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Journal of Physics Conference Series

A traveling wave Zeeman decelerator

Tomislav Damjanović, Nicolas Vanhaecke, JeanPaul Cromiéres, Henrik Haak, Gerard Meijer, Stefan Willitsch, Dongdong Zhang

Published Jan 1, 2020• 0 citations
Publisher

Abstract

Synopsis We developed a new-concept Zeeman decelerator which produces a traveling magnetic trap. Atoms and molecules possessing a magnetic dipole moment, in so-called low field seeking states, are trapped around a node of a propagating wave provided that the initial velocity of the wave matches a velocity populated in the supersonic beam. In addition, three dimensional confinement is achieved by controlling the radial orientation of the trap, which can be done fully independently from its longitudinal motion.

DOI
10.1088/1742-6596/1412/12/122014

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A new design for a traveling-wave Zeeman decelerator: I. Theory

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The concept of a novel traveling wave Zeeman deccelerator based on a double-helix wire geometry capable of decelerating paramagnetic species with high efficiency is presented. A moving magnetic trap is created by running time-dependent currents through the decelerator coils. Paramagnetic species in low-field-seeking Zeeman states are confined in the moving traps and transported to the end of the decelerator with programmable velocities. Here, we present the theoretical foundations underlying the working principle of the traveling-trap decelerator. Using trajectory simulations, we characterise the performance of the new device and explore the conditions for phase-space stability of the transported molecules.

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A new design for a traveling-wave Zeeman decelerator: II. Experiment

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A novel traveling-wave Zeeman decelerator based on a double-helix coil geometry capable of decelerating paramagnetic molecules with high efficiency is presented. Moving magnetic traps are generated by applying time-dependent currents through the decelerator coils. Paramagnetic molecules in low-field-seeking Zeeman states are confined inside the moving traps which are decelerated to lower forward velocities. As a prototypical example, we demonstrate the deceleration of OH radicals from an initial velocity of 445 m s −1 down to various final velocities. The experimental results are analyzed and numerically reproduced with the help of trajectory simulations confirming the phase-space stability and efficiency of the deceleration of the molecules in the new device.

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Caught In A Trap: Monte Carlo Methods For A Travelling-Wave Zeeman Decelerator

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Our group has previously designed and constructed a modular travelling-wave Zeeman decelerator, incorporating a pulsed-valve supersonic expansion source with dielectric barrier discharge. The decelerator comprises a set of custom-made flattened helical coils driven by a bespoke programmable power electronics control system. The decelerator produces very strong longitudinal magnetic forces that generate an effectively three-dimensional moving trapping potential, compared to a `conventional' Zeeman decelerator. An additional transverse focussing field was originally added by a wire quadrupole circuit, with a possible replacement being constructed from permanent magnets. The decelerator is intended as a loading stage for a novel hybrid magnetic trap / magneto-optical trap, designed for the sympathetic cooling of atomic or molecular species for which conventional laser cooling is not an option. This thesis describes some proposed adjustments and enhancements to the experimental apparatus. A suite of software tools has been developed in order to simulate the various components of the experiment. By its nature the travelling-wave decelerator produces a complex signal at a detector and Monte Carlo analysis has been necessary to understand it. For each part of our simulation process the theoretical underpinning and physical implementation is detailed. Models for fitting and characterising supersonic beam data are developed and demonstrated, and applied to the generation of simulated expansions. Key to the operation of the dynamical simulations is a novel interpolation library which is used to solve particle motion in time-dependent magnetic fields. Computer codes for calculating Stern-Gerlach type forces in different species are described. The phase-space acceptance of the decelerator and proposed trap are estimated. Metastable argon has been guided and decelerated for the purposes of characterising the decelerator; data is presented and analysed, showing a demonstrable reduction by $\approx$ 6~\% of the kinetic energy of a portion of the beam. Predictions show that this could be increased to ~ 14% for the decelerator in its current state, and up to ~ 30% should be achievable for the case of argon with a proposed extension of the deceleration stage. Further predictions are made regarding the deceleration of other species of scientific interest, including a dual-species beam of hydrogen and lithium. Some preliminary results regarding the suggested hybrid trap design are given.

