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The AC-MOT cold atom electron source

Michael E. Jones

Published May 31, 2019• 2 citations
OpenAlex

Abstract

This thesis presents details of a new type of cold atom electron source (CAES) that uses an alternating current magneto-optical trap (AC-MOT). Cold atom electron sources produce electron bunches by threshold photoionisation from a population of laser-cooled atoms. The electrons extracted from the atoms are made into a beam which has an ultra-low transverse velocity distribution. The low velocity distribution increases the brightness and reduces the energy spread of the beam, making CAESs potential sources for future electron diffraction and electron spectroscopy studies. The newly commissioned source detailed here produces focused electron bunches with characteristic temperatures as low as 12.6 ± 0.8 K, at a rate of up to 10 Hz, and with a bunch charge of 2.2±0.2fC, comparable to other state-of-the-art cold atom electron sources. The AC-MOT CAES can also produce cold bunches with energies <200 eV, which is lower than has previously been reported for this type of source. The electrons originate from a population of laser-cooled 85Rb atoms held in an AC- MOT. The AC-MOT CAES design allows the magnetic trapping fields to be switched to zero in 32 μs, more than �ˆ�175 times faster than is possible using a comparable DC-MOT source. This means that electron bunches can be created and extracted in a magnetic field-free environment without any reduction in trap density and hence bunch charge, as occurs in conventional DC-MOT sources. The design, development, characterisation, and operation of the systems needed for the new AC-MOT CAES are detailed in this thesis. This includes details of the atom trapping system, the photoionisation system, the electron extraction system and the various timing systems for creation and characterisation of the resultant cold electron bunches. Finally, the characterisation of the AC-MOT CAES itself is presented, and the measured beam metrics are used to compare the new AC-MOT CAES with other CAES designs which are under development.

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In a Magneto-Optical Trap (MOT), realized for the first time in 1987, one can trap and cool neutral atoms to temperatures below a mK. The invention of this device caused a revolution in atomic physics. With an MOT collision and spectroscopy experiments could be performed with unprecedented accuracy. The main breakthrough came about 10 years later when the atom from an MOT were used to create a Bose Einstein Condensate (BEC), a source of coherent matter. All this work has been rewarded with no less than two Nobel prizes: in 1997, Chu, Cohen-Tannoudji and Phillips received the Nobel prize for the practical and theoretical development of laser cooling, and in 2001 Cornell, Ketterle and Wieman received the Nobel prize for the realization of a Bose Einstein Condensate in a dilute gas. In 1999 Killian et al. took trapped atoms in an entirely new direction when they succeeded in creating the very first Ultracold Plasma (UCP) from the atoms in an MOT. 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As a main result, we were one of the ¯rst to show how the ions and electrons from a UCP can be imaged on a phosphor screen. From these images we can deduct an upper value for the electron and the ion temperature, we extract for both the electrons and the ions an upper temperature of 40 K, when we translate this temperature to an initial emittance, we obtain a value of 0.01 mm mrad, proving the potential of the electrons extracted from atoms excited out of an MOT as a low emittance source. Our measured current of 1 ¹A is not yet competitive and results from the small electric field, but is expected to increase dramatically in the near future. We will implement a 30 kV 10 ns risetime accelerator with which it should be possible to get a current of about 50 mA. Furthermore we will implement the proper diagnostics to measure the proper charge, current and emittance. 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A novel atom trap is described using alternating current to generate the magnetic B field, together with high speed polarization switching of the damping laser field. This combination produces a trap as effective as a standard magneto-optical trap (MOT), with the advantage that the average B field is zero. No net current is hence induced in surrounding conductive elements, and the B field produced by the ac MOT is found to switch off >300 times faster than a conventional MOT. New experiments can hence be performed, including charged particle probing or detection of the cold target ensemble.

