damping is primarily collisional and some primarily collisionless. We conducted experiments to compare the measured damping rate with theory predictions and found them to be in good agreement. Extension of our measurements to different parameter ranges should enable studies for strong-coupling influence on electron-ion collision rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Influence of electron evaporative cooling on ultracold plasma expansion

The expansion of ultracold neutral plasmas (UCP) is driven primarily by the thermal pressure of the electron component and is therefore sensitive to the electron temperature. For typical UCP spatial extents, evaporative cooling has a significant influence on the UCP expansion rate at lower densities (less than 10{sup 8}/cm{sup 3}). We studied the effect of electron evaporation in this density range. Owing to the low density, the effects of three-body recombination were negligible. We modeled the expansion by taking into account the change in electron temperature owing to evaporation as well as adiabatic expansion and found good agreement with our data. We also developed a simple model for initial evaporation over a range of ultracold plasma densities, sizes, and electron temperatures to determine over what parameter range electron evaporation is expected to have a significant effect. We also report on a signal calibration technique, which relates the signal at our detector to the total number of ions and electrons in the ultracold plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Ultracold plasma expansion as a function of charge neutrality

Ultracold plasmas (UCPs) are created under conditions of near but not perfect neutrality. In the limit of zero electron temperature, electron screening results in non-neutrality manifesting itself as an interior region of the UCP with both electrons and ions and an exterior region composed primarily of ions. The interior region is the region of the most scientific interest for 2-component ultracold plasma physics. This work presents a theoretical model through which the time evolution of non-neutral UCPs is calculated. Despite Debye screening lengths much smaller than the characteristic plasma spatial size, model calculations predict that the expansion rate and the electron temperature of the UCP interior is sensitive to the neutrality of the UCP. The predicted UCP dependence on neutrality has implications for the correct measurement of several UCP properties, such as electron temperature, and a proper understanding of evaporative cooling of the electrons in the UCP.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Characteristic features of temperature evolution in ultracold plasmas

The theoretical interpretation of recent experiments on the time evolution of the temperature in freely expanding, ultracold plasma clouds released from a magneto-optical trap is discussed. The most interesting result of those experiments was the asymptotic behavior T{sub e} {proportional_to} t{sup -(1,2{+-}0.1)}, instead of the behavior proportional to t{sup -2}, which was expected for a rarefied monatomic gas in the inertial expansion stage. It is shown that such a substantially slower temperature fall can be well explained by the specific properties of the equation of state of ultracold plasma with a large Coulomb coupling parameter; whereas the heat release in inelastic processes (in particular, three-body recombination) turns out to be relatively unimportant in the first approximation. This conclusion is confirmed by approximate analytic estimates from the model of virializing the energies of charged particles and also by the results of ab initio computer simulations; moreover, the computations demonstrate that the law of decrease in the electron temperature is established very rapidly, when the virialization criterion begins to be satisfied only to within a factor of order unity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Electron distribution function and recombination coefficient in ultracold plasma in a magnetic field

The electron distribution function and diffusion coefficient in energy space have been calculated for the first time for a weakly coupled ultracold plasma in a magnetic field in the range of magnetic fields B = 100-50000 G for various temperatures. The dependence of these characteristics on the magnetic field is analyzed and the distribution function is shown to depend on the electron energy shift in a magnetic field. The position of the 'bottleneck' of the distribution function has been found to be shifted toward negative energies with increasing magnetic field. The electron velocity autocorrelators as a function of the magnetic field have been calculated; their behavior suggests that the frequency of collisions between charged particles decreases significantly with increasing magnetic field. The collisional recombination coefficient {alpha}{sub B} has been calculated in the diffusion approximation for a weakly coupled ultracold plasma in a magnetic field. An increase in magnetic field is shown to lead to a decrease in {alpha}{sub B} and this decrease can be several orders of magnitude.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Electron evaporation from an ultracold plasma in a uniform electric field

Electrons in an expanding ultracold plasma are expected to be in quasiequilibrium since the collision times are short compared to the plasma lifetime, yet electron evaporation is observed as the ion density decreases during expansion. A small electric field that shifts the electron cloud with respect to the ions increases the evaporation rate. Treating the electrons as a zero-temperature fluid, their spatial distribution is calculated as a function of the applied field strength and the ion density. The zero-temperature approximation gives the maximum number of electrons the plasma can hold in the absence of evaporation. By applying this calculation at all times during expansion, the flux of cold electrons from the plasma is predicted and found to be in good agreement with the observed electron signal.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Absorption of resonance laser radiation in ultracold plasma

