Microscopic cathodes based on field emission (in contradistinction to thermionic emission) are undergoing development with a view toward using them as miniature or scalable sources of electrons in diverse applications that could include spacecraft thrusters, semiconductor-fabrication equipment, flat-panel display devices, miniature x-ray sources, and electrodynamic tethers and mass spectrometers. Increasing current levels can be accomplished by increasing the number of tips in an array. The basic concepts of utilizing field-emission cathodes for such applications and of scaling up by enlarging arrays are not new; the novel aspect of the present developmental cold cathodes lies in a microfabricated cathode lens and ion repeller (CLAIR) similar to an Einzel lens which will enable the integration of field-emission cathodes with electric propulsion systems, electrodynamic tethers, and instruments while meeting performance and lifetime requirements.
The performance requirements include emitter currents in the milliampere range at gate-electrode potentials between 10 and 70 V, high efficiency, and ability of emitter tips to strongly resist sputtering by impinging ions. More specifically:
- Gate potentials are required to be low in order to minimize the kinetic energies of ions bombarding emitter tips.
- In some applications, electron energies >20 eV are required to increase the space-charge current limit in plasma environments.
- For efficiency, leakage currents through gate electrodes must be kept to small fractions of emitted currents; it should not be difficult to satisfy this requirement in that gate leakage current is typically as little as a thousandth of the emitted current.
- The ability of an emitter tip to resist sputtering and poisoning with oxygen depends largely on the emitter material. It is desirable to choose an emitter material which is stable in an oxygen-rich environment, is not easily sputtered away when bombarded with ions, and has a low work function. Several materials are under investigation to meet these demands.
The figure depicts (not to scale) the configuration of an array of field-emission cathodes with CLAIR. Typical electrode thicknesses (ti), interelectrode distances (di), aperture diameter (f), and operating potentials (Vi) are shown according to one of the design concepts. Acting in concert, the V1, V2, and V3 electrodes would accelerate or decelerate and focus the emitted electron beam. Singly-charged ions entering the cathode with kinetic energies below 65 eV would be retarded by the electric field between V2 and V3. In the case of a spacecraft thruster, the V3 electrode would also shield the ion-retarding electrodes from electrons in the thruster discharge. The CLAIR configuration without the field emission tips can be used as an ion energy analyzer in plasma environments.
This work was done by Colleen Marrese of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.