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Compact ultracold electron source based on a grating magneto-optical trap

J. G. H. Franssen, T. C. H. de Raadt, M. A. W. van Ninhuijs +1 more

The ultrafast and ultracold electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge.Its use is however still limited because of the required cold-atom laser-cooling techniques.Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module.This makes the technique more widely accessible and increases its applicability.We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches.The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition.The electron beams extracted from the GMOT have been characterized.Beam energies up to 10 keV were measured using a time-of-flight method.The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in ϵ ¼ 1.9 nm rad.The root-mean-squared transverse size of the ionization volume is 30 μm or larger, implying an electron source temperature in the few-10 K range, 2-3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.

arXiv:1806.10504•Physical Review Accelerators and Beams•Feb 8, 2019•40 citations
This thesis presents details of a new type of cold atom electron source (CAES) that uses an alternating current magneto-optical trap (AC-MOT). Cold atom electron sources produce electron bunches by threshold photoionisation from a population of laser-cooled atoms. The electrons extracted from the atoms are made into a beam which has an ultra-low transverse velocity distribution. The low velocity distribution increases the brightness and reduces the energy spread of the beam, making CAESs potential sources for future electron diffraction and electron spectroscopy studies.
The newly commissioned source detailed here produces focused electron bunches with characteristic temperatures as low as 12.6 ± 0.8 K, at a rate of up to 10 Hz, and with a bunch charge of 2.2±0.2fC, comparable to other state-of-the-art cold atom electron sources. The AC-MOT CAES can also produce cold bunches with energies <200 eV, which is lower than has previously been reported for this type of source.
The electrons originate from a population of laser-cooled 85Rb atoms held in an AC- MOT. The AC-MOT CAES design allows the magnetic trapping fields to be switched to zero in 32 μs, more than �ˆ�175 times faster than is possible using a comparable DC-MOT source. This means that electron bunches can be created and extracted in a magnetic field-free environment without any reduction in trap density and hence bunch charge, as occurs in conventional DC-MOT sources.
The design, development, characterisation, and operation of the systems needed for the new AC-MOT CAES are detailed in this thesis. This includes details of the atom trapping system, the photoionisation system, the electron extraction system and the various timing systems for creation and characterisation of the resultant cold electron bunches.
Finally, the characterisation of the AC-MOT CAES itself is presented, and the measured beam metrics are used to compare the new AC-MOT CAES with other CAES designs which are under development.
In a Magneto-Optical Trap (MOT), realized for the first time in 1987, one can trap and cool neutral atoms to temperatures below a mK. The invention of this device caused a revolution in atomic physics. With an MOT collision and spectroscopy experiments could be performed with unprecedented accuracy. The main breakthrough came about 10 years later when the atom from an MOT were used to create a Bose Einstein Condensate (BEC), a source of coherent matter. All this work has been rewarded with no less than two Nobel prizes: in 1997, Chu, Cohen-Tannoudji and Phillips received the Nobel prize for the practical and theoretical development of laser cooling, and in 2001 Cornell, Ketterle and Wieman received the Nobel prize for the realization of a Bose Einstein Condensate in a dilute gas. In 1999 Killian et al. took trapped atoms in an entirely new direction when they succeeded in creating the very first Ultracold Plasma (UCP) from the atoms in an MOT. The temperature of such a plasma is more than 1000 times lower than the temperatures in conventional cold plasmas. As a result, this type of plasmas received a lot of attention, both theoretically and experimentally. So far studies of UCPs have mainly been of a fundamental kind. In this thesis we study both the fundamental aspects of atoms excited from a magneto-optical as well as the feasibility of using the electrons and ions from a UCP as a source for charged particle accelerators. The work in this thesis therefore pioneers the line between atom physics, plasma physics and accelerator physics. The first part of this thesis deals with an experimental study of the effect of an ionizing laser on cold neon atoms. Here we study the loss of atoms from a Magneto-Optical Trap caused by photionization. From these measurements we deduce a new value for the near threshold photoionization cross section 2:05±0:25x10-18 cm2 at ¿ = 351 nm and 2:05±0:25x10-18 cm2 at ¿ = 364 nm, which is a factor of four more accurate than previous measurements. The measured valuesagree with earlier measurements but differ signi¯cantly from theoretical values. The value of this cross section has a practical relevance for the realization of a continuous ion source. The second part of this thesis has been conducted at the University of Virginia. We study the role of dipole-dipole interactions in the formation of a UCP from a gas of cold Rydberg atoms, and the effect of dipole-dipole interactions on the broadening of microwave transitions. We find that the broadening scales linearly with the density en scales with the fourth power of n, the principal quantum number. These results show the great importance of dipole-dipole interactions in gas of cold Rydberg atoms and are also relevant for the possible realization of a quantum computer from Rydberg atoms. In a third part we studied the potential