Miniature ring-orbitron getter ion pumps have been proposed for supplying high vacuums to advanced scientific instruments expected to be developed in the next few years. Examples of such instruments include electron microscopes, ion mass spectrometers, and instruments based on electron probes.

A conventional orbitron getter ion pump (shown in the top part of the figure) includes a positively biased rod electrode on the axis of a cylindrical cavity with typical dimensions of tens of centimeters. Electrons are injected into the cavity, where they collide with and thereby ionize residual gas molecules. The resulting ions are accelerated toward, and become buried in, an ion-getter material on cavity surfaces. Because of the positively biased rod, the injected electrons get caught in orbits around the rod. The orbiting confines the electrons in a region away from the walls, thus increasing the electron path lengths and the probability that the electrons collide with the gas molecules, leading to increased efficiency of pumping.

The Positive Electrode in the Proposed Orbitron Pump would be a ring around the axis, instead of an axial rod as in the conventional orbitron. In the proposed orbitron, unlike in the conventional orbitron, there would be no need for negative bias on the end walls to reflect escaping electrons back into the cylindrical cavity.

In a conventional large orbitron getter ion pump, a negative bias is applied to the flat end walls of the cylindrical cavity to deflect the approaching electrons back into the cavity. However, the required relatively large voltage becomes increasingly impractical as the size of the pump is reduced. Thus, miniaturization must entail elimination of negative bias on the end walls; this makes it necessary to find another way to confine electrons in the cavity.

The proposed ring orbitron configuration would provide the needed confinement. The rod electrode of the conventional orbitron would be replaced with a wire ring electrode, as shown in the bottom part of the figure. As in the case of the rod electrode, positive bias on the ring electrode would create a potential well, causing the electrons to spiral around the ring, and the electrons would be injected slightly off-ring to give them enough angular momentum to go into the orbits.

Unlike a conventional orbitron, a ring orbitron would be scalable to subcentimeter dimensions. In the fabrication of miniature orbitron pumps, bulk and surface micromachining and lithography could be used to define ring electrodes, ring-supporting posts, and electron emitters. Cavities could be fabricated from stacks of micromachined wafers.

This work was done by Jaroslava Z. Wilcox, Thomas George, and Jason Feldman of Caltech for NASA's Jet Propulsion Laboratory. NPO-20436

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Miniature ring-orbitron getter ion vacuum pumps

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