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Evaporative Cooling in Electromagnetic Radio Frequency Traps

Lohrasp Seify

Published Jan 1, 2016• 0 citations
Publisher

Abstract

Paul traps are used in many different fields of physics to contain ions for different purposes. Give the oscillating fields that this type of trap employs to trap an ensemble and the heating that the interactions of these fields with multiple ions can cause, cooling the system could be of great interest, and there are several methods of doing so. This study explores on of these methods, evaporative cooling, applied to a type of Paul trap, a Linear Paul Trap (LPT), using computer simulations. This thesis will first describe a trapped ion ensemble and an evaporation event both physically and how it is computationally modeled. IT then proceeds to discuss the conditions for which evaporative cooling applied to an LPT could be optimized to maximize the chances of achieving highest temperature per particle evaporated.

DOI
10.11575/prism/25996

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Paul traps are used in many different fields of physics to contain ions for different purposes. Give the oscillating fields that this type of trap employs to trap an ensemble and the heating that the interactions of these fields with multiple ions can cause, cooling the system could be of great interest, and there are several methods of doing so. This study explores on of these methods, evaporative cooling, applied to a type of Paul trap, a Linear Paul Trap (LPT), using computer simulations. This thesis will first describe a trapped ion ensemble and an evaporation event both physically and how it is computationally modeled. IT then proceeds to discuss the conditions for which evaporative cooling applied to an LPT could be optimized to maximize the chances of achieving highest temperature per particle evaporated.
A theoretical description of a specific method of laser cooling of negative ions is presented based on evaporation through photodetachment. The ideal case of harmonically bound ions and the case of RF traps are considered in the weak space charge limit. Evolution equations of the ion cloud in the quasistatic regime are derived, in which eventually other cooling or heating processes could be inserted. These equations are analytically solved in a particular case where the laser is tuned slightly below photodetachment threshold with a gradually decreasing absolute value of the detuning. Numerical results concerning the evolution of ion cloud temperature and density are given and discussed for either s or p photodetached electrons. The influence of laser beam geometry and in particular the existence, beside the previously considered 'Doppler' cooling, of 'geometrical' cooling is investigated. In Paul traps, RF heating might impose a preliminary cooling by another method.
Sympathetic laser cooling of ions stored within a linear-geometry, radio frequency, electric-quadrupole trap has been investigated using computational and theoretical techniques. The simulation, which allows five sample ions to interact with 35 laser-cooled atomic ions, revealed an instability heating mechanism, which can prevent ions below a certain critical mass from being sympathetically cooled. This critical mass can, however, be varied by changing the trapping field parameters, thus allowing ions with a very large range of masses to be sympathetically cooled using a single ion species. A theoretical explanation of this instability heating mechanism is presented which predicts that the cooling-heating boundary in trapping parameter space is a line of constant qu{q}_{u}qu​ (ion trap stability coefficient), a result supported by the computational results. The threshold value of qu{q}_{u}qu​ depends on the masses of the interacting ions. A functional form of this dependence is given.
Linear Paul traps use radio frequency fields to confine ions for long times in deep potentials. The simplicity and versatility of these devices has led to their use in a wide variety of fields, from mass spectroscopy to quantum information. Within the past fifteen years, beginning with pioneering work at UConn, Paul traps have been combined with atom traps to form hybrid atom-ion traps. Hybrid traps serve as platforms for measuring ion-atom interaction rates, sympathetically cooling atomic and molecular ions, and studying the behavior of trapped ions. This work discusses simulations, analytical theory, and experiments in a hybrid trap that show a previously unknown dynamical effect in a population of trapped ions. The steady state trapped ion number in a Paul trap is non-monotonic with the loading rate of the Paul trap. This effect is universal to both Linear and 3D Paul traps and is likely to be found in other types of ion traps as well because it is the result of the Coulomb interaction in the presence of a trap. This behavior also affects the decay of ions from the trap. Understanding the loading and decay behavior from a saturated Linear Paul trap is important for the study of ion-atom interactions in a hybrid trap when the ions lack optical transitions.
In order to demonstrate laser cooling on negatively charged ions, it is necessary to confine the ions in a region free of strong magnetic fields. In the course of this thesis, the pre-existing design of a linear Paul trap was modified and taken into operation. The new design possesses two additional end cap electrodes whose shape provides effective axial confinement without blocking optical access to the trapped particles. The influence of these end cap electrodes on the trapping potential was simulated and compared to experimental results. Measurements with AU- ions show that the trap allows storage of ions for multiple hours. Analysis ofthe storage time suggests that the loading process heats up the trapped ions. The lifetime τHot of the hot ions was determined to be 23(3) min. After thermalization, the lifetime increases to 75(2) min. Measurements of the ion cloud’s radius provide an estimation of the equilibrium temperature T∞ ≈5000 K. The experimentally determined time constant of the cooling process is τcool = 41:6(4) s.
Ions in radio-frequency traps are widely used in various fields of applied and fundamental physics, such as metrology and quantum computing. One of the important tasks required for modern experiments is deep cooling of ion crystals. The results of simulations of an increase in the efficiency of deep cooling of linear ion crystals by the method of electromagnetically induced transparency (EIT cooling) by imposing an optical lattice on the radio-frequency trap have been reported. It has been shown that this method makes it possible to narrow the frequency range occupied by various vibrational modes of ions and to increase their axial frequencies of motion without violating the linear configuration of the crystal. Thus, for a crystal of eight ions in a Paul trap with secular frequencies ω z = 2 π × 100 kHz and ω r = 2 π × 650 kHz, the application of an optical lattice allows the reduction of the frequency range occupied by vibrational modes by a factor of 2. The dependence of the optimal power of the optical lattice for narrowing the vibrational spectrum on the number of particles in the trap and its parameters has been investigated.
We report the first cooling of atomic anions by laser radiation. O^{-} ions confined in a linear Paul trap were cooled by selectively photodetaching the hottest particles. For this purpose, anions with the highest total energy were illuminated with a 532 nm laser at their maximal radial excursion. Using laser-particle interaction, we realized a both colder and denser ion cloud, achieving a more than threefold temperature reduction from 1.15 to 0.33 eV. Compared with the interaction with a dilute buffer gas, the energy-selective addressing and removal of anions resulted in lower final temperatures, yet acted 10 times faster and preserved twice as large a fraction of ions in the final state. An ensemble of cold negative ions affords the ability to sympathetically cool any other negative ion species, enabling or facilitating a broad range of fundamental studies from interstellar chemistry to antimatter gravity. The technique can be extended to any negative ion species that can be neutralized via photodetachment.