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A travelling wave Zeeman decelerator for atoms and molecules

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The design of a modular moving trap Zeeman decelerator capable of decelerating gas pulses produced from a supersonic source is presented here. Unlike the conventional form of Zeeman decelerator, paramagnetic particles are confined in a 3D potential throughout the deceleration process. The decelerator field is produced by flattened helical coils and currents of up to 1000 A peak. As the coils are periodic in nature, each coil produces a number of deep, quadrupole traps along the molecular beam axis. The resultant periodic field is described as a travelling wave. The application of the appropriate time dependent current allows the traps to move through the four coil modules. In order to compensate for the weaker transverse confinement, a quadrupole guide, operating at 700 A DC, is required to prevent further losses during the deceleration process. The operation of the decelerator relies on the power electronics developed specifically for the quadrupole and the decelerator coils. Due to the electromagnetic interference generated through the switching of the large currents, much of the electronics used to control the power electronics had to be developed specifically. The quadrupole power electronics have been designed to produce fast switching edges. This is necessary to minimise the interaction of the particles within the fringe field regions while maximising the interaction time within the pure quadrupole field. Even at a modest voltage applied to the circuitry, the rise time in current to 700 A has been reduced by a half. The decelerator power electronics must be capable of producing an alternating waveform with an amplitude of at least 500 A for each of the coil phases. Furthermore, the frequency of the waveforms must be tunable within a range of 10 kHz to 0 Hz. Through a combination of pulse width modulation and knowledge of the electrical properties of the coil it is possible to synthesise an alternating current waveform from a 800 V DC supply using a suitable switching circuit. %The challenge of switching such high powers is the transient voltage spikes that can be produced, however, these can be avoided through careful consideration of the layout of the circuit. Decelerators such as this do not cool the sample but instead reduces the mean velocity of a subset of particles which remained trapped. This maintains the phase space density of trapped particles. Modelling the magnetic fields generated by the decelerator coils has been necessary in order to understand the phase space acceptance of the decelerator. The helical nature of the coils required the development of a specific algorithm in order to calculate the field generated by each wire element. The resultant potential can then be interpolated using a tricubic interpolator to extract the field gradients necessary for numerical simulations of the particle trajectories. Including the effects of the pulse width modulated on the trap facilitates the characterisation of the acceptance of the decelerator and the limitations of the current iteration of the design. The numerical simulations can also be compared to experimental results gathered for metastable argon. The 3D guiding, or velocity bunching, of the gas packet over a range of velocities has been demonstrated. The ability to 3D guide and decelerate were severely hampered by the failure of key electronic components, limiting three coils to 100 A peak, moreover, these traps were sub-optimally loaded. Deceleration from 350 to 347 m/s and 342 to 310 m/s has been observed. The design of a trap capable of simultaneously loading samples of decelerated CaH and Li while allowing the cooling of Li would potentially allow for the sympathetic cooling of a molecular species with an atomic refrigerant. This particular atom-molecule system would also facilitate the examination of controlled chemistry and collisions over a range of temperatures through state selection of the reactants. The loading of the trap has been optimised in 1D for CaH with a loading efficiency of 52.2 % while only 7.3 % of Li is loaded when each of the gas packets has a mean velocity of 11 m/s. This implies that the source of the Li must be at least 130 times brighter than that of the CaH.

•Jan 1, 2018•2 citations

Multistage Zeeman deceleration of atoms and molecules

A. W. Wiederkehr

A modular multistage Zeeman decelerator consisting of an array of solenoids has been developed to control the velocity of cold, paramagnetic atoms and molecules in supersonic beams. The translational motion these atoms and molecules was manipulated with high precision by exploiting the interaction between their magnetic moments and timedependent inhomogeneous magnetic elds. Deceleration was achieved by pulsing high currents (up to 300 A) through the solenoids of the decelerator with pulse sequences calculated to preserve the phase-space density of the atomic and molecular samples while removing a preset amount of kinetic energy. For the operation of the multistage Zeeman decelerator, solenoids and electronic units were developed to generate strong magneticeld pulses (> 2 T) with rise and fall times of only a few microseconds. An accurate three-dimensional particle-trajectory simulation program has been developed to analyze the deceleration process and optimize its e ciency. This software was thoroughly tested by systematic comparison with experimental results and enabled, in combination with measurements, the complete characterization of the spatial and velocity distributions of the decelerated and trapped atoms for di erent sets of experimental conditions. The program was essential to achieve a complete interpretation of the dynamics of the deceleration process and to devise a strategy to e ciently load the decelerated atoms and molecules into magnetic quadrupole traps. In a rst experiment, hydrogen atoms seeded in a supersonic expansion of Kr have been decelerated from an initial velocity of 435 m/s to nal velocities as low as 107 m/s in a 12-stage Zeeman decelerator. The operation of the decelerator has been analyzed by comparing the results of numerical particle-trajectory simulations with those of independent measurements of the velocity distribution by ion time-ofight mass spectrometry following photoionization of the decelerated atoms. The velocity distribution of the decelerated atom cloud was found to have a half width at half maximum of 25± 12 m/s corresponding to a temperature of ∼ 40 mK. The phase stability of the multistage-Zeeman-deceleration process has been investigated by numerical particle-trajectory simulations and experimental measurements on deuterium atoms in a 24-stage Zeeman decelerator. A one-dimensional analytical model of the phase stability in a multistage Zeeman decelerator was adapted from a one-dimensional model of phase stability in a multistage Stark decelerator and compared with the results of oneand three-dimensional particle-trajectory simulations. The comparison, which included the analysis of the e ects of nite switch-on and switch-o times of the deceleration pulses, revealed that transverse e ects in the decelerator lead to a considerable reduction of the