The absorption of resonance laser radiation in an expanding ultracold plasma is considered. Initially the optical thickness τ {sub 0} of the medium decreases due to correlation heating of the ions; the subsequent lowering of τ {sub 0} is due to the variation of radial ion velocities. The expanding optically thick medium is shown to become transparent when the ratio between the particle expansion velocity at the sphere boundary and the thermal velocity exceeds the optical thickness τ {sub 0}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Rydberg-atom formation in strongly correlated ultracold plasmas

In plasmas at very low temperatures, the formation of neutral atoms is dominated by collisional three-body recombination, owing to the strong {approx}T{sup -9/2} scaling of the corresponding recombination rate with the electron temperature T. While this law is well established at high temperatures, the unphysical divergence as T{yields}0 clearly suggests a breakdown in the low-temperature regime. Here, we present a combined molecular dynamics Monte Carlo study of electron-ion recombination over a wide range of temperatures and densities. Our results reproduce the known behavior of the recombination rate at high temperatures, but reveal significant deviations with decreasing temperature. We discuss the fate of the kinetic bottleneck and resolve the divergence problem as the plasma enters the ultracold, strongly coupled domain.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Distribution function and diffusion of electrons in the Rydberg energy space in a nonideal ultracold plasma

The distribution functions and diffusion coefficient of electrons in the Rydberg energy space in an ultracold plasma are obtained as functions of temperature by numerically solving a system of kinetic balance equations. As the initial conditions, the results obtained in previous papers by the molecular dynamics method are used. From calculation of Rydberg electron fluxes, the temperature dependence of the recombination coefficient is obtained, in good agreement with the recombination coefficient calculated previously by the molecular dynamics method.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Probing the fields in an ultracold plasma by microwave spectroscopy

By observing the Stark shift of the Rb 42s{sub 1/2}-42p{sub 1/2} microwave transition we have measured the electric fields in an ultracold plasma formed by photoionizing Rb atoms in a magneto-optical trap. Using this approach and a model of the electron charge distribution we have been able to measure both the microscopic fields due to nearest-neighbor ions and the macroscopic fields due to the charge imbalance in the plasma.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Three-dimensional imaging of the ultracold plasma formed in a supersonic molecular beam

Double-resonant excitation of nitric oxide in a seeded supersonic molecular beam forms a state-selected Rydberg gas that evolves to form an ultracold plasma. This plasma travels with the propagation of the molecular beam in z over a variable distance as great as 600 mm to strike an imaging detector, which records the charge distribution in the dimensions, x and y. The ω{sub 1} + ω{sub 2} laser crossed molecular beam excitation geometry convolutes the axial Gaussian distribution of NO in the molecular beam with the Gaussian intensity distribution of the perpendicularly aligned laser beam to create an ellipsoidal volume of Rydberg gas. Detected images describe the evolution of this initial density as a function of selected Rydberg gas initial principal quantum number, n{sub 0}, ω{sub 1} laser pulse energy (linearly related to Rydberg gas density, ρ{sub 0}) and flight time. Low-density Rydberg gases of lower principal quantum number produce uniformly expanding, ellipsoidal charge-density distributions. Increase either of n{sub 0} or ρ{sub 0} breaks the ellipsoidal symmetry of plasma expansion. The volume bifurcates to form repelling plasma volumes. The velocity of separation depends on n{sub 0} and ρ{sub 0} in a way that scales uniformly with ρ{sub e}, the density of electrons formed in the core of the Rydberg gas by prompt Penning ionization. Conditions under which this electron gas drives expansion in the long axis dimension of the ellipsoid favours the formation of counter-propagating shock waves.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Dynamical control in a prethermalized molecular ultracold plasma: Local dissipation drives global relaxation

