Turbomolecular Pumps for Holding Gases in Open Containers

Thermal gas atoms would be trapped; much faster atoms would pass through.

Proposed special-purpose turbomolecular pumps denoted turbotraps would be designed, along with mating open containers, to prevent the escape of relatively slowly (thermal) moving gas molecules from the containers while allowing atoms moving at much greater speeds to pass through. In the original intended applications, the containers would be electron-attachment cells, and the contained gases would be vapors of alkali metal atoms moving at thermal speeds that would be of the order of a fraction of 300 meters per second. These cells would be parts of apparatuses used to measure fluxes of neutral atoms incident at kinetic energies in the approximate range of 10 eV to 10 keV (corresponding to typical speeds of the order of 40,000 m/s and higher). The incident energetic neutral atoms would pass through the cells, wherein charge-exchange reactions with the alkali metal atoms would convert the neutral atoms to negative ions, which, in turn, could then be analyzed by use of conventional charged-particle optics.

Rotating and Stationary concentric rows of blades would momentarily open paths along which high-speed atoms could travel through the cell bounded by the blade circles." class="caption" align="left">The figure depicts selected aspects of a turbotrap as part of such an apparatus. The turbotrap would exploit the large difference between the speed range of the incident energetic neutral atoms and the thermal speed range of the alkali metal atoms in the cell. The turbotrap would consist primarily of two or more rotating concentric rows of blades interspersed with two or more concentric stationary rows of blades. The relative positions of the blades, the sizes of the gaps between them, and the speed of rotation would be chosen so as to periodically open up one or more straight path(s) through the cell during a time interval long enough to allow the incident energetic neutral atoms to pass through but short enough so that there would be no clear path through the cell for the slower thermal alkali metal atoms. Moreover, the blades would be shaped and oriented to pump most of the incident thermal atoms back into the cell.

The feasibility of several turbotrap designs has been tentatively demonstrated by means of computational simulations. For example, in the case of one design involving two rows of stationary vanes and two rows of blades on a circle of about 10-cm diameter, a gap of 0.5 cm between blades, 50-percent open area, and a rotational speed of 32,000 rpm, the simulation showed that >99 percent of alkali metal atoms entering the turbotrap would be returned to the cell. The rotational speed in this example is well within that attainable by exploiting recent developments in the technological disciplines of reaction wheels, gyroscopes, and conventional turbomolecular pumps.

This work was done by John W. Keller of Goddard Space Flight Center and John E. Lorenz of Litton Industries. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Manufacturing & Prototyping category. GSC-14402-1

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