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AIP conference proceedings

The ultra-cold polarized hydrogen jet project

R. S. Raymond, Donald G. Crabb, T. Roser, James A. Stewart, G.R. Court

Published Jan 1, 1989• 0 citations
Publisher

Abstract

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.

DOI
10.1063/1.38354

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H↓ atomic beam source: A feasibility study

T. O. Nünikoski, S. Penttilä, J.M. Rieubland +1 more

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.

AIP conference proceedings•Jan 1, 1984•0 citations
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.
Spin-polarized atomic hydrogen (H↓) gas at low temperatures (<0.5 K) has been studied using Electron Spin Resonance (ESR) techniques which allow direct measurements of the densities of H atoms in each of the two lower hyperfine states (a and b). It has been demonstrated that ESR can be made to yield accurate and detailed information about the decay of H↓ densities in time. Crucial to the analysis of the data is the ability to confine the sample of H↓, gas inside the liquid ⁴He coated microwave cavity where the magnetic field and temperature are well known. Measurements of K[sub aa] and K[sub ab], the two surface recombination rate constants, and their ratio γ=K[sub aa]/K[sub ab] have been made and from these results the binding energy of H on liquid ⁴He has been extracted. Also reported are the results of the first detailed study of the one-body surface relaxation rate which we find is due to microscopic magnetic impurities in the cell walls. Theoretical calculations of the two-body dipole-dipole relaxation rate of H↓ in the gas and on the surface are also presented.
In this thesis three experiments with atomic hydrogen (H) at low temperatures T<1 K are presented. Experiments were carried out with two- (2D) and three-dimensional (3D) H gas, and with H atoms trapped in solid H2 matrix. The main focus of this work is on interatomic interactions, which have certain specific features in these three systems considered. A common feature is the very high density of atomic hydrogen, the systems are close to quantum degeneracy. Short range interactions in collisions between atoms are important in gaseous H. The system of H in H2 differ dramatically because atoms remain fixed in the H2 lattice and properties are governed by long-range interactions with the solid matrix and with H atoms. The main tools in our studies were the methods of magnetic resonance, with electron spin resonance (ESR) at 128 GHz being used as the principal detection method. For the first time in experiments with H in high magnetic fields and at low temperatures we combined ESR and NMR to perform electron-nuclear double resonance (ENDOR) as well as coherent two-photon spectroscopy. This allowed to distinguish between different types of interactions in the magnetic resonance spectra. Experiments with 2D H gas utilized the thermal compression method in homogeneous magnetic field, developed in our laboratory. In this work methods were developed for direct studies of 3D H at high density, and for creating high density samples of H in H2. We measured magnetic resonance line shifts due to collisions in the 2D and 3D H gases. First we observed that the cold collision shift in 2D H gas composed of atoms in a single hyperfine state is much smaller than predicted by the mean-field theory. This motivated us to carry out similar experiments with 3D H. In 3D H the cold collision shift was found to be an order of magnitude smaller for atoms in a single hyperfine state than that for a mixture of atoms in two different hyperfine states. The collisional shifts were found to be in fair agreement with the theory, which takes into account symmetrization of the wave functions of the colliding atoms. The origin of the small shift in the 2D H composed of single hyperfine state atoms is not yet understood. The measurement of the shift in 3D H provides experimental determination for the difference of the scattering lengths of ground state atoms. The experiment with H atoms captured in H2 matrix at temperatures below 1 K originated from our work with H gas. We found out that samples of H in H2 were formed during recombination of gas phase H, enabling sample preparation at temperatures below 0.5 K. Alternatively, we created the samples by electron impact dissociation of H2 molecules in situ in the solid. By the latter method we reached highest densities of H atoms reported so far, 3.5(5)x1019 cm-3. The H atoms were found to be stable for weeks at temperatures below 0.5 K. The observation of dipolar interaction effects provides a verification for the density measurement. Our results point to two different sites for H atoms in H2 lattice. The steady-state nuclear polarizations of the atoms were found to be non-thermal. The possibility for further increase of the impurity H density is considered. At higher densities and lower temperatures it might be possible to observe phenomena related to quantum degeneracy in solid.
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.