A Field Emission Ionization Source Using Spindt Cathodes, 15-9159

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Principal Investigators
Martin P. Wüest
Gregory P. Miller

Inclusive Dates: 09/15/99 - Current

Background - Electron impact is the most common form of ionization for the neutral gas analysis of small molecules by mass spectrometry. Traditionally, thermionic electron emission from a filament is used to generate the electrons. Power is a premium resource on scientific space vehicles, and scientists strive to reduce power consumption as much as possible. Field emission cathodes provide an alternative approach at generating electrons without the heating power needed in a filament-based design. Field emitter cathodes are electron sources, in the form of arrays of microfabricated sharp tips. Field emission is used to extract the electrons without heating the cathodes. As a solid-state device, the field emission cathodes should be perfect for a space application. However, one issue that needs to be investigated is the lifetime of the device. Emission depends upon tip radius and work function, which may change during operation and degrade the device characteristics. Sputtering of the tip by residual gas molecules ionized by the emitted electrons blunt the emission point and increases the required voltage. The sputtering is highly dependent on the gas environment. Hydrogen-rich molecules tend to increase the emission, while oxygen-based molecules tend to decrease the emission. No report could be found in the literature concerning nitrogen, the most abundant gas in the Earth's atmosphere and the second most abundant gas on Mars' atmosphere.

Approach - Because the lifetime depends on the gases present and the microtip material, the team intends to test three different types of field emission arrays. The team has chosen an original Spindt-type cathode array, in which the tips are made of molybdenum, an array where the microtip material is silicon, and a diamond-like carbon cathode. The researchers will subject the different microtip arrays to varying gases that are relevant to planetary neutral gas mass spectrometry and investigate the emission characteristics as function of time and gas exposure. In addition, the team will develop and build an ion source using microtips to get actual performance data of a microtip-based electron source.

Accomplishments - The team has tested samples of all three field-emission types. To date, the team has been unable to operate the cathodes for more than a few hours. Therefore, no lifetime tests with gases could be performed. Parts of the ion source have been fabricated and are ready to be assembled and tested.

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