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arXiv:1006.0250 · Physics of Plasmas

Electron evaporation from an ultracold plasma in a uniform electric field

Kevin A. Twedt, S. L. Rolston

Published Aug 1, 2010• 13 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1063/1.3466856

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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 108/cm3). 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.

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Dynamics of low-density ultracold plasmas in externally applied electric and magnetic fields

Truman Wilson, Roberts, Jacob, Krueger, David +2 more

The experiments described in this thesis were focused on the influence of external electric and magnetic fields and electron evaporation on the evolution of ultracold plasmas (UCPs). The UCPs were created from the photoionization of 85Rb which was first captured in a magneto-optical trap (MOT) and then magnetically trapped and transferred by a set of magnetic coils attached to a motorized translation stage to a region of the vacuum chamber with a set of electrodes. The first experiment studied the response of the UCP to sharp electric field pulses, which included 2 cycles of a sine wave pulse. These experiments showed a resonant response to the 2 cycles of rf that was density dependent, but was not a collision based mechanism. Instead, the response was caused by a rapid energy transfer to individual electrons through the collective motion of the electron cloud in the UCP. This density-dependent response allowed us to develop a technique for measuring the expansion rate of the UCPs in our system. It was also observed in second set of experiments that electron evaporation from the UCP had a significant effect on the amount of energy that was transferred to the ions to drive the UCP expansion. Model calculations show that we should expect electron evaporation to have a more significant influence on the UCP expansion rate at the relatively low densities of the UCPs that we create compared to other experiments. By modeling electron evaporation during expansion, our data are consistent with evaporation reducing the electron temperature significantly, which lowers the overall UCP expansion rate. In addition to these studies, we also performed an experiment in which it was observed that in the presence of a magnetic field there was a significant increase in the initial UCP expansion rate coupled with a deceleration of the ion expansion at later times in the UCP evolution. Our observations to date are consistent with the magnetic field influencing electron screening and UCP formation. By restricting the electrons motion in the direction transverse to the magnetic field lines to circular orbits around the magnetic field lines, the electrons cannot move appropriately to screen the internal radial electric fields produced by the excess of ions. Studies of this effect are currently under way. Future studies include direct measurements of the electron temperature and collision rates between the components of the UCP as we move towards trapping the UCP in a Penning trap.

•Jan 3, 2007•1 citation
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.
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 108/cm3). 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.
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.
Electron evaporation plays an important role in the electron temperature evolution and thus the expansion rate of low-density ultracold plasmas. In addition, evaporation is useful as a potential tool for obtaining colder electron temperatures and characterizing plasma parameters. A theoretical treatment of evaporation has been developed for atomic gases and has been applied to a one-component plasma system. We numerically investigate whether such an adapted theory is applicable to ultracold neutral plasmas. We find that it is not due to the violation of fundamental assumptions of the model. The details of our calculations and a discussion of the implications for a simple description of the electron evaporation rate in ultracold plasmas are presented.
Abstract The electron component of an ultracold neutral plasma (UCP) is modelled based on a scalable method using a self-consistently determined mean-field approximation. Representative sampling of discrete electrons within the UCP is used to project the electron spatial distribution onto an expansion of orthogonal basis functions. A collision operator acting on the sample electrons is employed in order to drive the distribution toward thermal equilibrium. These equilibrium distributions can be determined for non-zero electron temperatures even in the presence of spherical symmetry-breaking applied electric fields. This is useful for predicting key macroscopic UCP parameters, such as the depth of the electrons’ confining potential given other plasma parameters. Dynamics such as electron oscillations in UCPs with non-uniform density distributions can also be treated by this model.
The details of recent calculations of the expansion of ultra-cold, neutral plasmas are given. The calculations are performed at several levels. The simplest level assumes an ansatz for the form of the electron and ion density; the result is a simple, ordinary differential equation which can be easily solved. The medium level of sophistication assumes the electrons are in thermal equilibrium but does not assume a particular form for the ion density; the result is a partial differential equation which is solved numerically. For the highest level of sophistication, a Monte Carlo technique is used to solve for the electron phase space distribution and solve for the ion motion in the resulting mean field. All levels of simulation include three body recombination and electron-Rydberg scattering. This paper contains the results of our simulations and compares them to measurements made on ultra-cold plasmas. Three body recombination is found to be important at very low temperatures since it is a heating mechanism for the electron gas. The collisions between cold electrons and Rydberg atoms are another important source of electron heating and de-excitation of atoms formed in the plasma. The evolution of the distribution of atoms in the plasma is simulated and several counter-intuitive effects that have been observed can be explained. Our simulations show that the plasma coupling constant does not become larger than ∼1/5 for the reported experiments. The behavior of plasma processes are investigated, e.g., ion acoustic waves, spike formation, and electron evaporation. The evolution of a cold Rydberg gas into a plasma is also simulated but certain properties of our simulation do not agree with measurements.
The experiments described in this thesis were focused on the influence of external electric and magnetic fields and electron evaporation on the evolution of ultracold plasmas (UCPs). The UCPs were created from the photoionization of 85Rb which was first captured in a magneto-optical trap (MOT) and then magnetically trapped and transferred by a set of magnetic coils attached to a motorized translation stage to a region of the vacuum chamber with a set of electrodes. The first experiment studied the response of the UCP to sharp electric field pulses, which included 2 cycles of a sine wave pulse. These experiments showed a resonant response to the 2 cycles of rf that was density dependent, but was not a collision based mechanism. Instead, the response was caused by a rapid energy transfer to individual electrons through the collective motion of the electron cloud in the UCP. This density-dependent response allowed us to develop a technique for measuring the expansion rate of the UCPs in our system. It was also observed in second set of experiments that electron evaporation from the UCP had a significant effect on the amount of energy that was transferred to the ions to drive the UCP expansion. Model calculations show that we should expect electron evaporation to have a more significant influence on the UCP expansion rate at the relatively low densities of the UCPs that we create compared to other experiments. By modeling electron evaporation during expansion, our data are consistent with evaporation reducing the electron temperature significantly, which lowers the overall UCP expansion rate. In addition to these studies, we also performed an experiment in which it was observed that in the presence of a magnetic field there was a significant increase in the initial UCP expansion rate coupled with a deceleration of the ion expansion at later times in the UCP evolution. Our observations to date are consistent with the magnetic field influencing electron screening and UCP formation. By restricting the electrons motion in the direction transverse to the magnetic field lines to circular orbits around the magnetic field lines, the electrons cannot move appropriately to screen the internal radial electric fields produced by the excess of ions. Studies of this effect are currently under way. Future studies include direct measurements of the electron temperature and collision rates between the components of the UCP as we move towards trapping the UCP in a Penning trap.