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Three stainless-steel multichannel arrays with channels of 16, 50, and 140 μm diameter have been investigated experimentally. The whole flow pattern is measured, including the angular dependence of the velocity distribution. All measurements concern the opaque mode, with λ⩽L and λ⩾α, where λ is the mean free path in the source and α and L are the radius and length of the channel, respectively. The results are compared with the predictions for free molecular flow in the same channel, as a function of the reduced source density n*=L/λ. The peaking factor is 30% lower than predicted by the Giordmaine-Wang model. At angles ϑ larger than the half-width–half-maximum ϑ1/2 the angular distribution is not perturbed by the opaque conditions; for ϑ<ϑ1/2 it levels off to a lower peaking factor due to operation in the opaque mode. The center-line beam shows an increasing loss of slow molecules for increasing n*. At n*=10 the gain in mean translational energy is 15%, much larger than predicted by the model of Olander. The deformation of the velocity distribution decreases with increasing angle, and for ϑ≳ϑ1/2 the molecules again have an unperturbed Maxwell-Boltzmann distribution.

Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: This paper investigates the possibility of suppressing the ionization rate in a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms. Suppression of the ionization is critical for the possibility of reaching Bose-Einstein condensation with such atoms. We estimate the relevant long-range interactions for the system, consisting of electric quadrupole-quadrupole and dipole-induced dipole terms, and develop short-range potentials based on the Na 2 singlet and triplet potentials. The autoionization widths of the system are also calculated. With these ingredients we calculate the ionization rate for spin-polarized and for spin-isotropic samples, caused by anisotropy of the long-range interactions. We find that spin polarization may allow for four orders of magnitude suppression of the ionization rate for Ne. The results depend sensitively on a precise knowledge of the interaction potentials, however, pointing out the need for experimental input. The same model gives a suppression ratio close to unity for metastable xenon in accordance with experimental results, due to a much increased anisotropy in this case.

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