Nasa Tech Briefs

A method eliminates (or recovers from) residual methane buildup in getter-pumped atomic frequency standard systems by applying ionizing assistance. Ultra-high stability trapped ion frequency standards for applications requiring very high reliability, and/or low power and mass (both for ground-based and space-based platforms) benefit from using sealed vacuum systems. These systems require careful material selection and system processing (cleaning and high-temperature bake-out). Even under the most careful preparation, residual hydrogen outgassing from vacuum chamber walls typically limits the base pressure.

Non-evaporable getter pumps (NEGs) provide a convenient pumping option for sealed systems because of low mass and volume, and no power once activated. However, NEGs do not pump inert gases, methane, and some other hydrocarbon gases. For ultra-high vacuum applications, methane can become the single largest unpumped component. Methane collisions with trapped ions (such as 199Hg+) used for frequency standard applications can produce decoherence and a very large frequency shift, both significant limitations to high-performance frequency standard operation. Therefore, any methane presence, or buildup in the vacuum system over time, can negate the benefit of getter pumping and degrade frequency standard performance.

It is well known that the presence of a hot surface at or above a particular temperature threshold in a vacuum chamber can “crack” residual methane (CH4 or other similar hydrocarbons) , dissociating it into C and H2. Each of these can be readily removed by a getter pump. This cracking process can occur when methane molecules interact with the hot tungsten filament of an ion gauge (ionization-assisted gettering). In this case, methane molecules are dissociated either via direct interaction with the hot filament or via electron impact. Thus an ion gauge in conjunction with a NEG can be used to provide a low-mass, low-power method for avoiding the deleterious effects of methane buildup in high-performance frequency standard vacuum systems.

This work was done by Robert L. Tjoelker and Eric A. Burt of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at under the Physical Sciences category. NPO-46208

This Brief includes a Technical Support Package (TSP).

Ionization-Assisted Getter Pumping for Ultra-Stable Trapped Ion Frequency Standards (reference NPO-46208) is currently available for download from the TSP library.

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