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Study of background gas collisions in atomic traps

Janelle Van Dongen

Published Jan 1, 2014• 3 citations
Publisher

Abstract

This thesis describes an investigation and application of the loss of laser cooled atoms from a trap induced by background collisions. The loss rate constant depends on the density of background gas and the velocity averaged collisional loss cross section due to collisions. The velocity averaged collisional loss cross section can be calculated and its dependence on trap depth was verified using a magneto-optical trap. This verification involved measurements of the loss rate constant for a quadrupole magnetic and a magneto-optical trap and measurement of the density of Ar background gas using a residual gas analyzer. The second part of the thesis focuses on an application of these measurements of the loss rate constant to measure the pressure of the background gas. The experimental progress to date on the atom pressure sensor is provided.

DOI
10.14288/1.0167508

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This thesis describes an investigation and application of the loss of laser cooled atoms from a trap induced by background collisions. The loss rate constant depends on the density of background gas and the velocity averaged collisional loss cross section due to collisions. The velocity averaged collisional loss cross section can be calculated and its dependence on trap depth was verified using a magneto-optical trap. This verification involved measurements of the loss rate constant for a quadrupole magnetic and a magneto-optical trap and measurement of the density of Ar background gas using a residual gas analyzer. The second part of the thesis focuses on an application of these measurements of the loss rate constant to measure the pressure of the background gas. The experimental progress to date on the atom pressure sensor is provided.
This thesis describes a method of using an ultra-cold ensemble of atoms confined in a trap as an atomic primary pressure standard. The development of the standard and its current status are described in detail. This standard uses a 3D MOT to trap ⁸⁷Rb and then transfer them to a quadrupole magnetic trap where the atoms undergoes collisions with a background gas. By measuring the number of atoms left in the magnetic trap as a function of time one extract a loss rate and from this rate determine the background gas density. This loss rate is a product of the density of the background, multiplied by the loss cross section averaged over the velocity distribution of the background gas. By computing the average loss cross section in the magnetic trap and measuring the loss rate, the density of the background gas can be determined. This gives a calibration free measurement of density of a background gas in the UHV range (10-⁶ ‑ 10-⁹) Torr or (10-⁴ - 10-⁷) Pa which allows for it to be used as a standard. In conjunction with this, preparation of the atoms prior to the loss rate measurement is investigated to ensure accuracy and reproducibility of the standard. Finally a comparison between UBC's atomic standard and NIST's (National Institute for Standards and Technology) orifice flow standard is conducted via an ionization gauge which employed as a transfer standard.All measurement are carried out using Argon gas as the background gas of study.
The population dynamics of a Magneto-Optical Trap (MOT) make it a potential candidate for flux measurements of an atomic beam. This is achieved by determining the collisional cross section of the trapped atom and the beam particle which would result in ejection of a trapped atom. Due to the properties of a MOT it is possible to make spatial and time-of-flight profiles of the beam using this technique. The work of this thesis explores the collisional cross sections and flux profiles of several gaseous beams with a MOT of ⁸⁵Rb or ⁸⁷Rb. Each of the beams, generated through supersonic expansion, produced a loss cross section on the order of the combined van-der-Waals radii of the two particles. The flux and time-of-flight information of the beam was verified with a Residual Gas Analyser (RGA) and high beam rep rate pressure measurements. The MOT was characterized through a combination of fluorescence detection for population and a catalysis process for the trap's depth. A custom built translation mechanism for the MOT's optics and Helmholtz coils was constructed to perform the profiling measurements.
This thesis discusses a new technique for measuring collision cross sections, using a magnetooptical trap (MOT) or magnetic trap as well as theoretical predictions for the resulting measurements based on scattering theory. The initial discussion focuses on a theoretical overview of the principles involved in laser cooling and trapping, magnetic trapping, and scattering theory. Following this, a detailed overview of the experimental instruments and methods describing the vacuum system, laser light, magnetic coils, imaging and control systems will be presented. In addition to an experimental overview, the methods and mechanical formulation involved in predicting cross sections and loss rates based on scattering theory are discussed. Finally, an overview of both experimental and theoretical results including cross sections and loss rates for the case of trapped 87Rb and background argon or helium gas will be given. The results gathered in this thesis show a strong dependence of elastic collision cross section on trap depth, which indicates that the majority of elastic collisions between ultra-cold rubidium and room temperature argon impart energy below 1K. These results suggest that the magnetic trap may be a better tool for measuring total elastic collision cross sections.
We present a simple, reliable, and nondestructive method for the measurement of vacuum pressure in a magneto-optical trap. The vacuum pressure is verified to be proportional to the collision rate constant between cold atoms and the background gas with a coefficient k, which can be calculated by means of the simple ideal gas law. The rate constant for loss due to collisions with all background gases can be derived from the total collision loss rate by a series of loading curves of cold atoms under different trapping laser intensities. The presented method is also applicable for other cold atomic systems and meets the miniaturization requirement of commercial applications.
The loss rate of cold atoms in a trap due to residual gas collisions differs from that in a free state after the cold atoms are released from the trap. In this paper, the loss rate in a cold rubidium-87 atom cloud was measured in a magneto-optical trap (MOT) and during its free flight. The residual gas pressure was analyzed by a residual gas analyzer, and the pressure distribution in a vacuum chamber was numerically calculated by the angular coefficient method. The decay factor, which describes the decay behavior of cold atoms due to residual gas collisions during a free flight, was calculated. It was found that the decay factor agrees well with theoretical predictions under various vacuum conditions.
We present measurements and calculations of the trap loss rate for laser-cooled 87Rb^{87}\text{R}\text{b}87Rb atoms confined in either a magneto-optic trap (MOT) or a magnetic quadrupole trap when exposed to a room-temperature background gas of Ar. We study the loss rate as a function of trap depth and find that copious glancing elastic collisions, which occur in the so-called quantum diffractive regime and impart very little energy to the trapped atoms, result in significant differences in the loss rate for the MOT compared to a pure magnetic trap due solely to the difference in potential depth. This finding highlights the importance of knowing the trap depth when attempting to infer the total collision cross section from measurements of trap loss rates. Moreover, this variation in trap loss rate with trap depth can be used to extract information about the differential cross section.