Creare has developed the world’s smallest vacuum pump.

Creare has had a long and successful history building high speed rotating machinery, and therefore a natural choice for a NASA project to develop the world’s smallest turbomolecular vacuum pump (TMP). In preparation for a future Mars mission, NASA needs radically smaller vacuum pumps to use with the miniaturized mass spectrometers and other analytical instruments they have developed. The military have also become interested in small, portable vacuum pumps to support the use of mass spectrometers in analyzing potentially hazardous materials in the field.

Creare faced several design challenges in miniaturizing vacuum pumps. To provide good performance, TMPs must achieve rotor tip speeds that are a reasonably large fraction of the molecular velocity of the gasses being pumped. For a miniature pump, this means that the TMP rotor must spin at very high rotational rates (e.g., greater than 100,000 rpm). In addition, the clearances between the rotating and stationary parts of the pump must be quite small, necessitating tight controls on machining and rotor excursions during operation. Finally, the pumps must be very efficient, both to allow portable operation with limited power sources and to minimize internal heating and temperature rise.

Creare’s first miniature turbomolecular vacuum pump was very successful and led to a subsequent award and follow-up project to build a vacuum pump an order-of-magnitude smaller in volume. The result of this project was a very small TMP which incorporates an integral molecular drag pump (MDP) stage in a package approximately the size of a C-cell battery.

The ultraminiaturized TMP/MDP has a measured ultimate pressure near 10^-8 Torr and supports foreline pressures in excess of 10 Torr. The relatively high foreline pressure allows the pump to be used in a standalone fashion on the Martian surface. It pumps in excess of 4 L/sec and has a power consumption of just a few watts.

By far the single most difficult design goal for this pump was power consumption low enough to run on batteries and minimize internal heating. Our motor development effort consisted of a number of electromagnetic finite element analyses combined with the construction and testing of a series of prototype motors. Creare’s eventual motor design achieved a nine-fold reduction in power loss, compared to the only available off-the-shelf motor, by minimizing eddy current heating in the motor shell, core losses in the stator substrate, ohmic losses in the coils, and bearing drag. In parallel, we used analysis and bench-scale testing to design the molecular drag stage to optimize pumping capability and minimize viscous heating.

In a related development, we also began work on two rugged high-vacuum systems for use in high-vibration areas such as on spacecraft or the Space Shuttle launch pad. Both of these designs (a TMP and a companion MDP) are based on an innovative “inside-out” electric motor. In these pumps, the shaft is stationary while the outside “case” of the motor turns, functioning as the pump rotor. Our design allows the pump rotor to be attached at both the top and bottom of the rotor, making the assembly much stiffer than the cantilever arrangement of conventional pumps. This reduces the likelihood that vibration will cause the shaft/rotor assembly to bend, which would in turn probably lead to catastrophic contact between the stator and the high-speed rotor.

This project is ongoing, and we are currently beginning construction of the first complete ruggedized pumps.

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