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We present an experimental realization of a moving magnetic trap decelerator, where paramagnetic particles entrained in a cold supersonic beam are decelerated in a co-moving magnetic trap.Our method allows for an efficient slowing down of both paramagnetic atoms and molecules to near stopping velocities.We show that under realistic conditions we will be able to trap and decelerate a large fraction of the initial supersonic beam.We present our first results on deceleration in a moving magnetic trap by bringing metastable neon atoms to near rest.Our estimated phase space volume occupied by decelerated particles at a final velocity of 50 m s -1 shows an improvement of two orders of magnitude as compared to currently available deceleration techniques.Cold chemistry (at temperature of ∼1 K) is a fast developing field with many interesting theoretical predictions yet only a few experiments [1][2][3][4].It has been predicted that in the cold chemistry regime, reactions might proceed through resonance states [5] and both reaction rate and path could be controlled [6] via external magnetic or electrostatic fields.Low-temperature chemistry is not limited to artificially created conditions alone but occurs naturally at the low temperature of interstellar space [7] and the atmospheres of remote planets and moons [8].It is anticipated that cold chemistry will follow the enormous progress in molecular chemistry that was spurred by the advent of supersonic beams more than half a century ago [9].However, the progress in experimental cold chemistry is rather slow partly due to the absence of a general method of atomic and molecular cooling.Although the low-temperature environment created within a supersonic expansion has been used to observe many reactions [10], a direct 1 Author to whom any correspondence should be addressed.