Prethermalization occurs as an important phase in the dynamics of isolated many-body systems when coupled degrees of freedom relax in a subspace separated from a state of complete thermodynamic equilibrium by a gap in energy or other conserved quantity. Slow equilibration from a prethermal phase can localize the dynamics of natural and model systems despite high dimensionality and limited disorder. Here, we report the signature of an enduring prethermal regime of arrested relaxation in the molecular ultracold plasma that forms following the avalanche of a state-selected Rydberg gas of nitric oxide. For a wide range of initial conditions, this system enters a critical phase in which a density of NO + and electrons balances a population of Rydberg molecules. Electron collisions mix orbital angular momentum, scattering Rydberg electrons to states of very high ℓ . The rapid predissociation of molecules in states of low- ℓ purifies this high- ℓ ensemble. The angular momentum barrier that separates NO + ions and electrons creates an extraordinary gap between the plasma states of n ≈ ℓ , with measured n > 200 and penetrating states of ℓ = 0 , 1 , and 2. Evolution to a statistically equilibrated state of N and O atoms cannot occur without Rydberg electron penetration, and this gap blocks relaxation for a millisecond or more. A weak radiofrequency (RF) field drives ℓ -mixing electron collisions that erase this gap, causing wholesale dissipation. Remarkably, a local quantum-state transition promoting an exceedingly small fraction of the molecules in the prethermalized ensemble to a predissociative state also acts with a global effect, driving the entire system to a dissipative equilibrium. Using the Lindblad master equation, we illustrate qualitatively similar dynamics for a toy model of an open quantum system that consists of a localized set of spins on which dissipation acts at a single site as a gateway to equilibrium. Published by the American Physical Society 2025

Wang, Ruoxi↗
DOE OSTI2011

Density and temperature scaling of disorder-induced heating in ultracold plasmas

We report measurements and simulations of disorder-induced heating in ultracold neutral plasmas. Fluorescence from plasma ions is excited using a detuned probe laser beam while the plasma relaxes from its initially disordered nonequilibrium state. This method probes the wings of the ion velocity distribution. The simulations yield information on time-evolving plasma parameters that are difficult to measure directly and make it possible to connect the fluorescence signal to the rms velocity distribution. The disorder-induced heating signal can be used to estimate the electron and ion temperatures {approx}100 ns after the plasma is created. This is particularly interesting for plasmas in which the electron and ion temperatures are not known.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Exploring the crossover between high-energy-density plasma and ultracold neutral plasma physics

In this paper, we present ideas that were part of the miniconference on the crossover between High Energy Density Plasmas (HEDP) and Ultracold Neutral Plasmas (UNPs) at the 60th Annual Meeting of the American Physical Society Division of Plasma Physics, November 2018. We give an overview of UNP experiments with an emphasis on measurements of the time-evolving ion density and velocity distributions, the electron-ion thermalization rate, and plasma self-assembly—all just inside the strongly coupled plasma regime. We also present theoretical and computational models that were developed to understand a subset of HEDP experiments. However, because HEDP experiments display similar degrees of strong coupling, many aspects of these models can be vetted using precision studies of UNPs. This comparison is important because some statistical assumptions used for ideal plasmas are of questionable validity in the strongly coupled plasma regime. We summarize two theoretical approaches that extend kinetic theories into the strong-coupling regime and show good agreement for momentum transfer and self-diffusion. As capabilities improve, both computationally and experimentally, UNP measurements may help guide the ongoing development of HEDP-appropriate plasma models. Future opportunities in viscosity, energy relaxation, and magnetized plasmas are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Collisional recombination coefficient in an ultracold plasma: Calculation by the molecular dynamics method

New results of the calculations of the distribution function and the electron diffusion coefficient in energy space are presented. We analyze all of the data obtained and calculate the temperature dependence of the recombination coefficient. This dependence coincides with the Gurevich-Pitaevsky analytical formula in the region of a weakly coupled plasma and begins to differ in the direction of a decline in the region of a strongly coupled plasma; the difference can reach several orders of magnitude.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Mechanism for the Efficient Homogeneous Nucleation of Ice in a Weakly Ionized, Ultracold Plasma

Abstract It is proposed that the rapid observed homogeneous nucleation of ice dust in a cold, weakly ionized plasma depends on the formation of hydroxide (OH − ) by fast electrons impacting water molecules. These OH − ions attract neutral water molecules because of the high dipole moment of the water molecules and so hydrates of the form (OH) − (H 2 O) n are formed. The hydrates continuously grow in the cold environment to become macroscopic ice grains. These ice grains are negatively charged as a result of electron impact and so continue to attract water molecules. Because hydroxide is a negative ion, unlike positive ions, it does not suffer recombination loss from collision with plasma electrons. Recombination with positive ions is minimal because positive ions are few in number (weak ionization) and slow-moving as result of being in thermal equilibrium with the cold background gas.

79 ASTRONOMY AND ASTROPHYSICS↗