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AIP conference proceedings

H↓ atomic beam source: A feasibility study

T. O. Nünikoski, S. Penttilä, J.M. Rieubland, A. Rijllart

Published Jan 1, 1984• 0 citations
Publisher

Abstract

The possible application of the stable atomic hydrogen (H↓) in intense polarized atom or ion beam sources is discussed. The proposed scheme is based on flipping the electron spin of stored atoms by microwaves; this results in the magnetic acceleration and focusing of the flipped atoms. The main advantages of such beams are: accumulation and pulsing with time structure convenient for injection into synchrotrons via Linac injectors; high density due to the slow velocity and high initial density; and monochromaticity which allows atomic beam transport with focusing elements having no large chromatic errors. The possible problems and limitations in such an extraction scheme are: depolarization and slow spin diffusion due to spin exchange collisions in the dense gas phase, and loss of density due to recombination of the flipped atoms on the walls, before reaching the extraction aperture and accelerating field gradient.The results of the first tests of our apparatus are presented. The apparatus, based on a powerful dilution refrigerator, is described briefly.

DOI
10.1063/1.34604

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The possible application of the stable atomic hydrogen (H↓) in intense polarized atom or ion beam sources is discussed. The proposed scheme is based on flipping the electron spin of stored atoms by microwaves; this results in the magnetic acceleration and focusing of the flipped atoms. The main advantages of such beams are: accumulation and pulsing with time structure convenient for injection into synchrotrons via Linac injectors; high density due to the slow velocity and high initial density; and monochromaticity which allows atomic beam transport with focusing elements having no large chromatic errors. The possible problems and limitations in such an extraction scheme are: depolarization and slow spin diffusion due to spin exchange collisions in the dense gas phase, and loss of density due to recombination of the flipped atoms on the walls, before reaching the extraction aperture and accelerating field gradient.The results of the first tests of our apparatus are presented. The apparatus, based on a powerful dilution refrigerator, is described briefly.
We present a design for an atomic synchrotron consisting of 40 hybrid magnetic hexapole lenses arranged in a circle. We show that for realistic parameters, hydrogen atoms with a velocity up to 600 m s −1 can be stored in a 1 m diameter ring, which implies that the atoms can be injected in the ring directly from a pulsed supersonic beam source. This ring can be used to study collisions between stored hydrogen atoms and supersonic beams of many different atoms and molecules. The advantage of using a synchrotron is two-fold: (i) the collision partners move in the same direction as the stored atoms, resulting in a small relative velocity and thus a low collision energy, and (ii) by storing atoms for many round-trips, the sensitivity to collisions is enhanced by a factor of 100–1000. In the proposed ring, the cross-sections for collisions between hydrogen, the most abundant atom in the universe, with any atom or molecule that can be put in a beam, including He, H 2 , CO, ammonia and OH can be measured at energies below 100 K. We discuss the possibility of using optical transitions to load hydrogen atoms into the ring without influencing the atoms that are already stored. In this way it will be possible to reach high densities of stored hydrogen atoms.
Research is described on a jet of polarized atomic hydrogen.Atoms in the two lower hyperfine states are trapped at 0.4 k in a 5 T magnetic field.Microwaves at 140 GHz then drive transitions to the upper hyperfine states and these atoms are expelled from the field to form the beam.Atoms have been trapped, transitions driven, and atoms detected outside the trapping field, but preliminary measurements are not fully understood.Studies are continuing.Work is continuing to test the applicability of cryogenic atomic physics techniques to the production of dense polarized atomic hydrogen beams.At high magnetic fields (5 T) and low temperatures (0.5 k), the magnetic energy of a hydrogen atom, approximately #, • B, is considerably higher than the thermal energy, kT.When cold atoms approach a high magnetic field, atoms in the two upper hyperfme states see a potential hill and are repulsed.Atoms in the two lower hyperfine states see a potential well, are attracted in, thermalize by hitting cold walls and are trapped.These trapped atoms are then irradiated with microwaves of the proper frequency (approximately 140 GHz for a 5 T field) to drive transition to one of the upper hyperflne states.These flipped atoms, now on a potential hill, are accelerated out of the field to form the beam.