arXiv:1011.0418•New Journal of Physics•Oct 24, 2011•51 citations
Synopsis We developed a new-concept Zeeman decelerator which produces a traveling magnetic trap. Atoms and molecules possessing a magnetic dipole moment, in so-called low field seeking states, are trapped around a node of a propagating wave provided that the initial velocity of the wave matches a velocity populated in the supersonic beam. In addition, three dimensional confinement is achieved by controlling the radial orientation of the trap, which can be done fully independently from its longitudinal motion.
The concept of a novel traveling wave Zeeman deccelerator based on a double-helix wire geometry capable of decelerating paramagnetic species with high efficiency is presented. A moving magnetic trap is created by running time-dependent currents through the decelerator coils. Paramagnetic species in low-field-seeking Zeeman states are confined in the moving traps and transported to the end of the decelerator with programmable velocities. Here, we present the theoretical foundations underlying the working principle of the traveling-trap decelerator. Using trajectory simulations, we characterise the performance of the new device and explore the conditions for phase-space stability of the transported molecules.
A novel traveling-wave Zeeman decelerator based on a double-helix coil geometry capable of decelerating paramagnetic molecules with high efficiency is presented. Moving magnetic traps are generated by applying time-dependent currents through the decelerator coils. Paramagnetic molecules in low-field-seeking Zeeman states are confined inside the moving traps which are decelerated to lower forward velocities. As a prototypical example, we demonstrate the deceleration of OH radicals from an initial velocity of 445 m s −1 down to various final velocities. The experimental results are analyzed and numerically reproduced with the help of trajectory simulations confirming the phase-space stability and efficiency of the deceleration of the molecules in the new device.
Our group has previously designed and constructed a modular travelling-wave Zeeman decelerator, incorporating a pulsed-valve supersonic expansion source with dielectric barrier discharge. The decelerator comprises a set of custom-made flattened helical coils driven by a bespoke programmable power electronics control system. The decelerator produces very strong longitudinal magnetic forces that generate an effectively three-dimensional moving trapping potential, compared to a `conventional' Zeeman decelerator. An additional transverse focussing field was originally added by a wire quadrupole circuit, with a possible replacement being constructed from permanent magnets. The decelerator is intended as a loading stage for a novel hybrid magnetic trap / magneto-optical trap, designed for the sympathetic cooling of atomic or molecular species for which conventional laser cooling is not an option. This thesis describes some proposed adjustments and enhancements to the experimental apparatus. A suite of software tools has been developed in order to simulate the various components of the experiment. By its nature the travelling-wave decelerator produces a complex signal at a detector and Monte Carlo analysis has been necessary to understand it. For each part of our simulation process the theoretical underpinning and physical implementation is detailed. Models for fitting and characterising supersonic beam data are developed and demonstrated, and applied to the generation of simulated expansions. Key to the operation of the dynamical simulations is a novel interpolation library which is used to solve particle motion in time-dependent magnetic fields. Computer codes for calculating Stern-Gerlach type forces in different species are described. The phase-space acceptance of the decelerator and proposed trap are estimated. Metastable argon has been guided and decelerated for the purposes of characterising the decelerator; data is presented and analysed, showing a demonstrable reduction by ≈\approx≈ 6~\% of the kinetic energy of a portion of the beam. Predictions show that this could be increased to ~ 14% for the decelerator in its current state, and up to ~ 30% should be achievable for the case of argon with a proposed extension of the deceleration stage. Further predictions are made regarding the deceleration of other species of scientific interest, including a dual-species beam of hydrogen and lithium. Some preliminary results regarding the suggested hybrid trap design are given.
The design of a modular moving trap Zeeman decelerator capable of decelerating gas pulses produced from a supersonic source is presented here. Unlike the conventional form of Zeeman decelerator, paramagnetic particles are confined in a 3D potential throughout the deceleration process. The decelerator field is produced by flattened helical coils and currents of up to 1000 A peak. As the coils are periodic in nature, each coil produces a number of deep, quadrupole traps along the molecular beam axis. The resultant periodic field is described as a travelling wave. The application of the appropriate time dependent current allows the traps to move through the four coil modules. In order to compensate for the weaker transverse confinement, a quadrupole guide, operating at 700 A DC, is required to prevent further losses during the deceleration process. The operation of the decelerator relies on the power electronics developed specifically for the quadrupole and the decelerator coils. Due to the electromagnetic interference generated through the switching of the large currents, much of the electronics used to control the power electronics had to be developed specifically. The quadrupole power electronics have been designed to produce fast switching edges. This is necessary to minimise the interaction of the particles within the fringe field regions while maximising the interaction time within the pure quadrupole field. Even at a modest voltage applied to the circuitry, the rise time in current to 700 A has been reduced by a half. The decelerator power electronics must be capable of producing an alternating waveform with an amplitude of at least 500 A for each of the coil phases. Furthermore, the frequency of the waveforms must be tunable within a range of 10 kHz to 0 Hz. Through a combination of pulse width modulation and knowledge of the electrical properties of the coil it is possible to synthesise an alternating current waveform from a 800 V DC supply using a suitable switching circuit. %The challenge of switching such high powers is the transient voltage spikes that can be produced, however, these can be avoided through careful consideration