A Halbach array composed of 12 permanent magnets in a hexapole configuration is employed to deflect hydrogen atoms as they exit a Zeeman decelerator. The ability to preferentially manipulate H atoms is very useful, as there are currently very few techniques that are appropriate for purifying a beam of H atoms from precursor molecules (such as molecular hydrogen or ammonia), seed gases, and other contaminant species. The extent to which hydrogen atoms are deflected by a single Halbach array when it is tilted or shifted off the main beam axis is characterised experimentally and interpreted with the aid of a simple mathematical model. A radical beam filter is subsequently introduced, where four Halbach arrays arranged in series serve to deflect H atoms away from the main beam axis and around skimming blades; all other components of the incoming beam are blocked by the blades and are thus not transmitted through the magnetic guide. The properties of the guide, as established by experimental measurements and complemented by detailed simulations, confirm that it is a highly effective beam filter—successfully generating a pure and velocity-selected beam of H atoms.

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Note that the influence of a free flight region between the beam source and the HA was neglected as its contribution is small, thanks to the low divergence of the beam.

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