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Review of Scientific Instruments

Ion imaging in a high-gradient magnetic guide

Varun Vaidya, Mallory Traxler, Cornelius Hempel, Rahul R. Mhaskar, Georg A. Raithel

Published Apr 1, 2010• 3 citations
Publisher

Abstract

We study a photoionization method to detect and image a narrow beam of cold atoms traveling along a high-gradient two-wire magnetic guide that is continuously on. Ions are accelerated in a compact acceleration region, directed through a drift region several centimeters in length, and detected using a position-sensitive ion detector. The potentials of several electrodes can be varied to adjust the imaging properties. Using ion trajectory simulations as well as experiments, we study the passage of the ions through the detection system, the magnification of the detection system, and the time-of-flight characteristics.

DOI
10.1063/1.3386583

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We study a photoionization method to detect and image a narrow beam of cold atoms traveling along a high-gradient two-wire magnetic guide that is continuously on. Ions are accelerated in a compact acceleration region, directed through a drift region several centimeters in length, and detected using a position-sensitive ion detector. The potentials of several electrodes can be varied to adjust the imaging properties. Using ion trajectory simulations as well as experiments, we study the passage of the ions through the detection system, the magnification of the detection system, and the time-of-flight characteristics.
We demonstrate the continuous injection and propagation of a cold atomic beam in a high-gradient (up to 2.7kG∕cm2.7\phantom{\rule{0.3em}{0ex}}\mathrm{kG}∕\mathrm{cm}2.7kG∕cm) magnetic guide of 1.7m1.7\phantom{\rule{0.3em}{0ex}}\mathrm{m}1.7m length. Continuous injection is accomplished using a side-loading scheme that involves a sequence of two modified magneto-optic traps. Methods are developed to measure the atomic-flow temperatures and the flux under steady-state conditions. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of $420\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}\ifmmode\pm\else\textpm\fi{}40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, a longitudinal temperature of 1mK1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}1mK, and an average velocity of order 1m∕s1\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}1m∕s. Using a radio-frequency current of a fixed frequency $\ensuremath{\nu}$ coupled directly into the guide wires, atoms exceeding a transverse energy of $h\ensuremath{\nu}$ can be continuously and selectively removed from the atomic beam.
The continuous injection and propagation of a cold atomic beam in a 1.7 m long high-gradient (up to 2.7 kG/cm) magnetic guide is demonstrated. Continuous injection is accomplished by side-loading a magnetic guide with atoms from a sequence of two modified magneto-optical traps. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of 420 muK +/- 40 muK, a longitudinal temperature of 1 mK, and an average velocity of order 1 m/s. Essential for the characterization of this steady-state atomic beam, imaging methods compatible with the high field-gradients associated with this magnetic guide have been investigated and developed. By utilizing a probe laser tuned to an open transition, the fluorescence yield per atom is largely fixed throughout the trap volume, independent of the trapping field. This enables a reliable conversion of fluorescence images into atomic-density profiles. Using a fixed radio-frequency (nurf) current coupled directly onto the guide wires, continuous and selective removal of atoms with a transverse energy exceeding hnurf is demonstrated. This removal scheme is the essence of an rf filter, where the number of modes occupied by the atomic flow is reduced. Through sufficient rf-filtering, a near-single-mode guided atomic beam should be achievable, enabling basic atom-interferometric experiments. The extension of this fixed-frequency removal of atoms for continuous position-resolved evaporative cooling in a long magnetic guide is outlined. Employing an innovative gridless technique for performing Direct Simulation of rarefied gas dynamics via Monte Carlo (DSMC), the evaporative cooling process in a guided atomic beam has been simulated. Both fixed-frequency as well as variable-frequency evaporative cooling layouts have been explored and the results of these simulations are presented. The implementation of this gridless technique is presented and its benefits are briefly outlined. In parallel with the development of a continuous-wave atom laser, tools suited to manipulate cold atomic beams in atom guides have been explored. An atomic guide with a tunable aspect ratio, using rf-dressed state potentials, is presented.