of the layout of the circuit. Decelerators such as this do not cool the sample but instead reduces the mean velocity of a subset of particles which remained trapped. This maintains the phase space density of trapped particles. Modelling the magnetic fields generated by the decelerator coils has been necessary in order to understand the phase space acceptance of the decelerator. The helical nature of the coils required the development of a specific algorithm in order to calculate the field generated by each wire element. The resultant potential can then be interpolated using a tricubic interpolator to extract the field gradients necessary for numerical simulations of the particle trajectories. Including the effects of the pulse width modulated on the trap facilitates the characterisation of the acceptance of the decelerator and the limitations of the current iteration of the design. The numerical simulations can also be compared to experimental results gathered for metastable argon. The 3D guiding, or velocity bunching, of the gas packet over a range of velocities has been demonstrated. The ability to 3D guide and decelerate were severely hampered by the failure of key electronic components, limiting three coils to 100 A peak, moreover, these traps were sub-optimally loaded. Deceleration from 350 to 347 m/s and 342 to 310 m/s has been observed. The design of a trap capable of simultaneously loading samples of decelerated CaH and Li while allowing the cooling of Li would potentially allow for the sympathetic cooling of a molecular species with an atomic refrigerant. This particular atom-molecule system would also facilitate the examination of controlled chemistry and collisions over a range of temperatures through state selection of the reactants. The loading of the trap has been optimised in 1D for CaH with a loading efficiency of 52.2 % while only 7.3 % of Li is loaded when each of the gas packets has a mean velocity of 11 m/s. This implies that the source of the Li must be at least 130 times brighter than that of the CaH.
A modular multistage Zeeman decelerator consisting of an array of solenoids has been developed to control the velocity of cold, paramagnetic atoms and molecules in supersonic beams. The translational motion these atoms and molecules was manipulated with high precision by exploiting the interaction between their magnetic moments and timedependent inhomogeneous magnetic elds. Deceleration was achieved by pulsing high currents (up to 300 A) through the solenoids of the decelerator with pulse sequences calculated to preserve the phase-space density of the atomic and molecular samples while removing a preset amount of kinetic energy. For the operation of the multistage Zeeman decelerator, solenoids and electronic units were developed to generate strong magneticeld pulses (> 2 T) with rise and fall times of only a few microseconds. An accurate three-dimensional particle-trajectory simulation program has been developed to analyze the deceleration process and optimize its e ciency. This software was thoroughly tested by systematic comparison with experimental results and enabled, in combination with measurements, the complete characterization of the spatial and velocity distributions of the decelerated and trapped atoms for di erent sets of experimental conditions. The program was essential to achieve a complete interpretation of the dynamics of the deceleration process and to devise a strategy to e ciently load the decelerated atoms and molecules into magnetic quadrupole traps. In a rst experiment, hydrogen atoms seeded in a supersonic expansion of Kr have been decelerated from an initial velocity of 435 m/s to nal velocities as low as 107 m/s in a 12-stage Zeeman decelerator. The operation of the decelerator has been analyzed by comparing the results of numerical particle-trajectory simulations with those of independent measurements of the velocity distribution by ion time-ofight mass spectrometry following photoionization of the decelerated atoms. The velocity distribution of the decelerated atom cloud was found to have a half width at half maximum of 25± 12 m/s corresponding to a temperature of ∼ 40 mK. The phase stability of the multistage-Zeeman-deceleration process has been investigated by numerical particle-trajectory simulations and experimental measurements on deuterium atoms in a 24-stage Zeeman decelerator. A one-dimensional analytical model of the phase stability in a multistage Zeeman decelerator was adapted from a one-dimensional model of phase stability in a multistage Stark decelerator and compared with the results of oneand three-dimensional particle-trajectory simulations. The comparison, which included the analysis of the e ects of nite switch-on and switch-o times of the deceleration pulses, revealed that transverse e ects in the decelerator lead to a considerable reduction of the
We present an experimental realization of a moving magnetic trap decelerator, where paramagnetic particles entrained in a cold supersonic beam are decelerated in a co-moving magnetic trap.Our method allows for an efficient slowing down of both paramagnetic atoms and molecules to near stopping velocities.We show that under realistic conditions we will be able to trap and decelerate a large fraction of the initial supersonic beam.We present our first results on deceleration in a moving magnetic trap by bringing metastable neon atoms to near rest.Our estimated phase space volume occupied by decelerated particles at a final velocity of 50 m s -1 shows an improvement of two orders of magnitude as compared to currently available deceleration techniques.Cold chemistry (at temperature of ∼1 K) is a fast developing field with many interesting theoretical predictions yet only a few experiments [1][2][3][4].It has been predicted that in the cold chemistry regime, reactions might proceed through resonance states [5] and both reaction rate and path could be controlled [6] via external magnetic or electrostatic fields.Low-temperature chemistry is not limited to artificially created conditions alone but occurs naturally at the low temperature of interstellar space [7] and the atmospheres of remote planets and moons [8].It is anticipated that cold chemistry will follow the enormous progress in molecular chemistry that was spurred by the advent of supersonic beams more than half a century ago [9].However, the progress in experimental cold chemistry is rather slow partly due to the absence of a general method of atomic and molecular cooling.Although the low-temperature environment created within a supersonic expansion has been used to observe many reactions [10], a direct 1 Author to whom any correspondence should be addressed.