of using a UCP as a source for electron accelerators. This was done based upon elaborate simulations performed with the General Particle Tracer Code. The main result of these simulations is that, based upon realistic parameters, the brightness, i.e., the figure of merit for charged particle sources defined as the current divided by the emittance squared, of such a source can be up to two orders of magnitude larger than the brightness of conventional sources, which is a major advance. The emittance is a measure for the the focusability of the bunch, and scales as p T, with T the electron temperature. The increase in brightness is mainly attributed to the low electron temperatures of approximately 10 K in an UCP, resulting in an initial emittance of 0.01 mm mrad, compared to 1 mm mrad in a conventional source. With experimentally proven techniques it is possible to extract 10 nC/s from the source, which is a competitive repetition rate. In the next part we describe the design, construction and testing of the first setup with which we want to build an electron accelerator based upon cold atoms. For this we first built a vapor-cell magneto-optical trap, in which we can trap over 109 rubidium atoms with a temperature of 200 ¹K, straight from a background vapor, we also designed an atomic beam that will be implemented in the near future and can provide an increased repetition rate. In the last chapter we show how Rydberg atoms excited from an MOT, evolve into an UCP spontaneously. We extracted a minimum of 1 pC from the plasma. As a main result, we were one of the ¯rst to show how the ions and electrons from a UCP can be imaged on a phosphor screen. From these images we can deduct an upper value for the electron and the ion temperature, we extract for both the electrons and the ions an upper temperature of 40 K, when we translate this temperature to an initial emittance, we obtain a value of 0.01 mm mrad, proving the potential of the electrons extracted from atoms excited out of an MOT as a low emittance source. Our measured current of 1 ¹A is not yet competitive and results from the small electric field, but is expected to increase dramatically in the near future. We will implement a 30 kV 10 ns risetime accelerator with which it should be possible to get a current of about 50 mA. Furthermore we will implement the proper diagnostics to measure the proper charge, current and emittance. In the very last chapter we speculate charged particle sources from cold atoms might advance the state of the art in achievable brightness for both electron and ion beams.
A novel atom trap is described using alternating current to generate the magnetic B field, together with high speed polarization switching of the damping laser field. This combination produces a trap as effective as a standard magneto-optical trap (MOT), with the advantage that the average B field is zero. No net current is hence induced in surrounding conductive elements, and the B field produced by the ac MOT is found to switch off >300 times faster than a conventional MOT. New experiments can hence be performed, including charged particle probing or detection of the cold target ensemble.
The ongoing drive to transfer quantum technologies from research laboratories to practical, portable devices requires the development of multiple key enabling technologies. The field of cold atoms, which includes high-precision metrology and quantum simulation, has various real-world applications including time-keeping and navigation. A crucial component in these systems is a cold-atom source that generates a flux of low-velocity atoms suitable for subsequent capture. A large flux enables fast repetition rates, high duty cycles, long measurement times, and resistance to long-term performance degradation with a good signal-to-noise ratio, all of which are essential to produce cold-atom devices and technologies that are capable of out-performing the classical alternatives. This thesis presents the design, fabrication and characterisation of a novel cold-atom source using a compact pyramidal magneto-optical trap (MOT) with a unique adjustable-aperture mechanism. This allowed an investigation of the dependence of the flux characteristics on the aperture size, a parameter space that has not previously been explored. The measurements showed a strong dependence of the flux on the aperture size, with a negligible flux below 0.4mm, then an approximately linear increase up to 1.0mm, at which point the relationship showed possible signs of saturation. Using 87Rb, a flux of 1.18×1010 atoms/s was produced using a 1.27mm aperture, for which the mean velocity of the atoms was 21.6(3) m/s with a FWHM of 11.2(2) m/s, and the FWHM divergence of the beam was 86(2) mrad. This is the highest recorded flux from a pyramid or 2D+-MOT source, and amongst the highest reported fluxes from any MOT sources, and avenues for improvement still remain. This work fulfils the aim of demonstrating a compact and robust cold-atom source generating a high flux of laser-cooled atoms for real-world quantum technology applications.
The ultrafast and ultracold electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge.Its use is however still limited because of the required cold-atom laser-cooling techniques.Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module.This makes the technique more widely accessible and increases its applicability.We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches.The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition.The electron beams extracted from the GMOT have been characterized.Beam energies up to 10 keV were measured using a time-of-flight method.The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in ϵ ¼ 1.9 nm rad.The root-mean-squared transverse size of the ionization volume is 30 μm or larger, implying an electron source temperature in the few-10 K range, 2-3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.