Two ultra-cold methods of atomic hydrogen electron spin separation for polarized jet targets and sources will be described and compared. Both of them use a helium-film coated cell at 300 mK located in a high magnetic field. The first one, microwave driven extraction, uses a 212 GHz microwave system and 7.5 T magnetic field with a high uniformity ({Delta}B/B{approx} 10{sup {minus}5}). The second method employs the gradient of a high magnetic field for the state separation and beam formation. Experimental results from the two methods will be presented.
Magnetic confinement is used to obtain spin-polarized atomic hydrogen (H\ensuremath{\uparrow}) in an openended helium-coated chamber, at densities up to 0.8\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 atoms/cm3{\mathrm{cm}}^{3}cm3. At a field of 10 T and temperature of 0.3 K, confinement times as long as 4 h are achieved. A quantitative study of the confinement times as a function of magnetic field is in excellent agreement with the expected escape from the magnetic potential well accompanied by a very slow loss process. Heat transport measurements confirm that the H\ensuremath{\uparrow} is a gas and an upper limit is placed on the three-body recombination rate.
Laser cooling methods provide the possibility to obtain very well collimated and slow atomic beams. These intense sources have attracted new interest in the manipulation of atomic trajectories. Atom optical schemes based on the optical dipole force are very attractive, for example in atom lithography where atomic beams are focused by standing wave light elds, to create periodic nanostructures. However, in applications where more complex structures are desirable or decoherence due to spontaneous emission has to be avoided, magnetic atom optical components are more attractive. For example, at and curved atomic mirrors with high reflectivity and low surface rough ness can be built from periodically magnetized videotapes. First, with a collimated and polarized cesium atomic beam from a Zeeman-slower apparatus, we have investigated the deflection of the atomic beam by a sinusoidally magnetized videotape with a period length of 30 μm. From these measurements we have determined the magnetic eld strength at the surface to be 0.05 Tesla. The reflectivity has been found to be consistent with 100%. The very large eld gradients produced by the tape can be exploited to guide atoms on curved trajectories with small radii of curvature. With a radius of 9.5 cm we have obtained a maximum deflection angle of 25° which is limited by technical constraint. Second, we have built a setup based on a 2D-MOT for generating a slow atomic beam with a velocity of 20 m/s. After leaving the optical collimation and the optical polarization stages, the atomic beam is reflected after one bounce onto the videotape up to angles as large as 7°. We exploited the natural curvature of our videotape and we demonstrated the focusing of the atomic beam after one and two bounces depending on the curvature of the videotape. The focal length and the angles of deflection enable to deduce the local radii of curvature. We have confronted our results by testing the focusing of the videotape with a red laser and by performing a numerical simulation which prove the consistency with the atomic beam experiments.
We present a design and performance tests of an intense source of cold hydrogen atoms for loading large magnetic traps. Our source is based on a cryogenic dissociator of molecular hydrogen at 0.6 K followed by a series of thermal accommodators at 0.5, 0.2 and 0.13 K with inner surfaces covered by a superfluid helium film. All components are thermally anchored to corresponding stages of a dilution refrigerator. The source provides a continuous flux of 7⋅\cdot⋅101310^{13}1013 H atoms/s in a temperature range of 130-200 mK. We have successfully used the source for loading a large Ioffe-Pritchard magnetic trap recently built in our laboratory [arXiv:2108.09123 or Rev. Sci. Instr. 93 (2), 023201 (2022)]. Calorimetric measurements of the atomic recombination heat allow reliable determination of the atomic flux and H gas density in the trap. We have tested the performance of the source and loading of H atoms into the trap at various configurations of the trapping field, reducing the magnetic barrier height to 75% and 50% of the nominal value of 0.8 T (0.54 K) as well as at the open configuration of the trap at its lower end, when the atoms are in contact with the trapping cell walls covered by a superfluid helium film. In the latter case, raising the trapping cell temperature to 200-250 mK, the low-field seeking atoms at densities exceeding 1011^{11}11 cm−3^{-3}−3 can be stored for the time over 1000 s, sufficiently long for experiments on precision spectroscopy of cold H gas.