We study the guiding of 87{}^{87}87Rb 59DDD5/2{}_{5/2}5/2​ Rydberg atoms in a linear, high-gradient, two-wire magnetic guide. Time-delayed microwave ionization and ion detection are used to probe the Rydberg atom motion. We observe guiding of Rydberg atoms over a period of 5 ms following excitation. The decay time of the guided-atom signal is about five times that of the initial state. We attribute the lifetime increase to an initial phase of lll-changing collisions and thermally induced Rydberg-Rydberg transitions. Detailed simulations of Rydberg-atom guiding reproduce most experimental observations and offer insight into the internal-state evolution.
We have developed a method to image a beam of neutral atoms (chiefly usable for alkalis) that is essentially an extension to two dimensions of the usual hot wire method of detection. The beam of neutral atoms first passes through a a highly transparent, positively charged grid and then impinges on a heated grid of electroformed nickel with lower transparency and at ground potential. Atoms with ionization potential less than the work function of Ni that strike the heated grid are ionized at its surface and are then pushed through it by the 250-V/cm field produced by the voltage on the first grid. The ions are then accelerated through ~700 V to a pair of multichannel plates (MCPs) ~3 mm away where the single charged ion signal is amplified. The output electrons from the MCPs are accelerated by 1.7 kV to a phosphor screen ~3 mm away that glows when the electrons strike it. The phosphor screen is viewed by a TV camera whose output goes to a frame processor in a PC. The system has been used for studying transverse laser cooling (optical collimation) of a Rb beam done with diode lasers.
We report on an ion-optical system that serves as a microscope for ultracold ground state and Rydberg atoms. The system is designed to achieve a magnification of up to 1000 and a spatial resolution in the 100 nm range, thereby surpassing many standard imaging techniques for cold atoms. The microscope consists of four electrostatic lenses and a microchannel plate in conjunction with a delay line detector in order to achieve single particle sensitivity with high temporal and spatial resolution. We describe the design process of the microscope including ion-optical simulations of the imaging system and characterize aberrations and the resolution limit. Furthermore, we present the experimental realization of the microscope in a cold atom setup and investigate its performance by patterned ionization with a structure size down to 2.7 μ m. The microscope meets the requirements for studying various many-body effects, ranging from correlations in cold quantum gases up to Rydberg molecule formation.
With the continuous advancement and maturation of laser cooling techniques for atoms and molecules and full-dimensional electron and ion imaging technology, using momentum imaging techniques to investigate the characteristic properties of cold atoms and collision dynamics has emerged as a burgeoning research direction. This progress has driven the development of a series of high-resolution electron and ion detection devices, leading to innovative breakthroughs in fields such as cold molecule reactions, Rydberg atoms, nuclear decay, photoionization of Bose-Einstein condensates (BECs) and cold plasmas, collisions between cold atoms and ions/electrons, coherent control of cold atoms, and strong-field ultrafast physics. This article reviews representative instruments and their corresponding seminal achievements in the following domains: In cold molecular/cold chemical reactions, imaging technology has revealed new insights into reaction mechanisms; For cold Rydberg atom interactions, it demonstrates high-precision quantum state manipulation capabilities, advancing quantum information processing; In nuclear decay research, it provides ultra-sensitive detection methods, deepening understanding of decay processes; For BEC photoionization and cold plasma control, it can precisely monitor and manipulate microscopic processes; In cold atomic collision studies, it reveals new details in collision dynamics, refining collision theories; Regarding coherent control of cold atoms, it achieves accurate quantum state manipulation and interference; In strong-field ultrafast processes, it elucidates complex electron dynamics under intense fields, providing innovative methods for ultrafast laser control. Furthermore, this article summarizes the applications of imaging technologies in the aforementioned research areas involving cold atoms, and provides prospects for future developments in this evolving field.